diff --git a/tests/snippets/bools.py b/tests/snippets/bools.py
index 50bd39f7e..2aa817ca4 100644
--- a/tests/snippets/bools.py
+++ b/tests/snippets/bools.py
@@ -40,8 +40,6 @@ assert (True and True)
 assert not (False and fake)
 assert (True and 5) == 5
 
-assert bool.__doc__ == "bool(x) -> bool\n\nReturns True when the argument x is true, False otherwise.\nThe builtins True and False are the only two instances of the class bool.\nThe class bool is a subclass of the class int, and cannot be subclassed."
-
 # Bools are also ints.
 assert isinstance(True, int)
 assert True + True == 2
diff --git a/tests/snippets/builtin_enumerate.py b/tests/snippets/builtin_enumerate.py
new file mode 100644
index 000000000..75f6f7412
--- /dev/null
+++ b/tests/snippets/builtin_enumerate.py
@@ -0,0 +1,23 @@
+assert list(enumerate(['a', 'b', 'c'])) == [(0, 'a'), (1, 'b'), (2, 'c')]
+
+assert type(enumerate([])) == enumerate
+
+assert list(enumerate(['a', 'b', 'c'], -100)) == [(-100, 'a'), (-99, 'b'), (-98, 'c')]
+assert list(enumerate(['a', 'b', 'c'], 2**200)) == [(2**200, 'a'), (2**200 + 1, 'b'), (2**200 + 2, 'c')]
+
+
+# test infinite iterator
+class Counter(object):
+    counter = 0
+
+    def __next__(self):
+        self.counter += 1
+        return self.counter
+
+    def __iter__(self):
+        return self
+
+
+it = enumerate(Counter())
+assert next(it) == (0, 1)
+assert next(it) == (1, 2)
diff --git a/tests/snippets/builtin_range.py b/tests/snippets/builtin_range.py
index 5e58ae069..c8efb189d 100644
--- a/tests/snippets/builtin_range.py
+++ b/tests/snippets/builtin_range.py
@@ -18,10 +18,16 @@ def assert_raises(expr, exc_type):
 
 assert range(2**63+1)[2**63] == 9223372036854775808
 
+# len tests
+assert len(range(10, 5)) == 0, 'Range with no elements should have length = 0'
+assert len(range(10, 5, -2)) == 3, 'Expected length 3, for elements: 10, 8, 6'
+assert len(range(5, 10, 2)) == 3, 'Expected length 3, for elements: 5, 7, 9'
+
 # index tests
 assert range(10).index(6) == 6
 assert range(4, 10).index(6) == 2
 assert range(4, 10, 2).index(6) == 1
+assert range(10, 4, -2).index(8) == 1
 
 # index raises value error on out of bounds
 assert_raises(lambda _: range(10).index(-1), ValueError)
@@ -29,3 +35,25 @@ assert_raises(lambda _: range(10).index(10), ValueError)
 
 # index raises value error if out of step
 assert_raises(lambda _: range(4, 10, 2).index(5), ValueError)
+
+# index raises value error if needle is not an int
+assert_raises(lambda _: range(10).index('foo'), ValueError)
+
+# __bool__
+assert range(1).__bool__()
+assert range(1, 2).__bool__()
+
+assert not range(0).__bool__()
+assert not range(1, 1).__bool__()
+
+# __contains__
+assert range(10).__contains__(6)
+assert range(4, 10).__contains__(6)
+assert range(4, 10, 2).__contains__(6)
+assert range(10, 4, -2).__contains__(10)
+assert range(10, 4, -2).__contains__(8)
+
+assert not range(10).__contains__(-1)
+assert not range(10, 4, -2).__contains__(9)
+assert not range(10, 4, -2).__contains__(4)
+assert not range(10).__contains__('foo')
diff --git a/tests/snippets/builtin_slice.py b/tests/snippets/builtin_slice.py
new file mode 100644
index 000000000..b7c3922c0
--- /dev/null
+++ b/tests/snippets/builtin_slice.py
@@ -0,0 +1,77 @@
+
+a = []
+assert a[:] == []
+assert a[:2**100] == []
+assert a[-2**100:] == []
+assert a[::2**100] == []
+assert a[10:20] == []
+assert a[-20:-10] == []
+
+b = [1, 2]
+
+assert b[:] == [1, 2]
+assert b[:2**100] == [1, 2]
+assert b[-2**100:] == [1, 2]
+assert b[2**100:] == []
+assert b[::2**100] == [1]
+assert b[-10:1] == [1]
+assert b[0:0] == []
+assert b[1:0] == []
+
+try:
+    _ = b[::0]
+except ValueError:
+    pass
+else:
+    assert False, "Zero step slice should raise ValueError"
+
+assert b[::-1] == [2, 1]
+assert b[1::-1] == [2, 1]
+assert b[0::-1] == [1]
+assert b[0:-5:-1] == [1]
+assert b[:0:-1] == [2]
+assert b[5:0:-1] == [2]
+
+c = list(range(10))
+
+assert c[9:6:-3] == [9]
+assert c[9::-3] == [9, 6, 3, 0]
+assert c[9::-4] == [9, 5, 1]
+assert c[8::-2**100] == [8]
+
+assert c[7:7:-2] == []
+assert c[7:8:-2] == []
+
+d = "123456"
+
+assert d[3::-1] == "4321"
+assert d[4::-3] == "52"
+
+
+slice_a = slice(5)
+assert slice_a.start is None
+assert slice_a.stop == 5
+assert slice_a.step is None
+
+slice_b = slice(1, 5)
+assert slice_b.start == 1
+assert slice_b.stop == 5
+assert slice_b.step is None
+
+slice_c = slice(1, 5, 2)
+assert slice_c.start == 1
+assert slice_c.stop == 5
+assert slice_c.step == 2
+
+
+class SubScript(object):
+    def __getitem__(self, item):
+        assert type(item) == slice
+
+    def __setitem__(self, key, value):
+        assert type(key) == slice
+
+
+ss = SubScript()
+_ = ss[:]
+ss[:1] = 1
diff --git a/tests/snippets/builtin_zip.py b/tests/snippets/builtin_zip.py
new file mode 100644
index 000000000..3665c7702
--- /dev/null
+++ b/tests/snippets/builtin_zip.py
@@ -0,0 +1,24 @@
+assert list(zip(['a', 'b', 'c'], range(3), [9, 8, 7, 99])) == [('a', 0, 9), ('b', 1, 8), ('c', 2, 7)]
+
+assert list(zip(['a', 'b', 'c'])) == [('a',), ('b',), ('c',)]
+assert list(zip()) == []
+
+assert list(zip(*zip(['a', 'b', 'c'], range(1, 4)))) == [('a', 'b', 'c'), (1, 2, 3)]
+
+
+# test infinite iterator
+class Counter(object):
+    def __init__(self, counter=0):
+        self.counter = counter
+
+    def __next__(self):
+        self.counter += 1
+        return self.counter
+
+    def __iter__(self):
+        return self
+
+
+it = zip(Counter(), Counter(3))
+assert next(it) == (1, 4)
+assert next(it) == (2, 5)
diff --git a/tests/snippets/builtins.py b/tests/snippets/builtins.py
index 539b49ef7..76b28a7b9 100644
--- a/tests/snippets/builtins.py
+++ b/tests/snippets/builtins.py
@@ -5,12 +5,8 @@ assert callable(type)
 # TODO:
 # assert callable(callable)
 
-assert list(enumerate(['a', 'b', 'c'])) == [(0, 'a'), (1, 'b'), (2, 'c')]
-
 assert type(frozenset) is type
 
-assert list(zip(['a', 'b', 'c'], range(3), [9, 8, 7, 99])) == [('a', 0, 9), ('b', 1, 8), ('c', 2, 7)]
-
 assert 3 == eval('1+2')
 
 code = compile('5+3', 'x.py', 'eval')
diff --git a/tests/snippets/division_by_zero.py b/tests/snippets/division_by_zero.py
new file mode 100644
index 000000000..7cb68cd76
--- /dev/null
+++ b/tests/snippets/division_by_zero.py
@@ -0,0 +1,34 @@
+try:
+    5 / 0
+except ZeroDivisionError:
+    pass
+else:
+    assert False, 'Expected ZeroDivisionError'
+
+try:
+    5 / -0.0
+except ZeroDivisionError:
+    pass
+else:
+    assert False, 'Expected ZeroDivisionError'
+
+try:
+    5 / (2-2)
+except ZeroDivisionError:
+    pass
+else:
+    assert False, 'Expected ZeroDivisionError'
+
+try:
+    5 % 0
+except ZeroDivisionError:
+    pass
+else:
+    assert False, 'Expected ZeroDivisionError'
+
+try:
+    raise ZeroDivisionError('Is an ArithmeticError subclass?')
+except ArithmeticError:
+    pass
+else:
+    assert False, 'Expected ZeroDivisionError'
diff --git a/tests/snippets/numbers.py b/tests/snippets/numbers.py
index 12e892385..ecd2263af 100644
--- a/tests/snippets/numbers.py
+++ b/tests/snippets/numbers.py
@@ -2,8 +2,6 @@ x = 5
 x.__init__(6)
 assert x == 5
 
-assert int.__doc__ == "int(x=0) -> integer\nint(x, base=10) -> integer\n\nConvert a number or string to an integer, or return 0 if no arguments\nare given.  If x is a number, return x.__int__().  For floating point\nnumbers, this truncates towards zero.\n\nIf x is not a number or if base is given, then x must be a string,\nbytes, or bytearray instance representing an integer literal in the\ngiven base.  The literal can be preceded by '+' or '-' and be surrounded\nby whitespace.  The base defaults to 10.  Valid bases are 0 and 2-36.\nBase 0 means to interpret the base from the string as an integer literal.\n>>> int('0b100', base=0)\n4"
-
 
 class A(int):
     pass
diff --git a/tests/snippets/tuple.py b/tests/snippets/tuple.py
index 56a61f827..eb5102fa3 100644
--- a/tests/snippets/tuple.py
+++ b/tests/snippets/tuple.py
@@ -15,3 +15,7 @@ assert x * -1 == ()  # integers less than zero treated as 0
 
 assert y < x, "tuple __lt__ failed"
 assert x > y, "tuple __gt__ failed"
+
+
+b = (1,2,3)
+assert b.index(2) == 1
diff --git a/vm/src/builtins.rs b/vm/src/builtins.rs
index 2e789383e..cad71c42e 100644
--- a/vm/src/builtins.rs
+++ b/vm/src/builtins.rs
@@ -21,8 +21,7 @@ use super::pyobject::{
 use super::stdlib::io::io_open;
 
 use super::vm::VirtualMachine;
-use num_bigint::ToBigInt;
-use num_traits::{Signed, ToPrimitive, Zero};
+use num_traits::{Signed, ToPrimitive};
 
 fn get_locals(vm: &mut VirtualMachine) -> PyObjectRef {
     let d = vm.new_dict();
@@ -136,11 +135,11 @@ fn builtin_compile(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 
     let mode = {
         let mode = objstr::get_value(mode);
-        if mode == String::from("exec") {
+        if mode == "exec" {
             compile::Mode::Exec
-        } else if mode == "eval".to_string() {
+        } else if mode == "eval" {
             compile::Mode::Eval
-        } else if mode == "single".to_string() {
+        } else if mode == "single" {
             compile::Mode::Single
         } else {
             return Err(
@@ -180,29 +179,6 @@ fn builtin_divmod(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     }
 }
 
-fn builtin_enumerate(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
-    arg_check!(
-        vm,
-        args,
-        required = [(iterable, None)],
-        optional = [(start, None)]
-    );
-    let items = vm.extract_elements(iterable)?;
-    let start = if let Some(start) = start {
-        objint::get_value(start)
-    } else {
-        Zero::zero()
-    };
-    let mut new_items = vec![];
-    for (i, item) in items.into_iter().enumerate() {
-        let element = vm
-            .ctx
-            .new_tuple(vec![vm.ctx.new_int(i.to_bigint().unwrap() + &start), item]);
-        new_items.push(element);
-    }
-    Ok(vm.ctx.new_list(new_items))
-}
-
 /// Implements `eval`.
 /// See also: https://docs.python.org/3/library/functions.html#eval
 fn builtin_eval(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -359,7 +335,7 @@ fn builtin_hex(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 fn builtin_id(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(obj, None)]);
 
-    Ok(vm.context().new_int(obj.get_id().to_bigint().unwrap()))
+    Ok(vm.context().new_int(obj.get_id()))
 }
 
 // builtin_input
@@ -553,9 +529,7 @@ fn builtin_ord(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         )));
     }
     match string.chars().next() {
-        Some(character) => Ok(vm
-            .context()
-            .new_int((character as i32).to_bigint().unwrap())),
+        Some(character) => Ok(vm.context().new_int(character as i32)),
         None => Err(vm.new_type_error(
             "ord() could not guess the integer representing this character".to_string(),
         )),
@@ -635,7 +609,7 @@ fn builtin_sum(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     let items = vm.extract_elements(iterable)?;
 
     // Start with zero and add at will:
-    let mut sum = vm.ctx.new_int(Zero::zero());
+    let mut sum = vm.ctx.new_int(0);
     for item in items {
         sum = vm._add(sum, item)?;
     }
@@ -643,32 +617,6 @@ fn builtin_sum(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 }
 
 // builtin_vars
-
-fn builtin_zip(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
-    no_kwargs!(vm, args);
-
-    // TODO: process one element at a time from iterators.
-    let mut iterables = vec![];
-    for iterable in args.args.iter() {
-        let iterable = vm.extract_elements(iterable)?;
-        iterables.push(iterable);
-    }
-
-    let minsize: usize = iterables.iter().map(|i| i.len()).min().unwrap_or(0);
-
-    let mut new_items = vec![];
-    for i in 0..minsize {
-        let items = iterables
-            .iter()
-            .map(|iterable| iterable[i].clone())
-            .collect();
-        let element = vm.ctx.new_tuple(items);
-        new_items.push(element);
-    }
-
-    Ok(vm.ctx.new_list(new_items))
-}
-
 // builtin___import__
 
 pub fn make_module(ctx: &PyContext) -> PyObjectRef {
@@ -694,7 +642,7 @@ pub fn make_module(ctx: &PyContext) -> PyObjectRef {
     ctx.set_attr(&py_mod, "dict", ctx.dict_type());
     ctx.set_attr(&py_mod, "divmod", ctx.new_rustfunc(builtin_divmod));
     ctx.set_attr(&py_mod, "dir", ctx.new_rustfunc(builtin_dir));
-    ctx.set_attr(&py_mod, "enumerate", ctx.new_rustfunc(builtin_enumerate));
+    ctx.set_attr(&py_mod, "enumerate", ctx.enumerate_type());
     ctx.set_attr(&py_mod, "eval", ctx.new_rustfunc(builtin_eval));
     ctx.set_attr(&py_mod, "exec", ctx.new_rustfunc(builtin_exec));
     ctx.set_attr(&py_mod, "float", ctx.float_type());
@@ -729,13 +677,14 @@ pub fn make_module(ctx: &PyContext) -> PyObjectRef {
     ctx.set_attr(&py_mod, "repr", ctx.new_rustfunc(builtin_repr));
     ctx.set_attr(&py_mod, "set", ctx.set_type());
     ctx.set_attr(&py_mod, "setattr", ctx.new_rustfunc(builtin_setattr));
+    ctx.set_attr(&py_mod, "slice", ctx.slice_type());
     ctx.set_attr(&py_mod, "staticmethod", ctx.staticmethod_type());
     ctx.set_attr(&py_mod, "str", ctx.str_type());
     ctx.set_attr(&py_mod, "sum", ctx.new_rustfunc(builtin_sum));
     ctx.set_attr(&py_mod, "super", ctx.super_type());
     ctx.set_attr(&py_mod, "tuple", ctx.tuple_type());
     ctx.set_attr(&py_mod, "type", ctx.type_type());
-    ctx.set_attr(&py_mod, "zip", ctx.new_rustfunc(builtin_zip));
+    ctx.set_attr(&py_mod, "zip", ctx.zip_type());
 
     // Exceptions:
     ctx.set_attr(
@@ -744,6 +693,11 @@ pub fn make_module(ctx: &PyContext) -> PyObjectRef {
         ctx.exceptions.base_exception_type.clone(),
     );
     ctx.set_attr(&py_mod, "Exception", ctx.exceptions.exception_type.clone());
+    ctx.set_attr(
+        &py_mod,
+        "ArithmeticError",
+        ctx.exceptions.arithmetic_error.clone(),
+    );
     ctx.set_attr(
         &py_mod,
         "AssertionError",
@@ -755,6 +709,11 @@ pub fn make_module(ctx: &PyContext) -> PyObjectRef {
         ctx.exceptions.attribute_error.clone(),
     );
     ctx.set_attr(&py_mod, "NameError", ctx.exceptions.name_error.clone());
+    ctx.set_attr(
+        &py_mod,
+        "OverflowError",
+        ctx.exceptions.overflow_error.clone(),
+    );
     ctx.set_attr(
         &py_mod,
         "RuntimeError",
@@ -774,6 +733,11 @@ pub fn make_module(ctx: &PyContext) -> PyObjectRef {
         "StopIteration",
         ctx.exceptions.stop_iteration.clone(),
     );
+    ctx.set_attr(
+        &py_mod,
+        "ZeroDivisionError",
+        ctx.exceptions.zero_division_error.clone(),
+    );
 
     py_mod
 }
diff --git a/vm/src/compile.rs b/vm/src/compile.rs
index fb9215341..9050e59ce 100644
--- a/vm/src/compile.rs
+++ b/vm/src/compile.rs
@@ -45,9 +45,8 @@ pub fn compile(
         },
     };
 
-    match result {
-        Err(msg) => return Err(vm.new_exception(syntax_error.clone(), msg)),
-        _ => {}
+    if let Err(msg) = result {
+        return Err(vm.new_exception(syntax_error.clone(), msg));
     }
 
     let code = compiler.pop_code_object();
@@ -589,7 +588,7 @@ impl Compiler {
             ast::Statement::Assign { targets, value } => {
                 self.compile_expression(value)?;
 
-                for (i, target) in targets.into_iter().enumerate() {
+                for (i, target) in targets.iter().enumerate() {
                     if i + 1 != targets.len() {
                         self.emit(Instruction::Duplicate);
                     }
@@ -665,7 +664,7 @@ impl Compiler {
 
         let mut flags = bytecode::FunctionOpArg::empty();
         if have_kwargs {
-            flags = flags | bytecode::FunctionOpArg::HAS_DEFAULTS;
+            flags |= bytecode::FunctionOpArg::HAS_DEFAULTS;
         }
 
         Ok(flags)
diff --git a/vm/src/dictdatatype.rs b/vm/src/dictdatatype.rs
new file mode 100644
index 000000000..3c4dee22a
--- /dev/null
+++ b/vm/src/dictdatatype.rs
@@ -0,0 +1,230 @@
+use super::obj::objbool;
+use super::obj::objint;
+use super::pyobject::{IdProtocol, PyObjectRef, PyResult};
+use super::vm::VirtualMachine;
+use num_traits::ToPrimitive;
+/// Ordered dictionary implementation.
+/// Inspired by: https://morepypy.blogspot.com/2015/01/faster-more-memory-efficient-and-more.html
+/// And: https://www.youtube.com/watch?v=p33CVV29OG8
+/// And: http://code.activestate.com/recipes/578375/
+use std::collections::HashMap;
+
+pub struct Dict {
+    size: usize,
+    indices: HashMap<usize, usize>,
+    entries: Vec<Option<DictEntry>>,
+}
+
+struct DictEntry {
+    hash: usize,
+    key: PyObjectRef,
+    value: PyObjectRef,
+}
+
+impl Dict {
+    pub fn new() -> Self {
+        Dict {
+            size: 0,
+            indices: HashMap::new(),
+            entries: Vec::new(),
+        }
+    }
+
+    /// Store a key
+    pub fn insert(
+        &mut self,
+        vm: &mut VirtualMachine,
+        key: &PyObjectRef,
+        value: PyObjectRef,
+    ) -> Result<(), PyObjectRef> {
+        match self.lookup(vm, key)? {
+            LookupResult::Existing(index) => {
+                // Update existing key
+                if let Some(ref mut entry) = self.entries[index] {
+                    entry.value = value;
+                    Ok(())
+                } else {
+                    panic!("Lookup returned invalid index into entries!");
+                }
+            }
+            LookupResult::NewIndex {
+                hash_index,
+                hash_value,
+            } => {
+                // New key:
+                let entry = DictEntry {
+                    hash: hash_value,
+                    key: key.clone(),
+                    value,
+                };
+                let index = self.entries.len();
+                self.entries.push(Some(entry));
+                self.indices.insert(hash_index, index);
+                self.size += 1;
+                Ok(())
+            }
+        }
+    }
+
+    pub fn contains(
+        &self,
+        vm: &mut VirtualMachine,
+        key: &PyObjectRef,
+    ) -> Result<bool, PyObjectRef> {
+        if let LookupResult::Existing(_index) = self.lookup(vm, key)? {
+            Ok(true)
+        } else {
+            Ok(false)
+        }
+    }
+
+    /// Retrieve a key
+    pub fn get(&self, vm: &mut VirtualMachine, key: &PyObjectRef) -> PyResult {
+        if let LookupResult::Existing(index) = self.lookup(vm, key)? {
+            if let Some(entry) = &self.entries[index] {
+                Ok(entry.value.clone())
+            } else {
+                panic!("Lookup returned invalid index into entries!");
+            }
+        } else {
+            let key_repr = vm.to_pystr(key)?;
+            Err(vm.new_value_error(format!("Key not found: {}", key_repr)))
+        }
+    }
+
+    /// Delete a key
+    pub fn delete(
+        &mut self,
+        vm: &mut VirtualMachine,
+        key: &PyObjectRef,
+    ) -> Result<(), PyObjectRef> {
+        if let LookupResult::Existing(index) = self.lookup(vm, key)? {
+            self.entries[index] = None;
+            self.size -= 1;
+            Ok(())
+        } else {
+            let key_repr = vm.to_pystr(key)?;
+            Err(vm.new_value_error(format!("Key not found: {}", key_repr)))
+        }
+    }
+
+    pub fn len(&self) -> usize {
+        self.size
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    pub fn get_items(&self) -> Vec<(PyObjectRef, PyObjectRef)> {
+        self.entries
+            .iter()
+            .filter(|e| e.is_some())
+            .map(|e| e.as_ref().unwrap())
+            .map(|e| (e.key.clone(), e.value.clone()))
+            .collect()
+    }
+
+    /// Lookup the index for the given key.
+    fn lookup(
+        &self,
+        vm: &mut VirtualMachine,
+        key: &PyObjectRef,
+    ) -> Result<LookupResult, PyObjectRef> {
+        let hash_value = calc_hash(vm, key)?;
+        let perturb = hash_value;
+        let mut hash_index: usize = hash_value;
+        loop {
+            if self.indices.contains_key(&hash_index) {
+                // Now we have an index, lets check the key.
+                let index = self.indices[&hash_index];
+                if let Some(entry) = &self.entries[index] {
+                    // Okay, we have an entry at this place
+                    if entry.key.is(key) {
+                        // Literally the same object
+                        break Ok(LookupResult::Existing(index));
+                    } else if entry.hash == hash_value {
+                        if do_eq(vm, &entry.key, key)? {
+                            break Ok(LookupResult::Existing(index));
+                        } else {
+                            // entry mismatch.
+                        }
+                    } else {
+                        // entry mismatch.
+                    }
+                } else {
+                    // Removed entry, continue search...
+                }
+            } else {
+                // Hash not in table, we are at free slot now.
+                break Ok(LookupResult::NewIndex {
+                    hash_value,
+                    hash_index,
+                });
+            }
+
+            // Update i to next probe location:
+            hash_index = hash_index
+                .wrapping_mul(5)
+                .wrapping_add(perturb)
+                .wrapping_add(1);
+            // warn!("Perturb value: {}", i);
+        }
+    }
+}
+
+enum LookupResult {
+    NewIndex {
+        hash_value: usize,
+        hash_index: usize,
+    }, // return not found, index into indices
+    Existing(usize), // Existing record, index into entries
+}
+
+fn calc_hash(vm: &mut VirtualMachine, key: &PyObjectRef) -> Result<usize, PyObjectRef> {
+    let hash = vm.call_method(key, "__hash__", vec![])?;
+    Ok(objint::get_value(&hash).to_usize().unwrap())
+}
+
+/// Invoke __eq__ on two keys
+fn do_eq(
+    vm: &mut VirtualMachine,
+    key1: &PyObjectRef,
+    key2: &PyObjectRef,
+) -> Result<bool, PyObjectRef> {
+    let result = vm._eq(key1, key2.clone())?;
+    Ok(objbool::get_value(&result))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{Dict, VirtualMachine};
+
+    #[test]
+    fn test_insert() {
+        let mut vm = VirtualMachine::new();
+        let mut dict = Dict::new();
+        assert_eq!(0, dict.len());
+
+        let key1 = vm.new_bool(true);
+        let value1 = vm.new_str("abc".to_string());
+        dict.insert(&mut vm, &key1, value1.clone()).unwrap();
+        assert_eq!(1, dict.len());
+
+        let key2 = vm.new_str("x".to_string());
+        let value2 = vm.new_str("def".to_string());
+        dict.insert(&mut vm, &key2, value2.clone()).unwrap();
+        assert_eq!(2, dict.len());
+
+        dict.insert(&mut vm, &key1, value2.clone()).unwrap();
+        assert_eq!(2, dict.len());
+
+        dict.delete(&mut vm, &key1).unwrap();
+        assert_eq!(1, dict.len());
+
+        dict.insert(&mut vm, &key1, value2).unwrap();
+        assert_eq!(2, dict.len());
+
+        assert_eq!(true, dict.contains(&mut vm, &key1).unwrap());
+    }
+}
diff --git a/vm/src/exceptions.rs b/vm/src/exceptions.rs
index 305297919..be3652a73 100644
--- a/vm/src/exceptions.rs
+++ b/vm/src/exceptions.rs
@@ -81,6 +81,7 @@ fn exception_str(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 
 #[derive(Debug)]
 pub struct ExceptionZoo {
+    pub arithmetic_error: PyObjectRef,
     pub assertion_error: PyObjectRef,
     pub attribute_error: PyObjectRef,
     pub base_exception_type: PyObjectRef,
@@ -93,12 +94,14 @@ pub struct ExceptionZoo {
     pub name_error: PyObjectRef,
     pub not_implemented_error: PyObjectRef,
     pub os_error: PyObjectRef,
+    pub overflow_error: PyObjectRef,
     pub permission_error: PyObjectRef,
     pub runtime_error: PyObjectRef,
     pub stop_iteration: PyObjectRef,
     pub syntax_error: PyObjectRef,
     pub type_error: PyObjectRef,
     pub value_error: PyObjectRef,
+    pub zero_division_error: PyObjectRef,
 }
 
 impl ExceptionZoo {
@@ -113,6 +116,8 @@ impl ExceptionZoo {
 
         let exception_type = create_type("Exception", &type_type, &base_exception_type, &dict_type);
 
+        let arithmetic_error =
+            create_type("ArithmeticError", &type_type, &exception_type, &dict_type);
         let assertion_error =
             create_type("AssertionError", &type_type, &exception_type, &dict_type);
         let attribute_error =
@@ -128,8 +133,18 @@ impl ExceptionZoo {
         let type_error = create_type("TypeError", &type_type, &exception_type, &dict_type);
         let value_error = create_type("ValueError", &type_type, &exception_type, &dict_type);
 
+        let overflow_error =
+            create_type("OverflowError", &type_type, &arithmetic_error, &dict_type);
+        let zero_division_error = create_type(
+            "ZeroDivisionError",
+            &type_type,
+            &arithmetic_error,
+            &dict_type,
+        );
+
         let module_not_found_error =
             create_type("ModuleNotFoundError", &type_type, &import_error, &dict_type);
+
         let not_implemented_error = create_type(
             "NotImplementedError",
             &type_type,
@@ -142,6 +157,7 @@ impl ExceptionZoo {
         let permission_error = create_type("PermissionError", &type_type, &os_error, &dict_type);
 
         ExceptionZoo {
+            arithmetic_error,
             assertion_error,
             attribute_error,
             base_exception_type,
@@ -154,12 +170,14 @@ impl ExceptionZoo {
             name_error,
             not_implemented_error,
             os_error,
+            overflow_error,
             permission_error,
             runtime_error,
             stop_iteration,
             syntax_error,
             type_error,
             value_error,
+            zero_division_error,
         }
     }
 }
diff --git a/vm/src/format.rs b/vm/src/format.rs
index 5b4035ef7..6dfa77c7e 100644
--- a/vm/src/format.rs
+++ b/vm/src/format.rs
@@ -314,7 +314,7 @@ impl FormatSpec {
             }
             None => Ok(magnitude.to_str_radix(10)),
         };
-        if !raw_magnitude_string_result.is_ok() {
+        if raw_magnitude_string_result.is_err() {
             return raw_magnitude_string_result;
         }
         let magnitude_string = format!(
diff --git a/vm/src/frame.rs b/vm/src/frame.rs
index ac118dcb7..86c0bc29d 100644
--- a/vm/src/frame.rs
+++ b/vm/src/frame.rs
@@ -20,8 +20,7 @@ use super::pyobject::{
     PyResult, TypeProtocol,
 };
 use super::vm::VirtualMachine;
-use num_bigint::ToBigInt;
-use num_traits::ToPrimitive;
+use num_bigint::BigInt;
 
 #[derive(Clone, Debug)]
 enum Block {
@@ -121,7 +120,7 @@ impl Frame {
                     trace!("Adding to traceback: {:?} {:?}", traceback, lineno);
                     let pos = vm.ctx.new_tuple(vec![
                         vm.ctx.new_str(filename.clone()),
-                        vm.ctx.new_int(lineno.get_row().to_bigint().unwrap()),
+                        vm.ctx.new_int(lineno.get_row()),
                         vm.ctx.new_str(run_obj_name.clone()),
                     ]);
                     objlist::list_append(
@@ -263,22 +262,22 @@ impl Frame {
                 assert!(*size == 2 || *size == 3);
                 let elements = self.pop_multiple(*size);
 
-                let mut out: Vec<Option<i32>> = elements
+                let mut out: Vec<Option<BigInt>> = elements
                     .into_iter()
                     .map(|x| match x.borrow().payload {
-                        PyObjectPayload::Integer { ref value } => Some(value.to_i32().unwrap()),
+                        PyObjectPayload::Integer { ref value } => Some(value.clone()),
                         PyObjectPayload::None => None,
                         _ => panic!("Expect Int or None as BUILD_SLICE arguments, got {:?}", x),
                     })
                     .collect();
 
-                let start = out[0];
-                let stop = out[1];
-                let step = if out.len() == 3 { out[2] } else { None };
+                let start = out[0].take();
+                let stop = out[1].take();
+                let step = if out.len() == 3 { out[2].take() } else { None };
 
                 let obj = PyObject::new(
                     PyObjectPayload::Slice { start, stop, step },
-                    vm.ctx.type_type(),
+                    vm.ctx.slice_type(),
                 );
                 self.push_value(obj);
                 Ok(None)
@@ -611,7 +610,7 @@ impl Frame {
                         .iter()
                         .skip(*before)
                         .take(middle)
-                        .map(|x| x.clone())
+                        .cloned()
                         .collect();
                     let t = vm.ctx.new_list(middle_elements);
                     self.push_value(t);
@@ -887,25 +886,25 @@ impl Frame {
     ) -> FrameResult {
         let b_ref = self.pop_value();
         let a_ref = self.pop_value();
-        let value = match op {
-            &bytecode::BinaryOperator::Subtract => vm._sub(a_ref, b_ref),
-            &bytecode::BinaryOperator::Add => vm._add(a_ref, b_ref),
-            &bytecode::BinaryOperator::Multiply => vm._mul(a_ref, b_ref),
-            &bytecode::BinaryOperator::MatrixMultiply => {
+        let value = match *op {
+            bytecode::BinaryOperator::Subtract => vm._sub(a_ref, b_ref),
+            bytecode::BinaryOperator::Add => vm._add(a_ref, b_ref),
+            bytecode::BinaryOperator::Multiply => vm._mul(a_ref, b_ref),
+            bytecode::BinaryOperator::MatrixMultiply => {
                 vm.call_method(&a_ref, "__matmul__", vec![b_ref])
             }
-            &bytecode::BinaryOperator::Power => vm._pow(a_ref, b_ref),
-            &bytecode::BinaryOperator::Divide => vm._div(a_ref, b_ref),
-            &bytecode::BinaryOperator::FloorDivide => {
+            bytecode::BinaryOperator::Power => vm._pow(a_ref, b_ref),
+            bytecode::BinaryOperator::Divide => vm._div(a_ref, b_ref),
+            bytecode::BinaryOperator::FloorDivide => {
                 vm.call_method(&a_ref, "__floordiv__", vec![b_ref])
             }
-            &bytecode::BinaryOperator::Subscript => self.subscript(vm, a_ref, b_ref),
-            &bytecode::BinaryOperator::Modulo => vm._modulo(a_ref, b_ref),
-            &bytecode::BinaryOperator::Lshift => vm.call_method(&a_ref, "__lshift__", vec![b_ref]),
-            &bytecode::BinaryOperator::Rshift => vm.call_method(&a_ref, "__rshift__", vec![b_ref]),
-            &bytecode::BinaryOperator::Xor => vm._xor(a_ref, b_ref),
-            &bytecode::BinaryOperator::Or => vm._or(a_ref, b_ref),
-            &bytecode::BinaryOperator::And => vm._and(a_ref, b_ref),
+            bytecode::BinaryOperator::Subscript => self.subscript(vm, a_ref, b_ref),
+            bytecode::BinaryOperator::Modulo => vm._modulo(a_ref, b_ref),
+            bytecode::BinaryOperator::Lshift => vm.call_method(&a_ref, "__lshift__", vec![b_ref]),
+            bytecode::BinaryOperator::Rshift => vm.call_method(&a_ref, "__rshift__", vec![b_ref]),
+            bytecode::BinaryOperator::Xor => vm._xor(a_ref, b_ref),
+            bytecode::BinaryOperator::Or => vm._or(a_ref, b_ref),
+            bytecode::BinaryOperator::And => vm._and(a_ref, b_ref),
         }?;
 
         self.push_value(value);
@@ -918,11 +917,11 @@ impl Frame {
         op: &bytecode::UnaryOperator,
     ) -> FrameResult {
         let a = self.pop_value();
-        let value = match op {
-            &bytecode::UnaryOperator::Minus => vm.call_method(&a, "__neg__", vec![])?,
-            &bytecode::UnaryOperator::Plus => vm.call_method(&a, "__pos__", vec![])?,
-            &bytecode::UnaryOperator::Invert => vm.call_method(&a, "__invert__", vec![])?,
-            &bytecode::UnaryOperator::Not => {
+        let value = match *op {
+            bytecode::UnaryOperator::Minus => vm.call_method(&a, "__neg__", vec![])?,
+            bytecode::UnaryOperator::Plus => vm.call_method(&a, "__pos__", vec![])?,
+            bytecode::UnaryOperator::Invert => vm.call_method(&a, "__invert__", vec![])?,
+            bytecode::UnaryOperator::Not => {
                 let value = objbool::boolval(vm, a)?;
                 vm.ctx.new_bool(!value)
             }
@@ -995,17 +994,17 @@ impl Frame {
     ) -> FrameResult {
         let b = self.pop_value();
         let a = self.pop_value();
-        let value = match op {
-            &bytecode::ComparisonOperator::Equal => vm._eq(&a, b)?,
-            &bytecode::ComparisonOperator::NotEqual => vm._ne(&a, b)?,
-            &bytecode::ComparisonOperator::Less => vm._lt(&a, b)?,
-            &bytecode::ComparisonOperator::LessOrEqual => vm._le(&a, b)?,
-            &bytecode::ComparisonOperator::Greater => vm._gt(&a, b)?,
-            &bytecode::ComparisonOperator::GreaterOrEqual => vm._ge(&a, b)?,
-            &bytecode::ComparisonOperator::Is => vm.ctx.new_bool(self._is(a, b)),
-            &bytecode::ComparisonOperator::IsNot => self._is_not(vm, a, b)?,
-            &bytecode::ComparisonOperator::In => self._in(vm, a, b)?,
-            &bytecode::ComparisonOperator::NotIn => self._not_in(vm, a, b)?,
+        let value = match *op {
+            bytecode::ComparisonOperator::Equal => vm._eq(&a, b)?,
+            bytecode::ComparisonOperator::NotEqual => vm._ne(&a, b)?,
+            bytecode::ComparisonOperator::Less => vm._lt(&a, b)?,
+            bytecode::ComparisonOperator::LessOrEqual => vm._le(&a, b)?,
+            bytecode::ComparisonOperator::Greater => vm._gt(&a, b)?,
+            bytecode::ComparisonOperator::GreaterOrEqual => vm._ge(&a, b)?,
+            bytecode::ComparisonOperator::Is => vm.ctx.new_bool(self._is(a, b)),
+            bytecode::ComparisonOperator::IsNot => self._is_not(vm, a, b)?,
+            bytecode::ComparisonOperator::In => self._in(vm, a, b)?,
+            bytecode::ComparisonOperator::NotIn => self._not_in(vm, a, b)?,
         };
 
         self.push_value(value);
@@ -1036,7 +1035,7 @@ impl Frame {
 
     fn unwrap_constant(&self, vm: &VirtualMachine, value: &bytecode::Constant) -> PyObjectRef {
         match *value {
-            bytecode::Constant::Integer { ref value } => vm.ctx.new_int(value.to_bigint().unwrap()),
+            bytecode::Constant::Integer { ref value } => vm.ctx.new_int(value.clone()),
             bytecode::Constant::Float { ref value } => vm.ctx.new_float(*value),
             bytecode::Constant::Complex { ref value } => vm.ctx.new_complex(*value),
             bytecode::Constant::String { ref value } => vm.new_str(value.clone()),
diff --git a/vm/src/import.rs b/vm/src/import.rs
index e560d3b18..be8807881 100644
--- a/vm/src/import.rs
+++ b/vm/src/import.rs
@@ -103,7 +103,7 @@ fn find_source(vm: &VirtualMachine, current_path: PathBuf, name: &str) -> Result
         }
     }
 
-    match filepaths.iter().filter(|p| p.exists()).next() {
+    match filepaths.iter().find(|p| p.exists()) {
         Some(path) => Ok(path.to_path_buf()),
         None => Err(format!("No module named '{}'", name)),
     }
diff --git a/vm/src/obj/mod.rs b/vm/src/obj/mod.rs
index a60ce79ff..f08998fcb 100644
--- a/vm/src/obj/mod.rs
+++ b/vm/src/obj/mod.rs
@@ -6,6 +6,7 @@ pub mod objbytes;
 pub mod objcode;
 pub mod objcomplex;
 pub mod objdict;
+pub mod objenumerate;
 pub mod objfilter;
 pub mod objfloat;
 pub mod objframe;
@@ -21,7 +22,9 @@ pub mod objproperty;
 pub mod objrange;
 pub mod objsequence;
 pub mod objset;
+pub mod objslice;
 pub mod objstr;
 pub mod objsuper;
 pub mod objtuple;
 pub mod objtype;
+pub mod objzip;
diff --git a/vm/src/obj/objbool.rs b/vm/src/obj/objbool.rs
index f3a2fafb8..dbec65a7c 100644
--- a/vm/src/obj/objbool.rs
+++ b/vm/src/obj/objbool.rs
@@ -20,6 +20,7 @@ pub fn boolval(vm: &mut VirtualMachine, obj: PyObjectRef) -> Result<bool, PyObje
                     PyObjectPayload::Integer { ref value } => !value.is_zero(),
                     _ => return Err(vm.new_type_error(String::from("TypeError"))),
                 };
+
                 v
             } else {
                 true
@@ -81,7 +82,7 @@ fn bool_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     Ok(match val {
         Some(val) => {
             let bv = boolval(vm, val.clone())?;
-            vm.new_bool(bv.clone())
+            vm.new_bool(bv)
         }
         None => vm.context().new_bool(false),
     })
diff --git a/vm/src/obj/objbytearray.rs b/vm/src/obj/objbytearray.rs
index f62b689af..b4afaa639 100644
--- a/vm/src/obj/objbytearray.rs
+++ b/vm/src/obj/objbytearray.rs
@@ -9,7 +9,6 @@ use super::objint;
 use super::super::vm::VirtualMachine;
 use super::objbytes::get_value;
 use super::objtype;
-use num_bigint::ToBigInt;
 use num_traits::ToPrimitive;
 
 // Binary data support
@@ -17,6 +16,20 @@ use num_traits::ToPrimitive;
 /// Fill bytearray class methods dictionary.
 pub fn init(context: &PyContext) {
     let bytearray_type = &context.bytearray_type;
+
+    let bytearray_doc =
+        "bytearray(iterable_of_ints) -> bytearray\n\
+         bytearray(string, encoding[, errors]) -> bytearray\n\
+         bytearray(bytes_or_buffer) -> mutable copy of bytes_or_buffer\n\
+         bytearray(int) -> bytes array of size given by the parameter initialized with null bytes\n\
+         bytearray() -> empty bytes array\n\n\
+         Construct a mutable bytearray object from:\n  \
+         - an iterable yielding integers in range(256)\n  \
+         - a text string encoded using the specified encoding\n  \
+         - a bytes or a buffer object\n  \
+         - any object implementing the buffer API.\n  \
+         - an integer";
+
     context.set_attr(
         &bytearray_type,
         "__eq__",
@@ -37,6 +50,11 @@ pub fn init(context: &PyContext) {
         "__len__",
         context.new_rustfunc(bytesarray_len),
     );
+    context.set_attr(
+        &bytearray_type,
+        "__doc__",
+        context.new_str(bytearray_doc.to_string()),
+    );
     context.set_attr(
         &bytearray_type,
         "isalnum",
@@ -110,8 +128,7 @@ fn bytesarray_len(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(a, Some(vm.ctx.bytearray_type()))]);
 
     let byte_vec = get_value(a).to_vec();
-    let value = byte_vec.len().to_bigint();
-    Ok(vm.ctx.new_int(value.unwrap()))
+    Ok(vm.ctx.new_int(byte_vec.len()))
 }
 
 fn bytearray_eq(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -205,11 +222,12 @@ fn bytearray_istitle(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
                 }
             };
 
-            if is_cased(current) && next.is_uppercase() && !prev_cased {
-                return Ok(vm.new_bool(false));
-            } else if !is_cased(current) && next.is_lowercase() {
+            if (is_cased(current) && next.is_uppercase() && !prev_cased)
+                || (!is_cased(current) && next.is_lowercase())
+            {
                 return Ok(vm.new_bool(false));
             }
+
             prev_cased = is_cased(current);
         }
 
diff --git a/vm/src/obj/objbytes.rs b/vm/src/obj/objbytes.rs
index 6288264a2..1909247cb 100644
--- a/vm/src/obj/objbytes.rs
+++ b/vm/src/obj/objbytes.rs
@@ -4,7 +4,6 @@ use super::super::pyobject::{
 use super::super::vm::VirtualMachine;
 use super::objint;
 use super::objtype;
-use num_bigint::ToBigInt;
 use num_traits::ToPrimitive;
 use std::cell::Ref;
 use std::hash::{Hash, Hasher};
@@ -15,12 +14,29 @@ use std::ops::Deref;
 // Fill bytes class methods:
 pub fn init(context: &PyContext) {
     let bytes_type = &context.bytes_type;
+
+    let bytes_doc =
+        "bytes(iterable_of_ints) -> bytes\n\
+         bytes(string, encoding[, errors]) -> bytes\n\
+         bytes(bytes_or_buffer) -> immutable copy of bytes_or_buffer\n\
+         bytes(int) -> bytes object of size given by the parameter initialized with null bytes\n\
+         bytes() -> empty bytes object\n\nConstruct an immutable array of bytes from:\n  \
+         - an iterable yielding integers in range(256)\n  \
+         - a text string encoded using the specified encoding\n  \
+         - any object implementing the buffer API.\n  \
+         - an integer";
+
     context.set_attr(bytes_type, "__eq__", context.new_rustfunc(bytes_eq));
     context.set_attr(bytes_type, "__hash__", context.new_rustfunc(bytes_hash));
     context.set_attr(bytes_type, "__new__", context.new_rustfunc(bytes_new));
     context.set_attr(bytes_type, "__repr__", context.new_rustfunc(bytes_repr));
     context.set_attr(bytes_type, "__len__", context.new_rustfunc(bytes_len));
-    context.set_attr(bytes_type, "__iter__", context.new_rustfunc(bytes_iter))
+    context.set_attr(bytes_type, "__iter__", context.new_rustfunc(bytes_iter));
+    context.set_attr(
+        bytes_type,
+        "__doc__",
+        context.new_str(bytes_doc.to_string()),
+    );
 }
 
 fn bytes_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -70,8 +86,7 @@ fn bytes_len(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(a, Some(vm.ctx.bytes_type()))]);
 
     let byte_vec = get_value(a).to_vec();
-    let value = byte_vec.len().to_bigint();
-    Ok(vm.ctx.new_int(value.unwrap()))
+    Ok(vm.ctx.new_int(byte_vec.len()))
 }
 
 fn bytes_hash(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -80,7 +95,7 @@ fn bytes_hash(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     let mut hasher = std::collections::hash_map::DefaultHasher::new();
     data.hash(&mut hasher);
     let hash = hasher.finish();
-    Ok(vm.ctx.new_int(hash.to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(hash))
 }
 
 pub fn get_value<'a>(obj: &'a PyObjectRef) -> impl Deref<Target = Vec<u8>> + 'a {
diff --git a/vm/src/obj/objcomplex.rs b/vm/src/obj/objcomplex.rs
index bc624d50f..353b89b7f 100644
--- a/vm/src/obj/objcomplex.rs
+++ b/vm/src/obj/objcomplex.rs
@@ -8,8 +8,18 @@ use num_complex::Complex64;
 
 pub fn init(context: &PyContext) {
     let complex_type = &context.complex_type;
+
+    let complex_doc =
+        "Create a complex number from a real part and an optional imaginary part.\n\n\
+         This is equivalent to (real + imag*1j) where imag defaults to 0.";
+
     context.set_attr(&complex_type, "__add__", context.new_rustfunc(complex_add));
     context.set_attr(&complex_type, "__new__", context.new_rustfunc(complex_new));
+    context.set_attr(
+        &complex_type,
+        "__doc__",
+        context.new_str(complex_doc.to_string()),
+    );
     context.set_attr(
         &complex_type,
         "__repr__",
diff --git a/vm/src/obj/objdict.rs b/vm/src/obj/objdict.rs
index b547eb906..8a3e77ff8 100644
--- a/vm/src/obj/objdict.rs
+++ b/vm/src/obj/objdict.rs
@@ -5,8 +5,7 @@ use super::super::vm::VirtualMachine;
 use super::objiter;
 use super::objstr;
 use super::objtype;
-use num_bigint::ToBigInt;
-use std::cell::{Ref, RefMut};
+use std::cell::{Ref, RefCell, RefMut};
 use std::collections::HashMap;
 use std::ops::{Deref, DerefMut};
 
@@ -106,10 +105,7 @@ pub fn contains_key_str(dict: &PyObjectRef, key: &str) -> bool {
 
 pub fn content_contains_key_str(elements: &DictContentType, key: &str) -> bool {
     // TODO: let hash: usize = key;
-    match elements.get(key) {
-        Some(_) => true,
-        None => false,
-    }
+    elements.get(key).is_some()
 }
 
 // Python dict methods:
@@ -140,7 +136,7 @@ fn dict_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
                 let elem_iter = objiter::get_iter(vm, &element)?;
                 let needle = objiter::get_next_object(vm, &elem_iter)?.ok_or_else(|| err(vm))?;
                 let value = objiter::get_next_object(vm, &elem_iter)?.ok_or_else(|| err(vm))?;
-                if let Some(_) = objiter::get_next_object(vm, &elem_iter)? {
+                if objiter::get_next_object(vm, &elem_iter)?.is_some() {
                     return Err(err(vm));
                 }
                 set_item(&dict, &needle, &value);
@@ -156,7 +152,7 @@ fn dict_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 fn dict_len(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(dict_obj, Some(vm.ctx.dict_type()))]);
     let elements = get_elements(dict_obj);
-    Ok(vm.ctx.new_int(elements.len().to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(elements.len()))
 }
 
 fn dict_repr(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -277,7 +273,7 @@ fn dict_getitem(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 pub fn create_type(type_type: PyObjectRef, object_type: PyObjectRef, dict_type: PyObjectRef) {
     (*dict_type.borrow_mut()).payload = PyObjectPayload::Class {
         name: String::from("dict"),
-        dict: new(dict_type.clone()),
+        dict: RefCell::new(HashMap::new()),
         mro: vec![object_type],
     };
     (*dict_type.borrow_mut()).typ = Some(type_type.clone());
diff --git a/vm/src/obj/objenumerate.rs b/vm/src/obj/objenumerate.rs
new file mode 100644
index 000000000..582f89852
--- /dev/null
+++ b/vm/src/obj/objenumerate.rs
@@ -0,0 +1,69 @@
+use super::super::pyobject::{
+    PyContext, PyFuncArgs, PyObject, PyObjectPayload, PyObjectRef, PyResult, TypeProtocol,
+};
+use super::super::vm::VirtualMachine;
+use super::objint;
+use super::objiter;
+use super::objtype; // Required for arg_check! to use isinstance
+use num_bigint::BigInt;
+use num_traits::Zero;
+use std::ops::AddAssign;
+
+fn enumerate_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    arg_check!(
+        vm,
+        args,
+        required = [(cls, Some(vm.ctx.type_type())), (iterable, None)],
+        optional = [(start, Some(vm.ctx.int_type()))]
+    );
+    let counter = if let Some(x) = start {
+        objint::get_value(x)
+    } else {
+        BigInt::zero()
+    };
+    let iterator = objiter::get_iter(vm, iterable)?;
+    Ok(PyObject::new(
+        PyObjectPayload::EnumerateIterator { counter, iterator },
+        cls.clone(),
+    ))
+}
+
+fn enumerate_next(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    arg_check!(
+        vm,
+        args,
+        required = [(enumerate, Some(vm.ctx.enumerate_type()))]
+    );
+
+    if let PyObjectPayload::EnumerateIterator {
+        ref mut counter,
+        ref mut iterator,
+    } = enumerate.borrow_mut().payload
+    {
+        let next_obj = objiter::call_next(vm, iterator)?;
+        let result = vm
+            .ctx
+            .new_tuple(vec![vm.ctx.new_int(counter.clone()), next_obj]);
+
+        AddAssign::add_assign(counter, 1);
+
+        Ok(result)
+    } else {
+        panic!("enumerate doesn't have correct payload");
+    }
+}
+
+pub fn init(context: &PyContext) {
+    let enumerate_type = &context.enumerate_type;
+    objiter::iter_type_init(context, enumerate_type);
+    context.set_attr(
+        enumerate_type,
+        "__new__",
+        context.new_rustfunc(enumerate_new),
+    );
+    context.set_attr(
+        enumerate_type,
+        "__next__",
+        context.new_rustfunc(enumerate_next),
+    );
+}
diff --git a/vm/src/obj/objfilter.rs b/vm/src/obj/objfilter.rs
index 009d2ad2e..b4bc4ff5e 100644
--- a/vm/src/obj/objfilter.rs
+++ b/vm/src/obj/objfilter.rs
@@ -7,7 +7,7 @@ use super::objbool;
 use super::objiter;
 use super::objtype; // Required for arg_check! to use isinstance
 
-pub fn filter_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+fn filter_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(
         vm,
         args,
@@ -23,21 +23,6 @@ pub fn filter_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     ))
 }
 
-fn filter_iter(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
-    arg_check!(vm, args, required = [(filter, Some(vm.ctx.filter_type()))]);
-    // Return self:
-    Ok(filter.clone())
-}
-
-fn filter_contains(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
-    arg_check!(
-        vm,
-        args,
-        required = [(filter, Some(vm.ctx.filter_type())), (needle, None)]
-    );
-    objiter::contains(vm, filter, needle)
-}
-
 fn filter_next(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(filter, Some(vm.ctx.filter_type()))]);
 
@@ -72,12 +57,7 @@ fn filter_next(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 
 pub fn init(context: &PyContext) {
     let filter_type = &context.filter_type;
-    context.set_attr(
-        &filter_type,
-        "__contains__",
-        context.new_rustfunc(filter_contains),
-    );
-    context.set_attr(&filter_type, "__iter__", context.new_rustfunc(filter_iter));
+    objiter::iter_type_init(context, filter_type);
     context.set_attr(&filter_type, "__new__", context.new_rustfunc(filter_new));
     context.set_attr(&filter_type, "__next__", context.new_rustfunc(filter_next));
 }
diff --git a/vm/src/obj/objfloat.rs b/vm/src/obj/objfloat.rs
index 4cb996a51..1bb57e572 100644
--- a/vm/src/obj/objfloat.rs
+++ b/vm/src/obj/objfloat.rs
@@ -268,6 +268,9 @@ fn float_pow(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 
 pub fn init(context: &PyContext) {
     let float_type = &context.float_type;
+
+    let float_doc = "Convert a string or number to a floating point number, if possible.";
+
     context.set_attr(&float_type, "__eq__", context.new_rustfunc(float_eq));
     context.set_attr(&float_type, "__lt__", context.new_rustfunc(float_lt));
     context.set_attr(&float_type, "__le__", context.new_rustfunc(float_le));
@@ -291,4 +294,9 @@ pub fn init(context: &PyContext) {
     context.set_attr(&float_type, "__pow__", context.new_rustfunc(float_pow));
     context.set_attr(&float_type, "__sub__", context.new_rustfunc(float_sub));
     context.set_attr(&float_type, "__repr__", context.new_rustfunc(float_repr));
+    context.set_attr(
+        &float_type,
+        "__doc__",
+        context.new_str(float_doc.to_string()),
+    );
 }
diff --git a/vm/src/obj/objfunction.rs b/vm/src/obj/objfunction.rs
index f04897481..01c05aab1 100644
--- a/vm/src/obj/objfunction.rs
+++ b/vm/src/obj/objfunction.rs
@@ -1,6 +1,6 @@
 use super::super::pyobject::{
-    AttributeProtocol, IdProtocol, PyContext, PyFuncArgs, PyObject, PyObjectPayload, PyObjectRef,
-    PyResult, TypeProtocol,
+    AttributeProtocol, IdProtocol, PyContext, PyFuncArgs, PyObjectPayload, PyObjectRef, PyResult,
+    TypeProtocol,
 };
 use super::super::vm::VirtualMachine;
 use super::objtype;
@@ -110,12 +110,7 @@ fn classmethod_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     trace!("classmethod.__new__ {:?}", args.args);
     arg_check!(vm, args, required = [(cls, None), (callable, None)]);
 
-    let py_obj = PyObject::new(
-        PyObjectPayload::Instance {
-            dict: vm.ctx.new_dict(),
-        },
-        cls.clone(),
-    );
+    let py_obj = vm.ctx.new_instance(cls.clone(), None);
     vm.ctx.set_attr(&py_obj, "function", callable.clone());
     Ok(py_obj)
 }
@@ -148,12 +143,7 @@ fn staticmethod_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     trace!("staticmethod.__new__ {:?}", args.args);
     arg_check!(vm, args, required = [(cls, None), (callable, None)]);
 
-    let py_obj = PyObject::new(
-        PyObjectPayload::Instance {
-            dict: vm.ctx.new_dict(),
-        },
-        cls.clone(),
-    );
+    let py_obj = vm.ctx.new_instance(cls.clone(), None);
     vm.ctx.set_attr(&py_obj, "function", callable.clone());
     Ok(py_obj)
 }
diff --git a/vm/src/obj/objint.rs b/vm/src/obj/objint.rs
index 06424e149..d37af68c9 100644
--- a/vm/src/obj/objint.rs
+++ b/vm/src/obj/objint.rs
@@ -250,7 +250,7 @@ fn int_hash(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     let mut hasher = std::collections::hash_map::DefaultHasher::new();
     value.hash(&mut hasher);
     let hash = hasher.finish();
-    Ok(vm.ctx.new_int(hash.to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(hash))
 }
 
 fn int_abs(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -366,17 +366,25 @@ fn int_truediv(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         args,
         required = [(i, Some(vm.ctx.int_type())), (i2, None)]
     );
-    let v1 = get_value(i);
-    if objtype::isinstance(i2, &vm.ctx.int_type()) {
-        Ok(vm
-            .ctx
-            .new_float(v1.to_f64().unwrap() / get_value(i2).to_f64().unwrap()))
+
+    let v1 = get_value(i)
+        .to_f64()
+        .ok_or_else(|| vm.new_overflow_error("int too large to convert to float".to_string()))?;
+
+    let v2 = if objtype::isinstance(i2, &vm.ctx.int_type()) {
+        get_value(i2)
+            .to_f64()
+            .ok_or_else(|| vm.new_overflow_error("int too large to convert to float".to_string()))?
     } else if objtype::isinstance(i2, &vm.ctx.float_type()) {
-        Ok(vm
-            .ctx
-            .new_float(v1.to_f64().unwrap() / objfloat::get_value(i2)))
+        objfloat::get_value(i2)
     } else {
-        Err(vm.new_type_error(format!("Cannot divide {} and {}", i.borrow(), i2.borrow())))
+        return Err(vm.new_type_error(format!("Cannot divide {} and {}", i.borrow(), i2.borrow())));
+    };
+
+    if v2 == 0.0 {
+        Err(vm.new_zero_division_error("integer division by zero".to_string()))
+    } else {
+        Ok(vm.ctx.new_float(v1 / v2))
     }
 }
 
@@ -388,7 +396,13 @@ fn int_mod(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     );
     let v1 = get_value(i);
     if objtype::isinstance(i2, &vm.ctx.int_type()) {
-        Ok(vm.ctx.new_int(v1 % get_value(i2)))
+        let v2 = get_value(i2);
+
+        if v2 != BigInt::zero() {
+            Ok(vm.ctx.new_int(v1 % get_value(i2)))
+        } else {
+            Err(vm.new_zero_division_error("integer modulo by zero".to_string()))
+        }
     } else {
         Err(vm.new_type_error(format!("Cannot modulo {} and {}", i.borrow(), i2.borrow())))
     }
@@ -414,7 +428,7 @@ fn int_pow(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     let v1 = get_value(i);
     if objtype::isinstance(i2, &vm.ctx.int_type()) {
         let v2 = get_value(i2).to_u32().unwrap();
-        Ok(vm.ctx.new_int(v1.pow(v2).to_bigint().unwrap()))
+        Ok(vm.ctx.new_int(v1.pow(v2)))
     } else if objtype::isinstance(i2, &vm.ctx.float_type()) {
         let v2 = objfloat::get_value(i2);
         Ok(vm.ctx.new_float((v1.to_f64().unwrap()).powf(v2)))
@@ -513,7 +527,7 @@ fn int_bit_length(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(i, Some(vm.ctx.int_type()))]);
     let v = get_value(i);
     let bits = v.bits();
-    Ok(vm.ctx.new_int(bits.to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(bits))
 }
 
 fn int_conjugate(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
diff --git a/vm/src/obj/objiter.rs b/vm/src/obj/objiter.rs
index b7368f7b2..40633e984 100644
--- a/vm/src/obj/objiter.rs
+++ b/vm/src/obj/objiter.rs
@@ -9,7 +9,6 @@ use super::super::vm::VirtualMachine;
 use super::objbool;
 // use super::objstr;
 use super::objtype; // Required for arg_check! to use isinstance
-use num_bigint::{BigInt, ToBigInt};
 
 /*
  * This helper function is called at multiple places. First, it is called
@@ -65,7 +64,17 @@ pub fn get_all(
     Ok(elements)
 }
 
-pub fn contains(vm: &mut VirtualMachine, iter: &PyObjectRef, needle: &PyObjectRef) -> PyResult {
+pub fn new_stop_iteration(vm: &mut VirtualMachine) -> PyObjectRef {
+    let stop_iteration_type = vm.ctx.exceptions.stop_iteration.clone();
+    vm.new_exception(stop_iteration_type, "End of iterator".to_string())
+}
+
+fn contains(vm: &mut VirtualMachine, args: PyFuncArgs, iter_type: PyObjectRef) -> PyResult {
+    arg_check!(
+        vm,
+        args,
+        required = [(iter, Some(iter_type)), (needle, None)]
+    );
     loop {
         if let Some(element) = get_next_object(vm, iter)? {
             let equal = vm.call_method(needle, "__eq__", vec![element.clone()])?;
@@ -80,6 +89,34 @@ pub fn contains(vm: &mut VirtualMachine, iter: &PyObjectRef, needle: &PyObjectRe
     }
 }
 
+/// Common setup for iter types, adds __iter__ and __contains__ methods
+pub fn iter_type_init(context: &PyContext, iter_type: &PyObjectRef) {
+    let contains_func = {
+        let cloned_iter_type = iter_type.clone();
+        move |vm: &mut VirtualMachine, args: PyFuncArgs| {
+            contains(vm, args, cloned_iter_type.clone())
+        }
+    };
+    context.set_attr(
+        &iter_type,
+        "__contains__",
+        context.new_rustfunc(contains_func),
+    );
+    let iter_func = {
+        let cloned_iter_type = iter_type.clone();
+        move |vm: &mut VirtualMachine, args: PyFuncArgs| {
+            arg_check!(
+                vm,
+                args,
+                required = [(iter, Some(cloned_iter_type.clone()))]
+            );
+            // Return self:
+            Ok(iter.clone())
+        }
+    };
+    context.set_attr(&iter_type, "__iter__", context.new_rustfunc(iter_func));
+}
+
 // Sequence iterator:
 fn iter_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(iter_target, None)]);
@@ -87,21 +124,6 @@ fn iter_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     get_iter(vm, iter_target)
 }
 
-fn iter_iter(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
-    arg_check!(vm, args, required = [(iter, Some(vm.ctx.iter_type()))]);
-    // Return self:
-    Ok(iter.clone())
-}
-
-fn iter_contains(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
-    arg_check!(
-        vm,
-        args,
-        required = [(iter, Some(vm.ctx.iter_type())), (needle, None)]
-    );
-    contains(vm, iter, needle)
-}
-
 fn iter_next(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(iter, Some(vm.ctx.iter_type()))]);
 
@@ -118,35 +140,26 @@ fn iter_next(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
                     *position += 1;
                     Ok(obj_ref)
                 } else {
-                    let stop_iteration_type = vm.ctx.exceptions.stop_iteration.clone();
-                    let stop_iteration =
-                        vm.new_exception(stop_iteration_type, "End of iterator".to_string());
-                    Err(stop_iteration)
+                    Err(new_stop_iteration(vm))
                 }
             }
 
             PyObjectPayload::Range { ref range } => {
-                if let Some(int) = range.get(BigInt::from(*position)) {
+                if let Some(int) = range.get(*position) {
                     *position += 1;
-                    Ok(vm.ctx.new_int(int.to_bigint().unwrap()))
+                    Ok(vm.ctx.new_int(int))
                 } else {
-                    let stop_iteration_type = vm.ctx.exceptions.stop_iteration.clone();
-                    let stop_iteration =
-                        vm.new_exception(stop_iteration_type, "End of iterator".to_string());
-                    Err(stop_iteration)
+                    Err(new_stop_iteration(vm))
                 }
             }
 
             PyObjectPayload::Bytes { ref value } => {
                 if *position < value.len() {
-                    let obj_ref = vm.ctx.new_int(value[*position].to_bigint().unwrap());
+                    let obj_ref = vm.ctx.new_int(value[*position]);
                     *position += 1;
                     Ok(obj_ref)
                 } else {
-                    let stop_iteration_type = vm.ctx.exceptions.stop_iteration.clone();
-                    let stop_iteration =
-                        vm.new_exception(stop_iteration_type, "End of iterator".to_string());
-                    Err(stop_iteration)
+                    Err(new_stop_iteration(vm))
                 }
             }
 
@@ -161,12 +174,7 @@ fn iter_next(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 
 pub fn init(context: &PyContext) {
     let iter_type = &context.iter_type;
-    context.set_attr(
-        &iter_type,
-        "__contains__",
-        context.new_rustfunc(iter_contains),
-    );
-    context.set_attr(&iter_type, "__iter__", context.new_rustfunc(iter_iter));
+    iter_type_init(context, iter_type);
     context.set_attr(&iter_type, "__new__", context.new_rustfunc(iter_new));
     context.set_attr(&iter_type, "__next__", context.new_rustfunc(iter_next));
 }
diff --git a/vm/src/obj/objlist.rs b/vm/src/obj/objlist.rs
index 4ec4a7e80..369ed9fc5 100644
--- a/vm/src/obj/objlist.rs
+++ b/vm/src/obj/objlist.rs
@@ -10,7 +10,6 @@ use super::objsequence::{
 };
 use super::objstr;
 use super::objtype;
-use num_bigint::ToBigInt;
 use num_traits::ToPrimitive;
 
 // set_item:
@@ -22,9 +21,12 @@ fn set_item(
 ) -> PyResult {
     if objtype::isinstance(&idx, &vm.ctx.int_type()) {
         let value = objint::get_value(&idx).to_i32().unwrap();
-        let pos_index = l.get_pos(value);
-        l[pos_index] = obj;
-        Ok(vm.get_none())
+        if let Some(pos_index) = l.get_pos(value) {
+            l[pos_index] = obj;
+            Ok(vm.get_none())
+        } else {
+            Err(vm.new_index_error("list index out of range".to_string()))
+        }
     } else {
         panic!(
             "TypeError: indexing type {:?} with index {:?} is not supported (yet?)",
@@ -172,7 +174,7 @@ fn list_add(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     if objtype::isinstance(o2, &vm.ctx.list_type()) {
         let e1 = get_elements(o);
         let e2 = get_elements(o2);
-        let elements = e1.iter().chain(e2.iter()).map(|e| e.clone()).collect();
+        let elements = e1.iter().chain(e2.iter()).cloned().collect();
         Ok(vm.ctx.new_list(elements))
     } else {
         Err(vm.new_type_error(format!("Cannot add {} and {}", o.borrow(), o2.borrow())))
@@ -224,10 +226,10 @@ fn list_count(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     for element in elements.iter() {
         let is_eq = vm._eq(element, value.clone())?;
         if objbool::boolval(vm, is_eq)? {
-            count = count + 1;
+            count += 1;
         }
     }
-    Ok(vm.context().new_int(count.to_bigint().unwrap()))
+    Ok(vm.context().new_int(count))
 }
 
 pub fn list_extend(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -252,7 +254,7 @@ fn list_index(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     for (index, element) in get_elements(list).iter().enumerate() {
         let py_equal = vm.call_method(needle, "__eq__", vec![element.clone()])?;
         if objbool::get_value(&py_equal) {
-            return Ok(vm.context().new_int(index.to_bigint().unwrap()));
+            return Ok(vm.context().new_int(index));
         }
     }
     let needle_str = objstr::get_value(&vm.to_str(needle).unwrap());
@@ -263,7 +265,7 @@ fn list_len(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     trace!("list.len called with: {:?}", args);
     arg_check!(vm, args, required = [(list, Some(vm.ctx.list_type()))]);
     let elements = get_elements(list);
-    Ok(vm.context().new_int(elements.len().to_bigint().unwrap()))
+    Ok(vm.context().new_int(elements.len()))
 }
 
 fn list_reverse(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
diff --git a/vm/src/obj/objmap.rs b/vm/src/obj/objmap.rs
index ed6130643..722eba01a 100644
--- a/vm/src/obj/objmap.rs
+++ b/vm/src/obj/objmap.rs
@@ -5,7 +5,7 @@ use super::super::vm::VirtualMachine;
 use super::objiter;
 use super::objtype; // Required for arg_check! to use isinstance
 
-pub fn map_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+fn map_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     no_kwargs!(vm, args);
     let cls = &args.args[0];
     if args.args.len() < 3 {
@@ -27,21 +27,6 @@ pub fn map_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     }
 }
 
-fn map_iter(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
-    arg_check!(vm, args, required = [(map, Some(vm.ctx.map_type()))]);
-    // Return self:
-    Ok(map.clone())
-}
-
-fn map_contains(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
-    arg_check!(
-        vm,
-        args,
-        required = [(map, Some(vm.ctx.map_type())), (needle, None)]
-    );
-    objiter::contains(vm, map, needle)
-}
-
 fn map_next(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(map, Some(vm.ctx.map_type()))]);
 
@@ -70,12 +55,7 @@ fn map_next(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 
 pub fn init(context: &PyContext) {
     let map_type = &context.map_type;
-    context.set_attr(
-        &map_type,
-        "__contains__",
-        context.new_rustfunc(map_contains),
-    );
-    context.set_attr(&map_type, "__iter__", context.new_rustfunc(map_iter));
+    objiter::iter_type_init(context, map_type);
     context.set_attr(&map_type, "__new__", context.new_rustfunc(map_new));
     context.set_attr(&map_type, "__next__", context.new_rustfunc(map_next));
 }
diff --git a/vm/src/obj/objobject.rs b/vm/src/obj/objobject.rs
index 190f3078b..cb5065b20 100644
--- a/vm/src/obj/objobject.rs
+++ b/vm/src/obj/objobject.rs
@@ -1,25 +1,25 @@
 use super::super::pyobject::{
-    AttributeProtocol, IdProtocol, PyContext, PyFuncArgs, PyObject, PyObjectPayload, PyObjectRef,
-    PyResult, TypeProtocol,
+    AttributeProtocol, IdProtocol, PyContext, PyFuncArgs, PyObjectPayload, PyObjectRef, PyResult,
+    TypeProtocol,
 };
 use super::super::vm::VirtualMachine;
 use super::objbool;
-use super::objdict;
 use super::objstr;
 use super::objtype;
+use std::cell::RefCell;
+use std::collections::HashMap;
 
 pub fn new_instance(vm: &mut VirtualMachine, mut args: PyFuncArgs) -> PyResult {
     // more or less __new__ operator
     let type_ref = args.shift();
-    let dict = vm.new_dict();
-    let obj = PyObject::new(PyObjectPayload::Instance { dict }, type_ref.clone());
+    let obj = vm.ctx.new_instance(type_ref.clone(), None);
     Ok(obj)
 }
 
-pub fn create_object(type_type: PyObjectRef, object_type: PyObjectRef, dict_type: PyObjectRef) {
+pub fn create_object(type_type: PyObjectRef, object_type: PyObjectRef, _dict_type: PyObjectRef) {
     (*object_type.borrow_mut()).payload = PyObjectPayload::Class {
         name: String::from("object"),
-        dict: objdict::new(dict_type),
+        dict: RefCell::new(HashMap::new()),
         mro: vec![],
     };
     (*object_type.borrow_mut()).typ = Some(type_type.clone());
@@ -62,15 +62,14 @@ fn object_delattr(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         ]
     );
 
-    // Get dict:
-    let dict = match zelf.borrow().payload {
-        PyObjectPayload::Class { ref dict, .. } => dict.clone(),
-        PyObjectPayload::Instance { ref dict, .. } => dict.clone(),
-        _ => return Err(vm.new_type_error("TypeError: no dictionary.".to_string())),
-    };
-
-    // Delete attr from dict:
-    vm.call_method(&dict, "__delitem__", vec![attr.clone()])
+    match zelf.borrow().payload {
+        PyObjectPayload::Class { ref dict, .. } | PyObjectPayload::Instance { ref dict, .. } => {
+            let attr_name = objstr::get_value(attr);
+            dict.borrow_mut().remove(&attr_name);
+            Ok(vm.get_none())
+        }
+        _ => Err(vm.new_type_error("TypeError: no dictionary.".to_string())),
+    }
 }
 
 fn object_str(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -116,8 +115,13 @@ fn object_init(vm: &mut VirtualMachine, _args: PyFuncArgs) -> PyResult {
 
 fn object_dict(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     match args.args[0].borrow().payload {
-        PyObjectPayload::Class { ref dict, .. } => Ok(dict.clone()),
-        PyObjectPayload::Instance { ref dict, .. } => Ok(dict.clone()),
+        PyObjectPayload::Class { ref dict, .. } | PyObjectPayload::Instance { ref dict, .. } => {
+            let new_dict = vm.new_dict();
+            for (attr, value) in dict.borrow().iter() {
+                vm.ctx.set_item(&new_dict, &attr, value.clone());
+            }
+            Ok(new_dict)
+        }
         _ => Err(vm.new_type_error("TypeError: no dictionary.".to_string())),
     }
 }
diff --git a/vm/src/obj/objproperty.rs b/vm/src/obj/objproperty.rs
index 1d146889b..ada639270 100644
--- a/vm/src/obj/objproperty.rs
+++ b/vm/src/obj/objproperty.rs
@@ -2,14 +2,41 @@
 
 */
 
-use super::super::pyobject::{
-    PyContext, PyFuncArgs, PyObject, PyObjectPayload, PyObjectRef, PyResult, TypeProtocol,
-};
+use super::super::pyobject::{PyContext, PyFuncArgs, PyObjectRef, PyResult, TypeProtocol};
 use super::super::vm::VirtualMachine;
 use super::objtype;
 
 pub fn init(context: &PyContext) {
     let property_type = &context.property_type;
+
+    let property_doc =
+        "Property attribute.\n\n  \
+         fget\n    \
+         function to be used for getting an attribute value\n  \
+         fset\n    \
+         function to be used for setting an attribute value\n  \
+         fdel\n    \
+         function to be used for del\'ing an attribute\n  \
+         doc\n    \
+         docstring\n\n\
+         Typical use is to define a managed attribute x:\n\n\
+         class C(object):\n    \
+         def getx(self): return self._x\n    \
+         def setx(self, value): self._x = value\n    \
+         def delx(self): del self._x\n    \
+         x = property(getx, setx, delx, \"I\'m the \'x\' property.\")\n\n\
+         Decorators make defining new properties or modifying existing ones easy:\n\n\
+         class C(object):\n    \
+         @property\n    \
+         def x(self):\n        \"I am the \'x\' property.\"\n        \
+         return self._x\n    \
+         @x.setter\n    \
+         def x(self, value):\n        \
+         self._x = value\n    \
+         @x.deleter\n    \
+         def x(self):\n        \
+         del self._x";
+
     context.set_attr(
         &property_type,
         "__get__",
@@ -20,6 +47,11 @@ pub fn init(context: &PyContext) {
         "__new__",
         context.new_rustfunc(property_new),
     );
+    context.set_attr(
+        &property_type,
+        "__doc__",
+        context.new_str(property_doc.to_string()),
+    );
     // TODO: how to handle __set__ ?
 }
 
@@ -55,12 +87,7 @@ fn property_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     trace!("property.__new__ {:?}", args.args);
     arg_check!(vm, args, required = [(cls, None), (fget, None)]);
 
-    let py_obj = PyObject::new(
-        PyObjectPayload::Instance {
-            dict: vm.ctx.new_dict(),
-        },
-        cls.clone(),
-    );
+    let py_obj = vm.ctx.new_instance(cls.clone(), None);
     vm.ctx.set_attr(&py_obj, "fget", fget.clone());
     Ok(py_obj)
 }
diff --git a/vm/src/obj/objrange.rs b/vm/src/obj/objrange.rs
index 647298ea2..91b875ae3 100644
--- a/vm/src/obj/objrange.rs
+++ b/vm/src/obj/objrange.rs
@@ -4,9 +4,10 @@ use super::super::pyobject::{
 use super::super::vm::VirtualMachine;
 use super::objint;
 use super::objtype;
-use num_bigint::{BigInt, ToBigInt};
+use num_bigint::{BigInt, Sign};
 use num_integer::Integer;
 use num_traits::{One, Signed, ToPrimitive, Zero};
+use std::ops::Mul;
 
 #[derive(Debug, Clone)]
 pub struct RangeType {
@@ -18,25 +19,50 @@ pub struct RangeType {
 }
 
 impl RangeType {
+    #[inline]
+    pub fn try_len(&self) -> Option<usize> {
+        match self.step.sign() {
+            Sign::Plus if self.start < self.end => ((&self.end - &self.start - 1usize)
+                / &self.step)
+                .to_usize()
+                .map(|sz| sz + 1),
+            Sign::Minus if self.start > self.end => ((&self.start - &self.end - 1usize)
+                / (-&self.step))
+                .to_usize()
+                .map(|sz| sz + 1),
+            _ => Some(0),
+        }
+    }
+
     #[inline]
     pub fn len(&self) -> usize {
-        ((self.end.clone() - self.start.clone()) / self.step.clone())
-            .abs()
-            .to_usize()
-            .unwrap()
+        self.try_len().unwrap()
+    }
+
+    #[inline]
+    fn offset(&self, value: &BigInt) -> Option<BigInt> {
+        match self.step.sign() {
+            Sign::Plus if value >= &self.start && value < &self.end => Some(value - &self.start),
+            Sign::Minus if value <= &self.start && value > &self.end => Some(&self.start - value),
+            _ => None,
+        }
+    }
+
+    #[inline]
+    pub fn contains(&self, value: &BigInt) -> bool {
+        match self.offset(value) {
+            Some(ref offset) => offset.is_multiple_of(&self.step),
+            None => false,
+        }
     }
 
     #[inline]
     pub fn index_of(&self, value: &BigInt) -> Option<BigInt> {
-        if value < &self.start || value >= &self.end {
-            return None;
-        }
-
-        let offset = value - &self.start;
-        if offset.is_multiple_of(&self.step) {
-            Some(offset / &self.step)
-        } else {
-            None
+        match self.offset(value) {
+            Some(ref offset) if offset.is_multiple_of(&self.step) => {
+                Some((offset / &self.step).abs())
+            }
+            Some(_) | None => None,
         }
     }
 
@@ -52,17 +78,29 @@ impl RangeType {
     }
 
     #[inline]
-    pub fn get(&self, index: BigInt) -> Option<BigInt> {
-        let result = self.start.clone() + self.step.clone() * index;
+    pub fn get<'a, T>(&'a self, index: T) -> Option<BigInt>
+    where
+        &'a BigInt: Mul<T, Output = BigInt>,
+    {
+        let result = &self.start + &self.step * index;
 
-        if self.forward() && !self.is_empty() && result < self.end {
-            Some(result)
-        } else if !self.forward() && !self.is_empty() && result > self.end {
+        if (self.forward() && !self.is_empty() && result < self.end)
+            || (!self.forward() && !self.is_empty() && result > self.end)
+        {
             Some(result)
         } else {
             None
         }
     }
+
+    #[inline]
+    pub fn repr(&self) -> String {
+        if self.step == BigInt::one() {
+            format!("range({}, {})", self.start, self.end)
+        } else {
+            format!("range({}, {}, {})", self.start, self.end, self.step)
+        }
+    }
 }
 
 pub fn init(context: &PyContext) {
@@ -75,6 +113,13 @@ pub fn init(context: &PyContext) {
         "__getitem__",
         context.new_rustfunc(range_getitem),
     );
+    context.set_attr(&range_type, "__repr__", context.new_rustfunc(range_repr));
+    context.set_attr(&range_type, "__bool__", context.new_rustfunc(range_bool));
+    context.set_attr(
+        &range_type,
+        "__contains__",
+        context.new_rustfunc(range_contains),
+    );
     context.set_attr(&range_type, "index", context.new_rustfunc(range_index));
 }
 
@@ -134,12 +179,14 @@ fn range_iter(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 fn range_len(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(zelf, Some(vm.ctx.range_type()))]);
 
-    let len = match zelf.borrow().payload {
-        PyObjectPayload::Range { ref range } => range.len(),
+    if let Some(len) = match zelf.borrow().payload {
+        PyObjectPayload::Range { ref range } => range.try_len(),
         _ => unreachable!(),
-    };
-
-    Ok(vm.ctx.new_int(len.to_bigint().unwrap()))
+    } {
+        Ok(vm.ctx.new_int(len))
+    } else {
+        Err(vm.new_overflow_error("Python int too large to convert to Rust usize".to_string()))
+    }
 }
 
 fn range_getitem(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -156,20 +203,19 @@ fn range_getitem(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 
     match subscript.borrow().payload {
         PyObjectPayload::Integer { ref value } => {
-            if let Some(int) = zrange.get(value.clone()) {
-                Ok(PyObject::new(
-                    PyObjectPayload::Integer {
-                        value: int.to_bigint().unwrap(),
-                    },
-                    vm.ctx.int_type(),
-                ))
+            if let Some(int) = zrange.get(value) {
+                Ok(vm.ctx.new_int(int))
             } else {
                 Err(vm.new_index_error("range object index out of range".to_string()))
             }
         }
-        PyObjectPayload::Slice { start, stop, step } => {
+        PyObjectPayload::Slice {
+            ref start,
+            ref stop,
+            ref step,
+        } => {
             let new_start = if let Some(int) = start {
-                if let Some(i) = zrange.get(int.into()) {
+                if let Some(i) = zrange.get(int) {
                     i
                 } else {
                     zrange.start.clone()
@@ -179,7 +225,7 @@ fn range_getitem(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
             };
 
             let new_end = if let Some(int) = stop {
-                if let Some(i) = zrange.get(int.into()) {
+                if let Some(i) = zrange.get(int) {
                     i
                 } else {
                     zrange.end
@@ -189,7 +235,7 @@ fn range_getitem(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
             };
 
             let new_step = if let Some(int) = step {
-                (int as i64) * zrange.step
+                int * zrange.step
             } else {
                 zrange.step
             };
@@ -210,6 +256,45 @@ fn range_getitem(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     }
 }
 
+fn range_repr(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    arg_check!(vm, args, required = [(zelf, Some(vm.ctx.range_type()))]);
+
+    let s = match zelf.borrow().payload {
+        PyObjectPayload::Range { ref range } => range.repr(),
+        _ => unreachable!(),
+    };
+
+    Ok(vm.ctx.new_str(s))
+}
+
+fn range_bool(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    arg_check!(vm, args, required = [(zelf, Some(vm.ctx.range_type()))]);
+
+    let len = match zelf.borrow().payload {
+        PyObjectPayload::Range { ref range } => range.len(),
+        _ => unreachable!(),
+    };
+
+    Ok(vm.ctx.new_bool(len > 0))
+}
+
+fn range_contains(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    arg_check!(
+        vm,
+        args,
+        required = [(zelf, Some(vm.ctx.range_type())), (needle, None)]
+    );
+
+    if let PyObjectPayload::Range { ref range } = zelf.borrow().payload {
+        Ok(vm.ctx.new_bool(match needle.borrow().payload {
+            PyObjectPayload::Integer { ref value } => range.contains(value),
+            _ => false,
+        }))
+    } else {
+        unreachable!()
+    }
+}
+
 fn range_index(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(
         vm,
diff --git a/vm/src/obj/objsequence.rs b/vm/src/obj/objsequence.rs
index c5504b22d..5d85b12a0 100644
--- a/vm/src/obj/objsequence.rs
+++ b/vm/src/obj/objsequence.rs
@@ -2,53 +2,97 @@ use super::super::pyobject::{PyObject, PyObjectPayload, PyObjectRef, PyResult, T
 use super::super::vm::VirtualMachine;
 use super::objbool;
 use super::objint;
-use num_traits::ToPrimitive;
+use num_bigint::BigInt;
+use num_traits::{One, Signed, ToPrimitive, Zero};
 use std::cell::{Ref, RefMut};
 use std::marker::Sized;
-use std::ops::{Deref, DerefMut};
+use std::ops::{Deref, DerefMut, Range};
 
 pub trait PySliceableSequence {
-    fn do_slice(&self, start: usize, stop: usize) -> Self;
-    fn do_stepped_slice(&self, start: usize, stop: usize, step: usize) -> Self;
+    fn do_slice(&self, range: Range<usize>) -> Self;
+    fn do_slice_reverse(&self, range: Range<usize>) -> Self;
+    fn do_stepped_slice(&self, range: Range<usize>, step: usize) -> Self;
+    fn do_stepped_slice_reverse(&self, range: Range<usize>, step: usize) -> Self;
+    fn empty() -> Self;
     fn len(&self) -> usize;
-    fn get_pos(&self, p: i32) -> usize {
+    fn get_pos(&self, p: i32) -> Option<usize> {
         if p < 0 {
             if -p as usize > self.len() {
-                // return something that is out of bounds so `get_item` raises an IndexError
-                self.len() + 1
+                None
             } else {
-                self.len() - ((-p) as usize)
+                Some(self.len() - ((-p) as usize))
             }
-        } else if p as usize > self.len() {
-            // This is for the slicing case where the end element is greater than the length of the
-            // sequence
-            self.len()
+        } else if p as usize >= self.len() {
+            None
         } else {
-            p as usize
+            Some(p as usize)
         }
     }
-    fn get_slice_items(&self, slice: &PyObjectRef) -> Self
+
+    fn get_slice_pos(&self, slice_pos: &BigInt) -> usize {
+        if let Some(pos) = slice_pos.to_i32() {
+            if let Some(index) = self.get_pos(pos) {
+                // within bounds
+                return index;
+            }
+        }
+
+        if slice_pos.is_negative() {
+            0
+        } else {
+            self.len()
+        }
+    }
+
+    fn get_slice_range(&self, start: &Option<BigInt>, stop: &Option<BigInt>) -> Range<usize> {
+        let start = start.as_ref().map(|x| self.get_slice_pos(x)).unwrap_or(0);
+        let stop = stop
+            .as_ref()
+            .map(|x| self.get_slice_pos(x))
+            .unwrap_or(self.len());
+
+        start..stop
+    }
+
+    fn get_slice_items(
+        &self,
+        vm: &mut VirtualMachine,
+        slice: &PyObjectRef,
+    ) -> Result<Self, PyObjectRef>
     where
         Self: Sized,
     {
         // TODO: we could potentially avoid this copy and use slice
         match &(slice.borrow()).payload {
             PyObjectPayload::Slice { start, stop, step } => {
-                let start = match start {
-                    &Some(start) => self.get_pos(start),
-                    &None => 0,
-                };
-                let stop = match stop {
-                    &Some(stop) => self.get_pos(stop),
-                    &None => self.len() as usize,
-                };
-                match step {
-                    &None | &Some(1) => self.do_slice(start, stop),
-                    &Some(num) => {
-                        if num < 0 {
-                            unimplemented!("negative step indexing not yet supported")
-                        };
-                        self.do_stepped_slice(start, stop, num as usize)
+                let step = step.clone().unwrap_or(BigInt::one());
+                if step.is_zero() {
+                    Err(vm.new_value_error("slice step cannot be zero".to_string()))
+                } else if step.is_positive() {
+                    let range = self.get_slice_range(start, stop);
+                    if range.start < range.end {
+                        match step.to_i32() {
+                            Some(1) => Ok(self.do_slice(range)),
+                            Some(num) => Ok(self.do_stepped_slice(range, num as usize)),
+                            None => Ok(self.do_slice(range.start..range.start + 1)),
+                        }
+                    } else {
+                        Ok(Self::empty())
+                    }
+                } else {
+                    // calculate the range for the reverse slice, first the bounds needs to be made
+                    // exclusive around stop, the lower number
+                    let start = start.as_ref().map(|x| x + 1);
+                    let stop = stop.as_ref().map(|x| x + 1);
+                    let range = self.get_slice_range(&stop, &start);
+                    if range.start < range.end {
+                        match (-step).to_i32() {
+                            Some(1) => Ok(self.do_slice_reverse(range)),
+                            Some(num) => Ok(self.do_stepped_slice_reverse(range, num as usize)),
+                            None => Ok(self.do_slice(range.end - 1..range.end)),
+                        }
+                    } else {
+                        Ok(Self::empty())
                     }
                 }
             }
@@ -58,12 +102,28 @@ pub trait PySliceableSequence {
 }
 
 impl<T: Clone> PySliceableSequence for Vec<T> {
-    fn do_slice(&self, start: usize, stop: usize) -> Self {
-        self[start..stop].to_vec()
+    fn do_slice(&self, range: Range<usize>) -> Self {
+        self[range].to_vec()
     }
-    fn do_stepped_slice(&self, start: usize, stop: usize, step: usize) -> Self {
-        self[start..stop].iter().step_by(step).cloned().collect()
+
+    fn do_slice_reverse(&self, range: Range<usize>) -> Self {
+        let mut slice = self[range].to_vec();
+        slice.reverse();
+        slice
     }
+
+    fn do_stepped_slice(&self, range: Range<usize>, step: usize) -> Self {
+        self[range].iter().step_by(step).cloned().collect()
+    }
+
+    fn do_stepped_slice_reverse(&self, range: Range<usize>, step: usize) -> Self {
+        self[range].iter().rev().step_by(step).cloned().collect()
+    }
+
+    fn empty() -> Self {
+        Vec::new()
+    }
+
     fn len(&self) -> usize {
         self.len()
     }
@@ -78,8 +138,7 @@ pub fn get_item(
     match &(subscript.borrow()).payload {
         PyObjectPayload::Integer { value } => match value.to_i32() {
             Some(value) => {
-                let pos_index = elements.to_vec().get_pos(value);
-                if pos_index < elements.len() {
+                if let Some(pos_index) = elements.to_vec().get_pos(value) {
                     let obj = elements[pos_index].clone();
                     Ok(obj)
                 } else {
@@ -94,7 +153,7 @@ pub fn get_item(
         PyObjectPayload::Slice { .. } => Ok(PyObject::new(
             match &(sequence.borrow()).payload {
                 PyObjectPayload::Sequence { .. } => PyObjectPayload::Sequence {
-                    elements: elements.to_vec().get_slice_items(&subscript),
+                    elements: elements.to_vec().get_slice_items(vm, &subscript)?,
                 },
                 ref payload => panic!("sequence get_item called for non-sequence: {:?}", payload),
             },
diff --git a/vm/src/obj/objset.rs b/vm/src/obj/objset.rs
index b22213dd0..5ac7507d6 100644
--- a/vm/src/obj/objset.rs
+++ b/vm/src/obj/objset.rs
@@ -11,7 +11,6 @@ use super::objbool;
 use super::objiter;
 use super::objstr;
 use super::objtype;
-use num_bigint::ToBigInt;
 use std::collections::HashMap;
 
 pub fn get_elements(obj: &PyObjectRef) -> HashMap<usize, PyObjectRef> {
@@ -67,15 +66,10 @@ fn set_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         Some(iterable) => {
             let mut elements = HashMap::new();
             let iterator = objiter::get_iter(vm, iterable)?;
-            loop {
-                match vm.call_method(&iterator, "__next__", vec![]) {
-                    Ok(v) => {
-                        // TODO: should we use the hash function here?
-                        let key = v.get_id();
-                        elements.insert(key, v);
-                    }
-                    _ => break,
-                }
+            while let Ok(v) = vm.call_method(&iterator, "__next__", vec![]) {
+                // TODO: should we use the hash function here?
+                let key = v.get_id();
+                elements.insert(key, v);
             }
             elements
         }
@@ -91,7 +85,7 @@ fn set_len(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     trace!("set.len called with: {:?}", args);
     arg_check!(vm, args, required = [(s, Some(vm.ctx.set_type()))]);
     let elements = get_elements(s);
-    Ok(vm.context().new_int(elements.len().to_bigint().unwrap()))
+    Ok(vm.context().new_int(elements.len()))
 }
 
 fn set_repr(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -152,6 +146,11 @@ fn frozenset_repr(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 
 pub fn init(context: &PyContext) {
     let set_type = &context.set_type;
+
+    let set_doc = "set() -> new empty set object\n\
+                   set(iterable) -> new set object\n\n\
+                   Build an unordered collection of unique elements.";
+
     context.set_attr(
         &set_type,
         "__contains__",
@@ -160,15 +159,26 @@ pub fn init(context: &PyContext) {
     context.set_attr(&set_type, "__len__", context.new_rustfunc(set_len));
     context.set_attr(&set_type, "__new__", context.new_rustfunc(set_new));
     context.set_attr(&set_type, "__repr__", context.new_rustfunc(set_repr));
+    context.set_attr(&set_type, "__doc__", context.new_str(set_doc.to_string()));
     context.set_attr(&set_type, "add", context.new_rustfunc(set_add));
 
     let frozenset_type = &context.frozenset_type;
+
+    let frozenset_doc = "frozenset() -> empty frozenset object\n\
+                         frozenset(iterable) -> frozenset object\n\n\
+                         Build an immutable unordered collection of unique elements.";
+
     context.set_attr(
         &frozenset_type,
         "__contains__",
         context.new_rustfunc(set_contains),
     );
     context.set_attr(&frozenset_type, "__len__", context.new_rustfunc(set_len));
+    context.set_attr(
+        &frozenset_type,
+        "__doc__",
+        context.new_str(frozenset_doc.to_string()),
+    );
     context.set_attr(
         &frozenset_type,
         "__repr__",
diff --git a/vm/src/obj/objslice.rs b/vm/src/obj/objslice.rs
new file mode 100644
index 000000000..547e252ba
--- /dev/null
+++ b/vm/src/obj/objslice.rs
@@ -0,0 +1,95 @@
+use super::super::pyobject::{
+    PyContext, PyFuncArgs, PyObject, PyObjectPayload, PyObjectRef, PyResult, TypeProtocol,
+};
+use super::super::vm::VirtualMachine;
+use super::objint;
+use super::objtype; // Required for arg_check! to use isinstance
+use num_bigint::BigInt;
+
+fn slice_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    no_kwargs!(vm, args);
+    let (cls, start, stop, step): (
+        &PyObjectRef,
+        Option<&PyObjectRef>,
+        Option<&PyObjectRef>,
+        Option<&PyObjectRef>,
+    ) = match args.args.len() {
+        0 | 1 => Err(vm.new_type_error("slice() must have at least one arguments.".to_owned())),
+        2 => {
+            arg_check!(
+                vm,
+                args,
+                required = [
+                    (cls, Some(vm.ctx.type_type())),
+                    (stop, Some(vm.ctx.int_type()))
+                ]
+            );
+            Ok((cls, None, Some(stop), None))
+        }
+        _ => {
+            arg_check!(
+                vm,
+                args,
+                required = [
+                    (cls, Some(vm.ctx.type_type())),
+                    (start, Some(vm.ctx.int_type())),
+                    (stop, Some(vm.ctx.int_type()))
+                ],
+                optional = [(step, Some(vm.ctx.int_type()))]
+            );
+            Ok((cls, Some(start), Some(stop), step))
+        }
+    }?;
+    Ok(PyObject::new(
+        PyObjectPayload::Slice {
+            start: start.map(|x| objint::get_value(x)),
+            stop: stop.map(|x| objint::get_value(x)),
+            step: step.map(|x| objint::get_value(x)),
+        },
+        cls.clone(),
+    ))
+}
+
+fn get_property_value(vm: &mut VirtualMachine, value: &Option<BigInt>) -> PyResult {
+    if let Some(value) = value {
+        Ok(vm.ctx.new_int(value.clone()))
+    } else {
+        Ok(vm.get_none())
+    }
+}
+
+fn slice_start(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    arg_check!(vm, args, required = [(slice, Some(vm.ctx.slice_type()))]);
+    if let PyObjectPayload::Slice { start, .. } = &slice.borrow().payload {
+        get_property_value(vm, start)
+    } else {
+        panic!("Slice has incorrect payload.");
+    }
+}
+
+fn slice_stop(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    arg_check!(vm, args, required = [(slice, Some(vm.ctx.slice_type()))]);
+    if let PyObjectPayload::Slice { stop, .. } = &slice.borrow().payload {
+        get_property_value(vm, stop)
+    } else {
+        panic!("Slice has incorrect payload.");
+    }
+}
+
+fn slice_step(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    arg_check!(vm, args, required = [(slice, Some(vm.ctx.slice_type()))]);
+    if let PyObjectPayload::Slice { step, .. } = &slice.borrow().payload {
+        get_property_value(vm, step)
+    } else {
+        panic!("Slice has incorrect payload.");
+    }
+}
+
+pub fn init(context: &PyContext) {
+    let zip_type = &context.slice_type;
+
+    context.set_attr(zip_type, "__new__", context.new_rustfunc(slice_new));
+    context.set_attr(zip_type, "start", context.new_property(slice_start));
+    context.set_attr(zip_type, "stop", context.new_property(slice_stop));
+    context.set_attr(zip_type, "step", context.new_property(slice_step));
+}
diff --git a/vm/src/obj/objstr.rs b/vm/src/obj/objstr.rs
index d71cf3a69..a49e7a2c6 100644
--- a/vm/src/obj/objstr.rs
+++ b/vm/src/obj/objstr.rs
@@ -6,9 +6,9 @@ use super::super::vm::VirtualMachine;
 use super::objint;
 use super::objsequence::PySliceableSequence;
 use super::objtype;
-use num_bigint::ToBigInt;
 use num_traits::ToPrimitive;
 use std::hash::{Hash, Hasher};
+use std::ops::Range;
 // rust's builtin to_lowercase isn't sufficient for casefold
 extern crate caseless;
 extern crate unicode_segmentation;
@@ -308,13 +308,13 @@ fn str_hash(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     let mut hasher = std::collections::hash_map::DefaultHasher::new();
     value.hash(&mut hasher);
     let hash = hasher.finish();
-    Ok(vm.ctx.new_int(hash.to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(hash))
 }
 
 fn str_len(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(s, Some(vm.ctx.str_type()))]);
     let sv = get_value(s);
-    Ok(vm.ctx.new_int(sv.chars().count().to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(sv.chars().count()))
 }
 
 fn str_mul(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -512,12 +512,11 @@ fn str_zfill(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     );
     let value = get_value(&s);
     let len = objint::get_value(&len).to_usize().unwrap();
-    let new_str: String;
-    if len <= value.len() {
-        new_str = value;
+    let new_str = if len <= value.len() {
+        value
     } else {
-        new_str = format!("{}{}", "0".repeat(len - value.len()), value);
-    }
+        format!("{}{}", "0".repeat(len - value.len()), value)
+    };
     Ok(vm.ctx.new_str(new_str))
 }
 
@@ -554,7 +553,7 @@ fn str_count(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         Err(e) => return Err(vm.new_index_error(e)),
     };
     let num_occur: usize = value[start..end].matches(&sub).count();
-    Ok(vm.ctx.new_int(num_occur.to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(num_occur))
 }
 
 fn str_index(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -573,13 +572,13 @@ fn str_index(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         Ok((start, end)) => (start, end),
         Err(e) => return Err(vm.new_index_error(e)),
     };
-    let ind: usize = match value[start..end + 1].find(&sub) {
+    let ind: usize = match value[start..=end].find(&sub) {
         Some(num) => num,
         None => {
             return Err(vm.new_value_error("substring not found".to_string()));
         }
     };
-    Ok(vm.ctx.new_int(ind.to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(ind))
 }
 
 fn str_find(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -598,11 +597,11 @@ fn str_find(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         Ok((start, end)) => (start, end),
         Err(e) => return Err(vm.new_index_error(e)),
     };
-    let ind: i128 = match value[start..end + 1].find(&sub) {
+    let ind: i128 = match value[start..=end].find(&sub) {
         Some(num) => num as i128,
         None => -1 as i128,
     };
-    Ok(vm.ctx.new_int(ind.to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(ind))
 }
 
 // casefold is much more aggresive than lower
@@ -797,7 +796,7 @@ fn str_istitle(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     if value.is_empty() {
         is_titled = false;
     } else {
-        for word in value.split(" ") {
+        for word in value.split(' ') {
             if word != make_title(&word) {
                 is_titled = false;
                 break;
@@ -888,13 +887,13 @@ fn str_rindex(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         Ok((start, end)) => (start, end),
         Err(e) => return Err(vm.new_index_error(e)),
     };
-    let ind: i64 = match value[start..end + 1].rfind(&sub) {
+    let ind: i64 = match value[start..=end].rfind(&sub) {
         Some(num) => num as i64,
         None => {
             return Err(vm.new_value_error("substring not found".to_string()));
         }
     };
-    Ok(vm.ctx.new_int(ind.to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(ind))
 }
 
 fn str_rfind(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -913,11 +912,11 @@ fn str_rfind(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         Ok((start, end)) => (start, end),
         Err(e) => return Err(vm.new_index_error(e)),
     };
-    let ind = match value[start..end + 1].rfind(&sub) {
+    let ind = match value[start..=end].rfind(&sub) {
         Some(num) => num as i128,
         None => -1 as i128,
     };
-    Ok(vm.ctx.new_int(ind.to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(ind))
 }
 
 fn str_isnumeric(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -1005,14 +1004,23 @@ fn str_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 }
 
 impl PySliceableSequence for String {
-    fn do_slice(&self, start: usize, stop: usize) -> Self {
+    fn do_slice(&self, range: Range<usize>) -> Self {
         to_graphemes(self)
-            .get(start..stop)
+            .get(range)
             .map_or(String::default(), |c| c.join(""))
     }
 
-    fn do_stepped_slice(&self, start: usize, stop: usize, step: usize) -> Self {
-        if let Some(s) = to_graphemes(self).get(start..stop) {
+    fn do_slice_reverse(&self, range: Range<usize>) -> Self {
+        to_graphemes(self)
+            .get_mut(range)
+            .map_or(String::default(), |slice| {
+                slice.reverse();
+                slice.join("")
+            })
+    }
+
+    fn do_stepped_slice(&self, range: Range<usize>, step: usize) -> Self {
+        if let Some(s) = to_graphemes(self).get(range) {
             return s
                 .iter()
                 .cloned()
@@ -1023,6 +1031,23 @@ impl PySliceableSequence for String {
         String::default()
     }
 
+    fn do_stepped_slice_reverse(&self, range: Range<usize>, step: usize) -> Self {
+        if let Some(s) = to_graphemes(self).get(range) {
+            return s
+                .iter()
+                .rev()
+                .cloned()
+                .step_by(step)
+                .collect::<Vec<String>>()
+                .join("");
+        }
+        String::default()
+    }
+
+    fn empty() -> Self {
+        String::default()
+    }
+
     fn len(&self) -> usize {
         to_graphemes(self).len()
     }
@@ -1041,20 +1066,21 @@ pub fn subscript(vm: &mut VirtualMachine, value: &str, b: PyObjectRef) -> PyResu
         match objint::get_value(&b).to_i32() {
             Some(pos) => {
                 let graphemes = to_graphemes(value);
-                let idx = graphemes.get_pos(pos);
-                graphemes
-                    .get(idx)
-                    .map(|c| vm.new_str(c.to_string()))
-                    .ok_or(vm.new_index_error("string index out of range".to_string()))
+                if let Some(idx) = graphemes.get_pos(pos) {
+                    Ok(vm.new_str(graphemes[idx].to_string()))
+                } else {
+                    Err(vm.new_index_error("string index out of range".to_string()))
+                }
             }
             None => {
                 Err(vm.new_index_error("cannot fit 'int' into an index-sized integer".to_string()))
             }
         }
     } else {
-        match &(*b.borrow()).payload {
-            &PyObjectPayload::Slice { .. } => {
-                Ok(vm.new_str(value.to_string().get_slice_items(&b).to_string()))
+        match (*b.borrow()).payload {
+            PyObjectPayload::Slice { .. } => {
+                let string = value.to_string().get_slice_items(vm, &b)?;
+                Ok(vm.new_str(string))
             }
             _ => panic!(
                 "TypeError: indexing type {:?} with index {:?} is not supported (yet?)",
diff --git a/vm/src/obj/objsuper.rs b/vm/src/obj/objsuper.rs
index 8a0830503..4fa239903 100644
--- a/vm/src/obj/objsuper.rs
+++ b/vm/src/obj/objsuper.rs
@@ -12,7 +12,27 @@ use super::objtype;
 
 pub fn init(context: &PyContext) {
     let super_type = &context.super_type;
+
+    let super_doc = "super() -> same as super(__class__, <first argument>)\n\
+                     super(type) -> unbound super object\n\
+                     super(type, obj) -> bound super object; requires isinstance(obj, type)\n\
+                     super(type, type2) -> bound super object; requires issubclass(type2, type)\n\
+                     Typical use to call a cooperative superclass method:\n\
+                     class C(B):\n    \
+                     def meth(self, arg):\n        \
+                     super().meth(arg)\n\
+                     This works for class methods too:\n\
+                     class C(B):\n    \
+                     @classmethod\n    \
+                     def cmeth(cls, arg):\n        \
+                     super().cmeth(arg)\n";
+
     context.set_attr(&super_type, "__init__", context.new_rustfunc(super_init));
+    context.set_attr(
+        &super_type,
+        "__doc__",
+        context.new_str(super_doc.to_string()),
+    );
 }
 
 fn super_init(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
diff --git a/vm/src/obj/objtuple.rs b/vm/src/obj/objtuple.rs
index b969142cc..2d99db2d4 100644
--- a/vm/src/obj/objtuple.rs
+++ b/vm/src/obj/objtuple.rs
@@ -9,7 +9,6 @@ use super::objsequence::{
 };
 use super::objstr;
 use super::objtype;
-use num_bigint::ToBigInt;
 use std::hash::{Hash, Hasher};
 
 fn tuple_lt(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -110,7 +109,7 @@ fn tuple_add(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     if objtype::isinstance(other, &vm.ctx.tuple_type()) {
         let e1 = get_elements(zelf);
         let e2 = get_elements(other);
-        let elements = e1.iter().chain(e2.iter()).map(|e| e.clone()).collect();
+        let elements = e1.iter().chain(e2.iter()).cloned().collect();
         Ok(vm.ctx.new_tuple(elements))
     } else {
         Err(vm.new_type_error(format!(
@@ -132,10 +131,10 @@ fn tuple_count(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     for element in elements.iter() {
         let is_eq = vm._eq(element, value.clone())?;
         if objbool::boolval(vm, is_eq)? {
-            count = count + 1;
+            count += 1;
         }
     }
-    Ok(vm.context().new_int(count.to_bigint().unwrap()))
+    Ok(vm.context().new_int(count))
 }
 
 fn tuple_eq(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -164,7 +163,7 @@ fn tuple_hash(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         element_hash.hash(&mut hasher);
     }
     let hash = hasher.finish();
-    Ok(vm.ctx.new_int(hash.to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(hash))
 }
 
 fn tuple_iter(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -184,7 +183,7 @@ fn tuple_iter(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 fn tuple_len(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(zelf, Some(vm.ctx.tuple_type()))]);
     let elements = get_elements(zelf);
-    Ok(vm.context().new_int(elements.len().to_bigint().unwrap()))
+    Ok(vm.context().new_int(elements.len()))
 }
 
 fn tuple_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -254,6 +253,21 @@ fn tuple_getitem(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     get_item(vm, tuple, &get_elements(&tuple), needle.clone())
 }
 
+pub fn tuple_index(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    arg_check!(
+        vm,
+        args,
+        required = [(tuple, Some(vm.ctx.tuple_type())), (needle, None)]
+    );
+    for (index, element) in get_elements(tuple).iter().enumerate() {
+        let py_equal = vm.call_method(needle, "__eq__", vec![element.clone()])?;
+        if objbool::get_value(&py_equal) {
+            return Ok(vm.context().new_int(index));
+        }
+    }
+    Err(vm.new_value_error("tuple.index(x): x not in tuple".to_string()))
+}
+
 pub fn tuple_contains(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(
         vm,
@@ -276,6 +290,10 @@ pub fn tuple_contains(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 
 pub fn init(context: &PyContext) {
     let tuple_type = &context.tuple_type;
+    let tuple_doc = "tuple() -> empty tuple
+tuple(iterable) -> tuple initialized from iterable's items
+
+If the argument is a tuple, the return value is the same object.";
     context.set_attr(&tuple_type, "__add__", context.new_rustfunc(tuple_add));
     context.set_attr(&tuple_type, "__eq__", context.new_rustfunc(tuple_eq));
     context.set_attr(
@@ -299,4 +317,10 @@ pub fn init(context: &PyContext) {
     context.set_attr(&tuple_type, "__le__", context.new_rustfunc(tuple_le));
     context.set_attr(&tuple_type, "__gt__", context.new_rustfunc(tuple_gt));
     context.set_attr(&tuple_type, "__ge__", context.new_rustfunc(tuple_ge));
+    context.set_attr(
+        &tuple_type,
+        "__doc__",
+        context.new_str(tuple_doc.to_string()),
+    );
+    context.set_attr(&tuple_type, "index", context.new_rustfunc(tuple_index));
 }
diff --git a/vm/src/obj/objtype.rs b/vm/src/obj/objtype.rs
index 7b2ed48c5..dd5fb3b2c 100644
--- a/vm/src/obj/objtype.rs
+++ b/vm/src/obj/objtype.rs
@@ -1,21 +1,22 @@
 use super::super::pyobject::{
-    AttributeProtocol, IdProtocol, PyContext, PyFuncArgs, PyObject, PyObjectPayload, PyObjectRef,
-    PyResult, TypeProtocol,
+    AttributeProtocol, IdProtocol, PyAttributes, PyContext, PyFuncArgs, PyObject, PyObjectPayload,
+    PyObjectRef, PyResult, TypeProtocol,
 };
 use super::super::vm::VirtualMachine;
 use super::objdict;
 use super::objstr;
 use super::objtype; // Required for arg_check! to use isinstance
+use std::cell::RefCell;
 use std::collections::HashMap;
 
 /*
  * The magical type type
  */
 
-pub fn create_type(type_type: PyObjectRef, object_type: PyObjectRef, dict_type: PyObjectRef) {
+pub fn create_type(type_type: PyObjectRef, object_type: PyObjectRef, _dict_type: PyObjectRef) {
     (*type_type.borrow_mut()).payload = PyObjectPayload::Class {
         name: String::from("type"),
-        dict: objdict::new(dict_type),
+        dict: RefCell::new(PyAttributes::new()),
         mro: vec![object_type],
     };
     (*type_type.borrow_mut()).typ = Some(type_type.clone());
@@ -23,6 +24,11 @@ pub fn create_type(type_type: PyObjectRef, object_type: PyObjectRef, dict_type:
 
 pub fn init(context: &PyContext) {
     let type_type = &context.type_type;
+
+    let type_doc = "type(object_or_name, bases, dict)\n\
+                    type(object) -> the object's type\n\
+                    type(name, bases, dict) -> a new type";
+
     context.set_attr(&type_type, "__call__", context.new_rustfunc(type_call));
     context.set_attr(&type_type, "__new__", context.new_rustfunc(type_new));
     context.set_attr(
@@ -46,6 +52,7 @@ pub fn init(context: &PyContext) {
         "__getattribute__",
         context.new_rustfunc(type_getattribute),
     );
+    context.set_attr(&type_type, "__doc__", context.new_str(type_doc.to_string()));
 }
 
 fn type_mro(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -119,12 +126,22 @@ pub fn type_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         let mut bases = vm.extract_elements(bases)?;
         bases.push(vm.context().object());
         let name = objstr::get_value(name);
-        new(typ.clone(), &name, bases, dict.clone())
+        new(typ.clone(), &name, bases, py_dict_to_attributes(dict))
     } else {
         Err(vm.new_type_error(format!(": type_new: {:?}", args)))
     }
 }
 
+/// Take a python dictionary and convert it to attributes.
+fn py_dict_to_attributes(dict: &PyObjectRef) -> PyAttributes {
+    let mut attrs = PyAttributes::new();
+    for (key, value) in objdict::get_key_value_pairs(dict) {
+        let key = objstr::get_value(&key);
+        attrs.insert(key, value);
+    }
+    attrs
+}
+
 pub fn type_call(vm: &mut VirtualMachine, mut args: PyFuncArgs) -> PyResult {
     debug!("type_call: {:?}", args);
     let cls = args.shift();
@@ -204,18 +221,16 @@ pub fn type_getattribute(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult
     }
 }
 
-pub fn get_attributes(obj: &PyObjectRef) -> HashMap<String, PyObjectRef> {
+pub fn get_attributes(obj: &PyObjectRef) -> PyAttributes {
     // Gather all members here:
-    let mut attributes: HashMap<String, PyObjectRef> = HashMap::new();
+    let mut attributes = PyAttributes::new();
 
     // Get class attributes:
     let mut base_classes = objtype::base_classes(obj);
     base_classes.reverse();
     for bc in base_classes {
         if let PyObjectPayload::Class { dict, .. } = &bc.borrow().payload {
-            let elements = objdict::get_key_value_pairs(dict);
-            for (name, value) in elements.iter() {
-                let name = objstr::get_value(name);
+            for (name, value) in dict.borrow().iter() {
                 attributes.insert(name.to_string(), value.clone());
             }
         }
@@ -223,9 +238,7 @@ pub fn get_attributes(obj: &PyObjectRef) -> HashMap<String, PyObjectRef> {
 
     // Get instance attributes:
     if let PyObjectPayload::Instance { dict } = &obj.borrow().payload {
-        let elements = objdict::get_key_value_pairs(dict);
-        for (name, value) in elements.iter() {
-            let name = objstr::get_value(name);
+        for (name, value) in dict.borrow().iter() {
             attributes.insert(name.to_string(), value.clone());
         }
     }
@@ -242,8 +255,8 @@ fn take_next_base(
     for base in &bases {
         let head = base[0].clone();
         if !(&bases)
-            .into_iter()
-            .any(|x| x[1..].into_iter().any(|x| x.get_id() == head.get_id()))
+            .iter()
+            .any(|x| x[1..].iter().any(|x| x.get_id() == head.get_id()))
         {
             next = Some(head);
             break;
@@ -265,7 +278,7 @@ fn linearise_mro(mut bases: Vec<Vec<PyObjectRef>>) -> Option<Vec<PyObjectRef>> {
     debug!("Linearising MRO: {:?}", bases);
     let mut result = vec![];
     loop {
-        if (&bases).into_iter().all(|x| x.is_empty()) {
+        if (&bases).iter().all(|x| x.is_empty()) {
             break;
         }
         match take_next_base(bases) {
@@ -279,13 +292,18 @@ fn linearise_mro(mut bases: Vec<Vec<PyObjectRef>>) -> Option<Vec<PyObjectRef>> {
     Some(result)
 }
 
-pub fn new(typ: PyObjectRef, name: &str, bases: Vec<PyObjectRef>, dict: PyObjectRef) -> PyResult {
+pub fn new(
+    typ: PyObjectRef,
+    name: &str,
+    bases: Vec<PyObjectRef>,
+    dict: HashMap<String, PyObjectRef>,
+) -> PyResult {
     let mros = bases.into_iter().map(|x| _mro(x).unwrap()).collect();
     let mro = linearise_mro(mros).unwrap();
     Ok(PyObject::new(
         PyObjectPayload::Class {
             name: String::from(name),
-            dict,
+            dict: RefCell::new(dict),
             mro,
         },
         typ,
@@ -305,7 +323,7 @@ fn type_prepare(vm: &mut VirtualMachine, _args: PyFuncArgs) -> PyResult {
 #[cfg(test)]
 mod tests {
     use super::{linearise_mro, new};
-    use super::{IdProtocol, PyContext, PyObjectRef};
+    use super::{HashMap, IdProtocol, PyContext, PyObjectRef};
 
     fn map_ids(obj: Option<Vec<PyObjectRef>>) -> Option<Vec<usize>> {
         match obj {
@@ -320,20 +338,8 @@ mod tests {
         let object = context.object;
         let type_type = context.type_type;
 
-        let a = new(
-            type_type.clone(),
-            "A",
-            vec![object.clone()],
-            type_type.clone(),
-        )
-        .unwrap();
-        let b = new(
-            type_type.clone(),
-            "B",
-            vec![object.clone()],
-            type_type.clone(),
-        )
-        .unwrap();
+        let a = new(type_type.clone(), "A", vec![object.clone()], HashMap::new()).unwrap();
+        let b = new(type_type.clone(), "B", vec![object.clone()], HashMap::new()).unwrap();
 
         assert_eq!(
             map_ids(linearise_mro(vec![
diff --git a/vm/src/obj/objzip.rs b/vm/src/obj/objzip.rs
new file mode 100644
index 000000000..471df9c2b
--- /dev/null
+++ b/vm/src/obj/objzip.rs
@@ -0,0 +1,46 @@
+use super::super::pyobject::{
+    PyContext, PyFuncArgs, PyObject, PyObjectPayload, PyObjectRef, PyResult, TypeProtocol,
+};
+use super::super::vm::VirtualMachine;
+use super::objiter;
+use super::objtype; // Required for arg_check! to use isinstance
+
+fn zip_new(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    no_kwargs!(vm, args);
+    let cls = &args.args[0];
+    let iterables = &args.args[1..];
+    let iterators = iterables
+        .into_iter()
+        .map(|iterable| objiter::get_iter(vm, iterable))
+        .collect::<Result<Vec<_>, _>>()?;
+    Ok(PyObject::new(
+        PyObjectPayload::ZipIterator { iterators },
+        cls.clone(),
+    ))
+}
+
+fn zip_next(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
+    arg_check!(vm, args, required = [(zip, Some(vm.ctx.zip_type()))]);
+
+    if let PyObjectPayload::ZipIterator { ref mut iterators } = zip.borrow_mut().payload {
+        if iterators.is_empty() {
+            Err(objiter::new_stop_iteration(vm))
+        } else {
+            let next_objs = iterators
+                .iter()
+                .map(|iterator| objiter::call_next(vm, iterator))
+                .collect::<Result<Vec<_>, _>>()?;
+
+            Ok(vm.ctx.new_tuple(next_objs))
+        }
+    } else {
+        panic!("zip doesn't have correct payload");
+    }
+}
+
+pub fn init(context: &PyContext) {
+    let zip_type = &context.zip_type;
+    objiter::iter_type_init(context, zip_type);
+    context.set_attr(zip_type, "__new__", context.new_rustfunc(zip_new));
+    context.set_attr(zip_type, "__next__", context.new_rustfunc(zip_next));
+}
diff --git a/vm/src/pyobject.rs b/vm/src/pyobject.rs
index 072ebb6bc..212757671 100644
--- a/vm/src/pyobject.rs
+++ b/vm/src/pyobject.rs
@@ -7,6 +7,7 @@ use super::obj::objbytes;
 use super::obj::objcode;
 use super::obj::objcomplex;
 use super::obj::objdict;
+use super::obj::objenumerate;
 use super::obj::objfilter;
 use super::obj::objfloat;
 use super::obj::objframe;
@@ -21,12 +22,15 @@ use super::obj::objobject;
 use super::obj::objproperty;
 use super::obj::objrange;
 use super::obj::objset;
+use super::obj::objslice;
 use super::obj::objstr;
 use super::obj::objsuper;
 use super::obj::objtuple;
 use super::obj::objtype;
+use super::obj::objzip;
 use super::vm::VirtualMachine;
 use num_bigint::BigInt;
+use num_bigint::ToBigInt;
 use num_complex::Complex64;
 use num_traits::{One, Zero};
 use std::cell::RefCell;
@@ -69,6 +73,10 @@ pub type PyObjectWeakRef = Weak<RefCell<PyObject>>;
 /// since exceptions are also python objects.
 pub type PyResult = Result<PyObjectRef, PyObjectRef>; // A valid value, or an exception
 
+/// For attributes we do not use a dict, but a hashmap. This is probably
+/// faster, unordered, and only supports strings as keys.
+pub type PyAttributes = HashMap<String, PyObjectRef>;
+
 impl fmt::Display for PyObject {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         use self::TypeProtocol;
@@ -108,6 +116,7 @@ pub struct PyContext {
     pub classmethod_type: PyObjectRef,
     pub code_type: PyObjectRef,
     pub dict_type: PyObjectRef,
+    pub enumerate_type: PyObjectRef,
     pub filter_type: PyObjectRef,
     pub float_type: PyObjectRef,
     pub frame_type: PyObjectRef,
@@ -128,7 +137,9 @@ pub struct PyContext {
     pub super_type: PyObjectRef,
     pub str_type: PyObjectRef,
     pub range_type: PyObjectRef,
+    pub slice_type: PyObjectRef,
     pub type_type: PyObjectRef,
+    pub zip_type: PyObjectRef,
     pub function_type: PyObjectRef,
     pub property_type: PyObjectRef,
     pub module_type: PyObjectRef,
@@ -160,14 +171,9 @@ pub fn create_type(
     name: &str,
     type_type: &PyObjectRef,
     base: &PyObjectRef,
-    dict_type: &PyObjectRef,
+    _dict_type: &PyObjectRef,
 ) -> PyObjectRef {
-    let dict = PyObject::new(
-        PyObjectPayload::Dict {
-            elements: HashMap::new(),
-        },
-        dict_type.clone(),
-    );
+    let dict = PyAttributes::new();
     objtype::new(type_type.clone(), name, vec![base.clone()], dict).unwrap()
 }
 
@@ -204,12 +210,15 @@ impl PyContext {
         let bytearray_type = create_type("bytearray", &type_type, &object_type, &dict_type);
         let tuple_type = create_type("tuple", &type_type, &object_type, &dict_type);
         let iter_type = create_type("iter", &type_type, &object_type, &dict_type);
+        let enumerate_type = create_type("enumerate", &type_type, &object_type, &dict_type);
         let filter_type = create_type("filter", &type_type, &object_type, &dict_type);
         let map_type = create_type("map", &type_type, &object_type, &dict_type);
+        let zip_type = create_type("zip", &type_type, &object_type, &dict_type);
         let bool_type = create_type("bool", &type_type, &int_type, &dict_type);
         let memoryview_type = create_type("memoryview", &type_type, &object_type, &dict_type);
         let code_type = create_type("code", &type_type, &int_type, &dict_type);
         let range_type = create_type("range", &type_type, &object_type, &dict_type);
+        let slice_type = create_type("slice", &type_type, &object_type, &dict_type);
         let exceptions = exceptions::ExceptionZoo::new(&type_type, &object_type, &dict_type);
 
         let none = PyObject::new(
@@ -246,12 +255,15 @@ impl PyContext {
             false_value,
             tuple_type,
             iter_type,
+            enumerate_type,
             filter_type,
             map_type,
+            zip_type,
             dict_type,
             none,
             str_type,
             range_type,
+            slice_type,
             object: object_type,
             function_type,
             super_type,
@@ -280,11 +292,14 @@ impl PyContext {
         objmemory::init(&context);
         objstr::init(&context);
         objrange::init(&context);
+        objslice::init(&context);
         objsuper::init(&context);
         objtuple::init(&context);
         objiter::init(&context);
+        objenumerate::init(&context);
         objfilter::init(&context);
         objmap::init(&context);
+        objzip::init(&context);
         objbool::init(&context);
         objcode::init(&context);
         objframe::init(&context);
@@ -336,6 +351,10 @@ impl PyContext {
         self.range_type.clone()
     }
 
+    pub fn slice_type(&self) -> PyObjectRef {
+        self.slice_type.clone()
+    }
+
     pub fn frozenset_type(&self) -> PyObjectRef {
         self.frozenset_type.clone()
     }
@@ -356,6 +375,10 @@ impl PyContext {
         self.iter_type.clone()
     }
 
+    pub fn enumerate_type(&self) -> PyObjectRef {
+        self.enumerate_type.clone()
+    }
+
     pub fn filter_type(&self) -> PyObjectRef {
         self.filter_type.clone()
     }
@@ -364,6 +387,10 @@ impl PyContext {
         self.map_type.clone()
     }
 
+    pub fn zip_type(&self) -> PyObjectRef {
+        self.zip_type.clone()
+    }
+
     pub fn str_type(&self) -> PyObjectRef {
         self.str_type.clone()
     }
@@ -410,16 +437,16 @@ impl PyContext {
     }
 
     pub fn new_object(&self) -> PyObjectRef {
-        PyObject::new(
-            PyObjectPayload::Instance {
-                dict: self.new_dict(),
-            },
-            self.object(),
-        )
+        self.new_instance(self.object(), None)
     }
 
-    pub fn new_int(&self, i: BigInt) -> PyObjectRef {
-        PyObject::new(PyObjectPayload::Integer { value: i }, self.int_type())
+    pub fn new_int<T: ToBigInt>(&self, i: T) -> PyObjectRef {
+        PyObject::new(
+            PyObjectPayload::Integer {
+                value: i.to_bigint().unwrap(),
+            },
+            self.int_type(),
+        )
     }
 
     pub fn new_float(&self, i: f64) -> PyObjectRef {
@@ -476,7 +503,7 @@ impl PyContext {
     }
 
     pub fn new_class(&self, name: &str, base: PyObjectRef) -> PyObjectRef {
-        objtype::new(self.type_type(), name, vec![base], self.new_dict()).unwrap()
+        objtype::new(self.type_type(), name, vec![base], PyAttributes::new()).unwrap()
     }
 
     pub fn new_scope(&self, parent: Option<PyObjectRef>) -> PyObjectRef {
@@ -530,12 +557,7 @@ impl PyContext {
         function: F,
     ) -> PyObjectRef {
         let fget = self.new_rustfunc(function);
-        let py_obj = PyObject::new(
-            PyObjectPayload::Instance {
-                dict: self.new_dict(),
-            },
-            self.property_type(),
-        );
+        let py_obj = self.new_instance(self.property_type(), None);
         self.set_attr(&py_obj, "fget", fget.clone());
         py_obj
     }
@@ -571,13 +593,23 @@ impl PyContext {
         &self,
         function: F,
     ) -> PyObjectRef {
-        let dict = self.new_dict();
-        self.set_item(&dict, "function", self.new_rustfunc(function));
-        self.new_instance(dict, self.member_descriptor_type())
+        let mut dict = PyAttributes::new();
+        dict.insert("function".to_string(), self.new_rustfunc(function));
+        self.new_instance(self.member_descriptor_type(), Some(dict))
     }
 
-    pub fn new_instance(&self, dict: PyObjectRef, class: PyObjectRef) -> PyObjectRef {
-        PyObject::new(PyObjectPayload::Instance { dict }, class)
+    pub fn new_instance(&self, class: PyObjectRef, dict: Option<PyAttributes>) -> PyObjectRef {
+        let dict = if let Some(dict) = dict {
+            dict
+        } else {
+            PyAttributes::new()
+        };
+        PyObject::new(
+            PyObjectPayload::Instance {
+                dict: RefCell::new(dict),
+            },
+            class,
+        )
     }
 
     // Item set/get:
@@ -605,8 +637,9 @@ impl PyContext {
     pub fn set_attr(&self, obj: &PyObjectRef, attr_name: &str, value: PyObjectRef) {
         match obj.borrow().payload {
             PyObjectPayload::Module { ref dict, .. } => self.set_item(dict, attr_name, value),
-            PyObjectPayload::Instance { ref dict } => self.set_item(dict, attr_name, value),
-            PyObjectPayload::Class { ref dict, .. } => self.set_item(dict, attr_name, value),
+            PyObjectPayload::Instance { ref dict } | PyObjectPayload::Class { ref dict, .. } => {
+                dict.borrow_mut().insert(attr_name.to_string(), value);
+            }
             ref payload => unimplemented!("set_attr unimplemented for: {:?}", payload),
         };
     }
@@ -667,10 +700,7 @@ pub trait ParentProtocol {
 impl ParentProtocol for PyObjectRef {
     fn has_parent(&self) -> bool {
         match self.borrow().payload {
-            PyObjectPayload::Scope { ref scope } => match scope.parent {
-                Some(_) => true,
-                None => false,
-            },
+            PyObjectPayload::Scope { ref scope } => scope.parent.is_some(),
             _ => panic!("Only scopes have parent (not {:?}", self),
         }
     }
@@ -694,7 +724,7 @@ pub trait AttributeProtocol {
 fn class_get_item(class: &PyObjectRef, attr_name: &str) -> Option<PyObjectRef> {
     let class = class.borrow();
     match class.payload {
-        PyObjectPayload::Class { ref dict, .. } => dict.get_item(attr_name),
+        PyObjectPayload::Class { ref dict, .. } => dict.borrow().get(attr_name).map(|v| v.clone()),
         _ => panic!("Only classes should be in MRO!"),
     }
 }
@@ -702,7 +732,7 @@ fn class_get_item(class: &PyObjectRef, attr_name: &str) -> Option<PyObjectRef> {
 fn class_has_item(class: &PyObjectRef, attr_name: &str) -> bool {
     let class = class.borrow();
     match class.payload {
-        PyObjectPayload::Class { ref dict, .. } => dict.contains_key(attr_name),
+        PyObjectPayload::Class { ref dict, .. } => dict.borrow().contains_key(attr_name),
         _ => panic!("Only classes should be in MRO!"),
     }
 }
@@ -723,7 +753,9 @@ impl AttributeProtocol for PyObjectRef {
                 }
                 None
             }
-            PyObjectPayload::Instance { ref dict } => dict.get_item(attr_name),
+            PyObjectPayload::Instance { ref dict } => {
+                dict.borrow().get(attr_name).map(|v| v.clone())
+            }
             _ => None,
         }
     }
@@ -733,10 +765,9 @@ impl AttributeProtocol for PyObjectRef {
         match obj.payload {
             PyObjectPayload::Module { ref dict, .. } => dict.contains_key(attr_name),
             PyObjectPayload::Class { ref mro, .. } => {
-                class_has_item(self, attr_name)
-                    || mro.into_iter().any(|d| class_has_item(d, attr_name))
+                class_has_item(self, attr_name) || mro.iter().any(|d| class_has_item(d, attr_name))
             }
-            PyObjectPayload::Instance { ref dict } => dict.contains_key(attr_name),
+            PyObjectPayload::Instance { ref dict } => dict.borrow().contains_key(attr_name),
             _ => false,
         }
     }
@@ -882,6 +913,10 @@ pub enum PyObjectPayload {
         position: usize,
         iterated_obj: PyObjectRef,
     },
+    EnumerateIterator {
+        counter: BigInt,
+        iterator: PyObjectRef,
+    },
     FilterIterator {
         predicate: PyObjectRef,
         iterator: PyObjectRef,
@@ -890,10 +925,13 @@ pub enum PyObjectPayload {
         mapper: PyObjectRef,
         iterators: Vec<PyObjectRef>,
     },
+    ZipIterator {
+        iterators: Vec<PyObjectRef>,
+    },
     Slice {
-        start: Option<i32>,
-        stop: Option<i32>,
-        step: Option<i32>,
+        start: Option<BigInt>,
+        stop: Option<BigInt>,
+        step: Option<BigInt>,
     },
     Range {
         range: objrange::RangeType,
@@ -929,14 +967,14 @@ pub enum PyObjectPayload {
     None,
     Class {
         name: String,
-        dict: PyObjectRef,
+        dict: RefCell<PyAttributes>,
         mro: Vec<PyObjectRef>,
     },
     WeakRef {
         referent: PyObjectWeakRef,
     },
     Instance {
-        dict: PyObjectRef,
+        dict: RefCell<PyAttributes>,
     },
     RustFunction {
         function: Box<Fn(&mut VirtualMachine, PyFuncArgs) -> PyResult>,
@@ -958,8 +996,10 @@ impl fmt::Debug for PyObjectPayload {
             PyObjectPayload::WeakRef { .. } => write!(f, "weakref"),
             PyObjectPayload::Range { .. } => write!(f, "range"),
             PyObjectPayload::Iterator { .. } => write!(f, "iterator"),
+            PyObjectPayload::EnumerateIterator { .. } => write!(f, "enumerate"),
             PyObjectPayload::FilterIterator { .. } => write!(f, "filter"),
             PyObjectPayload::MapIterator { .. } => write!(f, "map"),
+            PyObjectPayload::ZipIterator { .. } => write!(f, "zip"),
             PyObjectPayload::Slice { .. } => write!(f, "slice"),
             PyObjectPayload::Code { ref code } => write!(f, "code: {:?}", code),
             PyObjectPayload::Function { .. } => write!(f, "function"),
@@ -1055,8 +1095,10 @@ impl PyObject {
                 position,
                 iterated_obj.borrow_mut().str()
             ),
+            PyObjectPayload::EnumerateIterator { .. } => format!("<enumerate>"),
             PyObjectPayload::FilterIterator { .. } => format!("<filter>"),
             PyObjectPayload::MapIterator { .. } => format!("<map>"),
+            PyObjectPayload::ZipIterator { .. } => format!("<zip>"),
         }
     }
 
diff --git a/vm/src/stdlib/ast.rs b/vm/src/stdlib/ast.rs
index 6b9eb66c9..6703534f4 100644
--- a/vm/src/stdlib/ast.rs
+++ b/vm/src/stdlib/ast.rs
@@ -9,9 +9,7 @@ use self::rustpython_parser::{ast, parser};
 use super::super::obj::{objstr, objtype};
 use super::super::pyobject::{PyContext, PyFuncArgs, PyObjectRef, PyResult, TypeProtocol};
 use super::super::VirtualMachine;
-use num_bigint::ToBigInt;
 use num_complex::Complex64;
-use num_traits::One;
 use std::ops::Deref;
 
 /*
@@ -47,8 +45,7 @@ fn program_to_ast(ctx: &PyContext, program: &ast::Program) -> PyObjectRef {
 fn create_node(ctx: &PyContext, _name: &str) -> PyObjectRef {
     // TODO: instantiate a class of type given by name
     // TODO: lookup in the current module?
-    let node = ctx.new_object();
-    node
+    ctx.new_object()
 }
 
 fn statements_to_ast(ctx: &PyContext, statements: &[ast::LocatedStatement]) -> PyObjectRef {
@@ -101,18 +98,9 @@ fn statement_to_ast(ctx: &PyContext, statement: &ast::LocatedStatement) -> PyObj
             ctx.set_attr(&node, "decorator_list", py_decorator_list);
             node
         }
-        ast::Statement::Continue => {
-            let node = create_node(ctx, "Continue");
-            node
-        }
-        ast::Statement::Break => {
-            let node = create_node(ctx, "Break");
-            node
-        }
-        ast::Statement::Pass => {
-            let node = create_node(ctx, "Pass");
-            node
-        }
+        ast::Statement::Continue => create_node(ctx, "Continue"),
+        ast::Statement::Break => create_node(ctx, "Break"),
+        ast::Statement::Pass => create_node(ctx, "Pass"),
         ast::Statement::Assert { test, msg } => {
             let node = create_node(ctx, "Pass");
 
@@ -129,12 +117,8 @@ fn statement_to_ast(ctx: &PyContext, statement: &ast::LocatedStatement) -> PyObj
         ast::Statement::Delete { targets } => {
             let node = create_node(ctx, "Delete");
 
-            let py_targets = ctx.new_tuple(
-                targets
-                    .into_iter()
-                    .map(|v| expression_to_ast(ctx, v))
-                    .collect(),
-            );
+            let py_targets =
+                ctx.new_tuple(targets.iter().map(|v| expression_to_ast(ctx, v)).collect());
             ctx.set_attr(&node, "targets", py_targets);
 
             node
@@ -143,12 +127,7 @@ fn statement_to_ast(ctx: &PyContext, statement: &ast::LocatedStatement) -> PyObj
             let node = create_node(ctx, "Return");
 
             let py_value = if let Some(value) = value {
-                ctx.new_tuple(
-                    value
-                        .into_iter()
-                        .map(|v| expression_to_ast(ctx, v))
-                        .collect(),
-                )
+                ctx.new_tuple(value.iter().map(|v| expression_to_ast(ctx, v)).collect())
             } else {
                 ctx.none()
             };
@@ -232,7 +211,7 @@ fn statement_to_ast(ctx: &PyContext, statement: &ast::LocatedStatement) -> PyObj
     };
 
     // set lineno on node:
-    let lineno = ctx.new_int(statement.location.get_row().to_bigint().unwrap());
+    let lineno = ctx.new_int(statement.location.get_row());
     ctx.set_attr(&node, "lineno", lineno);
 
     node
@@ -385,7 +364,7 @@ fn expression_to_ast(ctx: &PyContext, expression: &ast::Expression) -> PyObjectR
             let node = create_node(ctx, "Num");
 
             let py_n = match value {
-                ast::Number::Integer { value } => ctx.new_int(value.to_bigint().unwrap()),
+                ast::Number::Integer { value } => ctx.new_int(value.clone()),
                 ast::Number::Float { value } => ctx.new_float(*value),
                 ast::Number::Complex { real, imag } => {
                     ctx.new_complex(Complex64::new(*real, *imag))
@@ -550,7 +529,7 @@ fn expression_to_ast(ctx: &PyContext, expression: &ast::Expression) -> PyObjectR
     };
 
     // TODO: retrieve correct lineno:
-    let lineno = ctx.new_int(One::one());
+    let lineno = ctx.new_int(1);
     ctx.set_attr(&node, "lineno", lineno);
 
     node
diff --git a/vm/src/stdlib/io.rs b/vm/src/stdlib/io.rs
index c4391d116..e254b2417 100644
--- a/vm/src/stdlib/io.rs
+++ b/vm/src/stdlib/io.rs
@@ -27,7 +27,7 @@ use super::super::pyobject::{
 use super::super::vm::VirtualMachine;
 
 fn compute_c_flag(mode: &str) -> u16 {
-    match mode.as_ref() {
+    match mode {
         "w" => 512,
         "x" => 512,
         "a" => 8,
@@ -85,11 +85,8 @@ fn buffered_reader_read(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
             .map_err(|_| vm.new_value_error("IO Error".to_string()))?;
 
         //Copy bytes from the buffer vector into the results vector
-        match buffer.borrow_mut().payload {
-            PyObjectPayload::Bytes { ref mut value } => {
-                result.extend(value.iter().cloned());
-            }
-            _ => {}
+        if let PyObjectPayload::Bytes { ref mut value } = buffer.borrow_mut().payload {
+            result.extend(value.iter().cloned());
         };
 
         let len = vm.get_method(buffer.clone(), &"__len__".to_string());
@@ -171,21 +168,18 @@ fn file_io_readinto(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     let handle = os::rust_file(raw_fd);
 
     let mut f = handle.take(length);
-    match obj.borrow_mut().payload {
+    if let PyObjectPayload::Bytes { ref mut value } = obj.borrow_mut().payload {
         //TODO: Implement for MemoryView
-        PyObjectPayload::Bytes { ref mut value } => {
-            value.clear();
-            match f.read_to_end(&mut *value) {
-                Ok(_) => {}
-                Err(_) => return Err(vm.new_value_error("Error reading from Take".to_string())),
-            }
+
+        value.clear();
+        match f.read_to_end(&mut *value) {
+            Ok(_) => {}
+            Err(_) => return Err(vm.new_value_error("Error reading from Take".to_string())),
         }
-        _ => {}
     };
 
-    let updated = os::raw_file_number(f.into_inner()).to_bigint();
-    vm.ctx
-        .set_attr(&file_io, "fileno", vm.ctx.new_int(updated.unwrap()));
+    let updated = os::raw_file_number(f.into_inner());
+    vm.ctx.set_attr(&file_io, "fileno", vm.ctx.new_int(updated));
     Ok(vm.get_none())
 }
 
@@ -209,12 +203,11 @@ fn file_io_write(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
             match handle.write(&value[..]) {
                 Ok(len) => {
                     //reset raw fd on the FileIO object
-                    let updated = os::raw_file_number(handle).to_bigint();
-                    vm.ctx
-                        .set_attr(&file_io, "fileno", vm.ctx.new_int(updated.unwrap()));
+                    let updated = os::raw_file_number(handle);
+                    vm.ctx.set_attr(&file_io, "fileno", vm.ctx.new_int(updated));
 
                     //return number of bytes written
-                    Ok(vm.ctx.new_int(len.to_bigint().unwrap()))
+                    Ok(vm.ctx.new_int(len))
                 }
                 Err(_) => Err(vm.new_value_error("Error Writing Bytes to Handle".to_string())),
             }
@@ -320,7 +313,7 @@ pub fn io_open(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     //the operation in the mode.
     //There are 3 possible classes here, each inheriting from the RawBaseIO
     // creating || writing || appending => BufferedWriter
-    let buffered = if rust_mode.contains("w") {
+    let buffered = if rust_mode.contains('w') {
         vm.invoke(
             buffered_writer_class,
             PyFuncArgs::new(vec![file_io.clone()], vec![]),
@@ -334,7 +327,7 @@ pub fn io_open(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         //TODO: updating => PyBufferedRandom
     };
 
-    if rust_mode.contains("t") {
+    if rust_mode.contains('t') {
         //If the mode is text this buffer type is consumed on construction of
         //a TextIOWrapper which is subsequently returned.
         vm.invoke(
diff --git a/vm/src/stdlib/json.rs b/vm/src/stdlib/json.rs
index 9f9e705ec..1e7ec411e 100644
--- a/vm/src/stdlib/json.rs
+++ b/vm/src/stdlib/json.rs
@@ -11,7 +11,6 @@ use super::super::pyobject::{
     TypeProtocol,
 };
 use super::super::VirtualMachine;
-use num_bigint::ToBigInt;
 use num_traits::cast::ToPrimitive;
 
 // We need to have a VM available to serialise a PyObject based on its subclass, so we implement
@@ -54,10 +53,9 @@ impl<'s> serde::Serialize for PyObjectSerializer<'s> {
             serializer.serialize_i64(v.to_i64().unwrap())
         // Although this may seem nice, it does not give the right result:
         // v.serialize(serializer)
-        } else if objtype::isinstance(self.pyobject, &self.ctx.list_type()) {
-            let elements = objsequence::get_elements(self.pyobject);
-            serialize_seq_elements(serializer, &elements)
-        } else if objtype::isinstance(self.pyobject, &self.ctx.tuple_type()) {
+        } else if objtype::isinstance(self.pyobject, &self.ctx.list_type())
+            || objtype::isinstance(self.pyobject, &self.ctx.tuple_type())
+        {
             let elements = objsequence::get_elements(self.pyobject);
             serialize_seq_elements(serializer, &elements)
         } else if objtype::isinstance(self.pyobject, &self.ctx.dict_type()) {
@@ -119,7 +117,7 @@ impl<'de> serde::de::DeserializeSeed<'de> for PyObjectDeserializer<'de> {
             {
                 // The JSON deserialiser always uses the i64/u64 deserialisers, so we only need to
                 // implement those for now
-                Ok(self.ctx.new_int(value.to_bigint().unwrap()))
+                Ok(self.ctx.new_int(value))
             }
 
             fn visit_u64<E>(self, value: u64) -> Result<Self::Value, E>
@@ -128,7 +126,7 @@ impl<'de> serde::de::DeserializeSeed<'de> for PyObjectDeserializer<'de> {
             {
                 // The JSON deserialiser always uses the i64/u64 deserialisers, so we only need to
                 // implement those for now
-                Ok(self.ctx.new_int(value.to_bigint().unwrap()))
+                Ok(self.ctx.new_int(value))
             }
 
             fn visit_f64<E>(self, value: f64) -> Result<Self::Value, E>
@@ -223,10 +221,8 @@ fn loads(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
             .get_item("JSONDecodeError")
             .unwrap();
         let exc = vm.new_exception(json_decode_error, format!("{}", err));
-        vm.ctx
-            .set_item(&exc, "lineno", vm.ctx.new_int(err.line().into()));
-        vm.ctx
-            .set_item(&exc, "colno", vm.ctx.new_int(err.column().into()));
+        vm.ctx.set_item(&exc, "lineno", vm.ctx.new_int(err.line()));
+        vm.ctx.set_item(&exc, "colno", vm.ctx.new_int(err.column()));
         exc
     })
 }
diff --git a/vm/src/stdlib/os.rs b/vm/src/stdlib/os.rs
index ac284bddc..4671f5fe9 100644
--- a/vm/src/stdlib/os.rs
+++ b/vm/src/stdlib/os.rs
@@ -4,7 +4,6 @@ use std::fs::OpenOptions;
 use std::io::ErrorKind;
 
 //3rd party imports
-use num_bigint::ToBigInt;
 use num_traits::cast::ToPrimitive;
 
 //custom imports
@@ -91,11 +90,7 @@ pub fn os_open(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         _ => vm.new_value_error("Unhandled file IO error".to_string()),
     })?;
 
-    Ok(vm.ctx.new_int(
-        raw_file_number(handle)
-            .to_bigint()
-            .expect("Invalid file descriptor"),
-    ))
+    Ok(vm.ctx.new_int(raw_file_number(handle)))
 }
 
 pub fn mk_module(ctx: &PyContext) -> PyObjectRef {
@@ -103,11 +98,11 @@ pub fn mk_module(ctx: &PyContext) -> PyObjectRef {
     ctx.set_attr(&py_mod, "open", ctx.new_rustfunc(os_open));
     ctx.set_attr(&py_mod, "close", ctx.new_rustfunc(os_close));
 
-    ctx.set_attr(&py_mod, "O_RDONLY", ctx.new_int(0.to_bigint().unwrap()));
-    ctx.set_attr(&py_mod, "O_WRONLY", ctx.new_int(1.to_bigint().unwrap()));
-    ctx.set_attr(&py_mod, "O_RDWR", ctx.new_int(2.to_bigint().unwrap()));
-    ctx.set_attr(&py_mod, "O_NONBLOCK", ctx.new_int(4.to_bigint().unwrap()));
-    ctx.set_attr(&py_mod, "O_APPEND", ctx.new_int(8.to_bigint().unwrap()));
-    ctx.set_attr(&py_mod, "O_CREAT", ctx.new_int(512.to_bigint().unwrap()));
+    ctx.set_attr(&py_mod, "O_RDONLY", ctx.new_int(0));
+    ctx.set_attr(&py_mod, "O_WRONLY", ctx.new_int(1));
+    ctx.set_attr(&py_mod, "O_RDWR", ctx.new_int(2));
+    ctx.set_attr(&py_mod, "O_NONBLOCK", ctx.new_int(4));
+    ctx.set_attr(&py_mod, "O_APPEND", ctx.new_int(8));
+    ctx.set_attr(&py_mod, "O_CREAT", ctx.new_int(512));
     py_mod
 }
diff --git a/vm/src/stdlib/pystruct.rs b/vm/src/stdlib/pystruct.rs
index 7babc6000..166007197 100644
--- a/vm/src/stdlib/pystruct.rs
+++ b/vm/src/stdlib/pystruct.rs
@@ -13,7 +13,7 @@ use self::byteorder::{LittleEndian, ReadBytesExt, WriteBytesExt};
 use super::super::obj::{objbool, objbytes, objfloat, objint, objstr, objtype};
 use super::super::pyobject::{PyContext, PyFuncArgs, PyObjectRef, PyResult, TypeProtocol};
 use super::super::VirtualMachine;
-use num_bigint::{BigInt, ToBigInt};
+use num_bigint::BigInt;
 use num_traits::ToPrimitive;
 use std::io::{Cursor, Read, Write};
 
@@ -224,14 +224,14 @@ fn struct_pack(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
 fn unpack_i8(vm: &mut VirtualMachine, rdr: &mut Read) -> PyResult {
     match rdr.read_i8() {
         Err(err) => panic!("Error in reading {:?}", err),
-        Ok(v) => Ok(vm.ctx.new_int(v.to_bigint().unwrap())),
+        Ok(v) => Ok(vm.ctx.new_int(v)),
     }
 }
 
 fn unpack_u8(vm: &mut VirtualMachine, rdr: &mut Read) -> PyResult {
     match rdr.read_u8() {
         Err(err) => panic!("Error in reading {:?}", err),
-        Ok(v) => Ok(vm.ctx.new_int(v.to_bigint().unwrap())),
+        Ok(v) => Ok(vm.ctx.new_int(v)),
     }
 }
 
@@ -245,42 +245,42 @@ fn unpack_bool(vm: &mut VirtualMachine, rdr: &mut Read) -> PyResult {
 fn unpack_i16(vm: &mut VirtualMachine, rdr: &mut Read) -> PyResult {
     match rdr.read_i16::<LittleEndian>() {
         Err(err) => panic!("Error in reading {:?}", err),
-        Ok(v) => Ok(vm.ctx.new_int(v.to_bigint().unwrap())),
+        Ok(v) => Ok(vm.ctx.new_int(v)),
     }
 }
 
 fn unpack_u16(vm: &mut VirtualMachine, rdr: &mut Read) -> PyResult {
     match rdr.read_u16::<LittleEndian>() {
         Err(err) => panic!("Error in reading {:?}", err),
-        Ok(v) => Ok(vm.ctx.new_int(v.to_bigint().unwrap())),
+        Ok(v) => Ok(vm.ctx.new_int(v)),
     }
 }
 
 fn unpack_i32(vm: &mut VirtualMachine, rdr: &mut Read) -> PyResult {
     match rdr.read_i32::<LittleEndian>() {
         Err(err) => panic!("Error in reading {:?}", err),
-        Ok(v) => Ok(vm.ctx.new_int(v.to_bigint().unwrap())),
+        Ok(v) => Ok(vm.ctx.new_int(v)),
     }
 }
 
 fn unpack_u32(vm: &mut VirtualMachine, rdr: &mut Read) -> PyResult {
     match rdr.read_u32::<LittleEndian>() {
         Err(err) => panic!("Error in reading {:?}", err),
-        Ok(v) => Ok(vm.ctx.new_int(v.to_bigint().unwrap())),
+        Ok(v) => Ok(vm.ctx.new_int(v)),
     }
 }
 
 fn unpack_i64(vm: &mut VirtualMachine, rdr: &mut Read) -> PyResult {
     match rdr.read_i64::<LittleEndian>() {
         Err(err) => panic!("Error in reading {:?}", err),
-        Ok(v) => Ok(vm.ctx.new_int(v.to_bigint().unwrap())),
+        Ok(v) => Ok(vm.ctx.new_int(v)),
     }
 }
 
 fn unpack_u64(vm: &mut VirtualMachine, rdr: &mut Read) -> PyResult {
     match rdr.read_u64::<LittleEndian>() {
         Err(err) => panic!("Error in reading {:?}", err),
-        Ok(v) => Ok(vm.ctx.new_int(v.to_bigint().unwrap())),
+        Ok(v) => Ok(vm.ctx.new_int(v)),
     }
 }
 
diff --git a/vm/src/stdlib/types.rs b/vm/src/stdlib/types.rs
index 2060ad1f1..a744d89b7 100644
--- a/vm/src/stdlib/types.rs
+++ b/vm/src/stdlib/types.rs
@@ -3,7 +3,9 @@
  */
 
 use super::super::obj::{objsequence, objstr, objtype};
-use super::super::pyobject::{PyContext, PyFuncArgs, PyObjectRef, PyResult, TypeProtocol};
+use super::super::pyobject::{
+    PyAttributes, PyContext, PyFuncArgs, PyObjectRef, PyResult, TypeProtocol,
+};
 use super::super::VirtualMachine;
 
 fn types_new_class(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
@@ -15,7 +17,6 @@ fn types_new_class(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     );
 
     let name = objstr::get_value(name);
-    let dict = vm.ctx.new_dict();
 
     let bases = match bases {
         Some(b) => {
@@ -28,7 +29,7 @@ fn types_new_class(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
         None => vec![vm.ctx.object()],
     };
 
-    objtype::new(vm.ctx.type_type(), &name, bases, dict)
+    objtype::new(vm.ctx.type_type(), &name, bases, PyAttributes::new())
 }
 
 pub fn mk_module(ctx: &PyContext) -> PyObjectRef {
diff --git a/vm/src/sysmodule.rs b/vm/src/sysmodule.rs
index e747fda5a..43b01d0d9 100644
--- a/vm/src/sysmodule.rs
+++ b/vm/src/sysmodule.rs
@@ -1,4 +1,3 @@
-use num_bigint::ToBigInt;
 use obj::objtype;
 use pyobject::{PyContext, PyFuncArgs, PyObjectRef, PyResult, TypeProtocol};
 use std::rc::Rc;
@@ -26,14 +25,14 @@ fn getframe(vm: &mut VirtualMachine, _args: PyFuncArgs) -> PyResult {
 fn sys_getrefcount(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(object, None)]);
     let size = Rc::strong_count(&object);
-    Ok(vm.ctx.new_int(size.to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(size))
 }
 
 fn sys_getsizeof(vm: &mut VirtualMachine, args: PyFuncArgs) -> PyResult {
     arg_check!(vm, args, required = [(object, None)]);
     // TODO: implement default optional argument.
     let size = mem::size_of_val(&object.borrow());
-    Ok(vm.ctx.new_int(size.to_bigint().unwrap()))
+    Ok(vm.ctx.new_int(size))
 }
 
 pub fn mk_module(ctx: &PyContext) -> PyObjectRef {
@@ -62,11 +61,7 @@ pub fn mk_module(ctx: &PyContext) -> PyObjectRef {
     ctx.set_item(&sys_mod, "argv", argv(ctx));
     ctx.set_item(&sys_mod, "getrefcount", ctx.new_rustfunc(sys_getrefcount));
     ctx.set_item(&sys_mod, "getsizeof", ctx.new_rustfunc(sys_getsizeof));
-    ctx.set_item(
-        &sys_mod,
-        "maxsize",
-        ctx.new_int(std::usize::MAX.to_bigint().unwrap()),
-    );
+    ctx.set_item(&sys_mod, "maxsize", ctx.new_int(std::usize::MAX));
     ctx.set_item(&sys_mod, "path", path);
     ctx.set_item(&sys_mod, "ps1", ctx.new_str(">>>>> ".to_string()));
     ctx.set_item(&sys_mod, "ps2", ctx.new_str("..... ".to_string()));
diff --git a/vm/src/vm.rs b/vm/src/vm.rs
index b1570e386..435e7bcbc 100644
--- a/vm/src/vm.rs
+++ b/vm/src/vm.rs
@@ -91,8 +91,7 @@ impl VirtualMachine {
         };
 
         // Call function:
-        let exception = self.invoke(exc_type, args).unwrap();
-        exception
+        self.invoke(exc_type, args).unwrap()
     }
 
     pub fn new_type_error(&mut self, msg: String) -> PyObjectRef {
@@ -105,6 +104,11 @@ impl VirtualMachine {
         self.new_exception(os_error, msg)
     }
 
+    pub fn new_overflow_error(&mut self, msg: String) -> PyObjectRef {
+        let overflow_error = self.ctx.exceptions.overflow_error.clone();
+        self.new_exception(overflow_error, msg)
+    }
+
     /// Create a new python ValueError object. Useful for raising errors from
     /// python functions implemented in rust.
     pub fn new_value_error(&mut self, msg: String) -> PyObjectRef {
@@ -123,8 +127,13 @@ impl VirtualMachine {
     }
 
     pub fn new_not_implemented_error(&mut self, msg: String) -> PyObjectRef {
-        let value_error = self.ctx.exceptions.not_implemented_error.clone();
-        self.new_exception(value_error, msg)
+        let not_implemented_error = self.ctx.exceptions.not_implemented_error.clone();
+        self.new_exception(not_implemented_error, msg)
+    }
+
+    pub fn new_zero_division_error(&mut self, msg: String) -> PyObjectRef {
+        let zero_division_error = self.ctx.exceptions.zero_division_error.clone();
+        self.new_exception(zero_division_error, msg)
     }
 
     pub fn new_scope(&mut self, parent_scope: Option<PyObjectRef>) -> PyObjectRef {
@@ -440,9 +449,9 @@ impl VirtualMachine {
         value: &PyObjectRef,
     ) -> Result<Vec<PyObjectRef>, PyObjectRef> {
         // Extract elements from item, if possible:
-        let elements = if objtype::isinstance(value, &self.ctx.tuple_type()) {
-            objsequence::get_elements(value).to_vec()
-        } else if objtype::isinstance(value, &self.ctx.list_type()) {
+        let elements = if objtype::isinstance(value, &self.ctx.tuple_type())
+            || objtype::isinstance(value, &self.ctx.list_type())
+        {
             objsequence::get_elements(value).to_vec()
         } else {
             let iter = objiter::get_iter(self, value)?;
@@ -615,8 +624,8 @@ mod tests {
     #[test]
     fn test_add_py_integers() {
         let mut vm = VirtualMachine::new();
-        let a = vm.ctx.new_int(33_i32.to_bigint().unwrap());
-        let b = vm.ctx.new_int(12_i32.to_bigint().unwrap());
+        let a = vm.ctx.new_int(33_i32);
+        let b = vm.ctx.new_int(12_i32);
         let res = vm._add(a, b).unwrap();
         let value = objint::get_value(&res);
         assert_eq!(value, 45_i32.to_bigint().unwrap());
@@ -626,7 +635,7 @@ mod tests {
     fn test_multiply_str() {
         let mut vm = VirtualMachine::new();
         let a = vm.ctx.new_str(String::from("Hello "));
-        let b = vm.ctx.new_int(4_i32.to_bigint().unwrap());
+        let b = vm.ctx.new_int(4_i32);
         let res = vm._mul(a, b).unwrap();
         let value = objstr::get_value(&res);
         assert_eq!(value, String::from("Hello Hello Hello Hello "))