Implement FileIO.read with nbytes

tests/snippets/stdlib_io.py

@@ -30,3 +30,9 @@ fi.read()
 fi.close()
 with assertRaises(ValueError):
     fi.read()
+
+with FileIO('README.md') as fio:
+	nres = fio.read(1)
+	assert len(nres) == 1
+	nres = fio.read(2)
+	assert len(nres) == 2

vm/src/stdlib/io.rs

@@ -346,18 +346,36 @@ fn file_io_init(vm: &VirtualMachine, args: PyFuncArgs) -> PyResult {
 }
 
 fn file_io_read(vm: &VirtualMachine, args: PyFuncArgs) -> PyResult {
-    arg_check!(vm, args, required = [(file_io, None)]);
+    arg_check!(
+        vm,
+        args,
+        required = [(file_io, None)],
+        optional = [(read_byte, Some(vm.ctx.int_type()))]
+    );
 
     let file_no = vm.get_attribute(file_io.clone(), "fileno")?;
     let raw_fd = objint::get_value(&file_no).to_i64().unwrap();
 
     let mut handle = os::rust_file(raw_fd);
 
-    let mut bytes = vec![];
-    match handle.read_to_end(&mut bytes) {
-        Ok(_) => {}
-        Err(_) => return Err(vm.new_value_error("Error reading from Buffer".to_string())),
-    }
+    let bytes = match read_byte {
+        None => {
+            let mut bytes = vec![];
+            handle
+                .read_to_end(&mut bytes)
+                .map_err(|_| vm.new_value_error("Error reading from Buffer".to_string()))?;
+            bytes
+        }
+        Some(read_byte) => {
+            let mut bytes = vec![0; objint::get_value(&read_byte).to_usize().unwrap()];
+            handle
+                .read_exact(&mut bytes)
+                .map_err(|_| vm.new_value_error("Error reading from Buffer".to_string()))?;
+            let updated = os::raw_file_number(handle);
+            vm.set_attr(file_io, "fileno", vm.ctx.new_int(updated))?;
+            bytes
+        }
+    };
 
     Ok(vm.ctx.new_bytes(bytes))
 }