Allow SyntaxError.msg to be writable and reflected in string formatting (#6493)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: youknowone <69878+youknowone@users.noreply.github.com>

DEVELOPMENT.md

@@ -130,7 +130,8 @@ repository's structure:
   - `stdlib`: Standard library parts implemented in rust.
 - `src`: using the other subcrates to bring rustpython to life.
 - `wasm`: Binary crate and resources for WebAssembly build
-- `extra_tests`: extra integration test snippets as a supplement to `Lib/test`
+- `extra_tests`: extra integration test snippets as a supplement to `Lib/test`.
+  Add new RustPython-only regression tests here; do not place new tests under `Lib/test`.
 
 ## Understanding Internals
 

crates/vm/src/exceptions.rs

@@ -9,7 +9,7 @@ use crate::{
     },
     class::{PyClassImpl, StaticType},
     convert::{ToPyException, ToPyObject},
-    function::{ArgIterable, FuncArgs, IntoFuncArgs},
+    function::{ArgIterable, FuncArgs, IntoFuncArgs, PySetterValue},
     py_io::{self, Write},
     stdlib::sys,
     suggestion::offer_suggestions,
@@ -994,7 +994,12 @@ impl ExceptionZoo {
         extend_exception!(PyRecursionError, ctx, excs.recursion_error);
 
         extend_exception!(PySyntaxError, ctx, excs.syntax_error, {
-            "msg" => ctx.new_readonly_getset("msg", excs.syntax_error, make_arg_getter(0)),
+            "msg" => ctx.new_static_getset(
+                "msg",
+                excs.syntax_error,
+                make_arg_getter(0),
+                syntax_error_set_msg,
+            ),
             // TODO: members
             "filename" => ctx.none(),
             "lineno" => ctx.none(),
@@ -1041,6 +1046,25 @@ fn make_arg_getter(idx: usize) -> impl Fn(PyBaseExceptionRef) -> Option<PyObject
     move |exc| exc.get_arg(idx)
 }
 
+fn syntax_error_set_msg(
+    exc: PyBaseExceptionRef,
+    value: PySetterValue,
+    vm: &VirtualMachine,
+) -> PyResult<()> {
+    let mut args = exc.args.write();
+    let mut new_args = args.as_slice().to_vec();
+    // Ensure the message slot at index 0 always exists for SyntaxError.args.
+    if new_args.is_empty() {
+        new_args.push(vm.ctx.none());
+    }
+    match value {
+        PySetterValue::Assign(value) => new_args[0] = value,
+        PySetterValue::Delete => new_args[0] = vm.ctx.none(),
+    }
+    *args = PyTuple::new_ref(new_args, &vm.ctx);
+    Ok(())
+}
+
 fn system_exit_code(exc: PyBaseExceptionRef) -> Option<PyObjectRef> {
     exc.args.read().first().map(|code| {
         match_class!(match code {
@@ -2048,15 +2072,14 @@ pub(super) mod types {
                     .unwrap_or_else(|_| vm.ctx.new_str("<filename str() failed>"))
             });
 
-            let args = zelf.args();
-
-            let msg = if args.len() == 1 {
-                vm.exception_args_as_string(args, false)
-                    .into_iter()
-                    .exactly_one()
-                    .unwrap()
-            } else {
-                return zelf.__str__(vm);
+            let msg = match zelf.as_object().get_attr("msg", vm) {
+                Ok(obj) => obj
+                    .str(vm)
+                    .unwrap_or_else(|_| vm.ctx.new_str("<msg str() failed>")),
+                Err(_) => {
+                    // Fallback to the base formatting if the msg attribute was deleted or attribute lookup fails for any reason.
+                    return Py::<PyBaseException>::__str__(zelf, vm);
+                }
             };
 
             let msg_with_location_info: String = match (maybe_lineno, maybe_filename) {

extra_tests/snippets/builtin_exceptions.py

@@ -85,6 +85,13 @@ assert exc.lineno is None
 assert exc.offset is None
 assert exc.text is None
 
+err = SyntaxError("bad bad", ("bad.py", 1, 2, "abcdefg"))
+err.msg = "changed"
+assert err.msg == "changed"
+assert str(err) == "changed (bad.py, line 1)"
+del err.msg
+assert err.msg is None
+
 # Regression to:
 # https://github.com/RustPython/RustPython/issues/2779