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Preserve str subclass results from __repr__ in plain str() #7454

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Preserve str subclass results from __repr__ in plain str() #7454

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8a56d78
Initial plan
Copilot Mar 17, 2026
ed2fd1e
Preserve str subclass returned by __repr__
Copilot Mar 17, 2026
2ff66e7
Format str constructor fast path
Copilot Mar 17, 2026
5140534
Tighten str subclass conversion regression
Copilot Mar 17, 2026
9a11d44
Align test_str with CPython and move local coverage
Copilot Mar 19, 2026
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1 change: 0 additions & 1 deletion Lib/test/test_str.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2414,7 +2414,6 @@ def test_ucs4(self):
else:
self.fail("Should have raised UnicodeDecodeError")

@unittest.expectedFailure # TODO: RUSTPYTHON; AssertionError: <class 'str'> is not <class 'test.test_str.StrSubclass'>
def test_conversion(self):
# Make sure __str__() works properly
class StrWithStr(str):
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9 changes: 3 additions & 6 deletions crates/vm/src/builtins/str.rs
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Expand Up @@ -394,13 +394,10 @@ impl Constructor for PyStr {
type Args = StrArgs;

fn slot_new(cls: PyTypeRef, func_args: FuncArgs, vm: &VirtualMachine) -> PyResult {
// Optimization: return exact str as-is (only when no encoding/errors provided)
if cls.is(vm.ctx.types.str_type)
&& func_args.args.len() == 1
&& func_args.kwargs.is_empty()
&& func_args.args[0].class().is(vm.ctx.types.str_type)
// Optimization: for exact str, return PyObject_Str result as-is
if cls.is(vm.ctx.types.str_type) && func_args.args.len() == 1 && func_args.kwargs.is_empty()
{
return Ok(func_args.args[0].clone());
return func_args.args[0].str(vm).map(Into::into);
}

let args: Self::Args = func_args.bind(vm)?;
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27 changes: 27 additions & 0 deletions extra_tests/snippets/builtin_str_subclass.py
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Expand Up @@ -19,6 +19,33 @@ def __init__(self, value):
assert y + " other" == "1 other"
assert y.x == "substr"


class ReprStrSubclass(str):
pass


class WithStr:
def __init__(self, value):
self.value = value

def __str__(self):
return self.value


class WithRepr:
def __init__(self, value):
self.value = value

def __repr__(self):
return self.value


str_value = ReprStrSubclass("abc")
assert str(WithStr(str_value)) is str_value

repr_value = ReprStrSubclass("<abc>")
assert str(WithRepr(repr_value)) is repr_value

## Base strings currently get an attribute dict, but shouldn't.
# with assert_raises(AttributeError):
# "hello".x = 5
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