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Preserve str subclass type returned by __str__ / __repr__ #7701

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Merged
youknowone merged 1 commit into from
Apr 28, 2026
Merged

Preserve str subclass type returned by __str__ / __repr__ #7701

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Preserve str subclass type returned by __str__ / __repr__ 
Closes #7450.

CPython's unicode_new_impl returns the PyObject_Str result as-is when
type == &PyUnicode_Type, only invoking unicode_subtype_new for actual
str subclasses. RustPython's PyStr::Constructor stripped the result via
Self::from(s.as_wtf8().to_owned()) and re-materialized through
into_ref_with_type, dropping the subclass type even when cls is exactly
str.

Add a slot_new branch that returns input.str(vm)? directly when cls is
str_type with no encoding. Subtype construction and the bytes-decoding
path are unchanged.

Unmasks test_str.StrTest.test_conversion (11 assertTypedEqual cases).
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@changjoon-park
changjoon-park committed Apr 27, 2026
commit f7870f64dfe806dec1b20fcfa5d579611960fba5
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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12 changes: 12 additions & 0 deletions crates/vm/src/builtins/str.rs
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Expand Up @@ -406,6 +406,18 @@ impl Constructor for PyStr {
}

let args: Self::Args = func_args.bind(vm)?;

// CPython parity: when cls is exactly str, return the __str__ / __repr__
// result as-is so any str subclass type the user returned is preserved
// (matches unicode_new_impl which only invokes unicode_subtype_new when
// type != &PyUnicode_Type).
if cls.is(vm.ctx.types.str_type)
&& args.encoding.is_missing()
&& let OptionalArg::Present(input) = &args.object
{
return Ok(input.str(vm)?.into());
}

let payload = Self::py_new(&cls, args, vm)?;
payload.into_ref_with_type(vm, cls).map(Into::into)
}
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