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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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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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