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changjoon-park · 2026-04-27T14:20:25Z

Closes #7450.

Background

CPython's unicode_new_impl (Objects/unicodeobject.c#L15575-L15596) only invokes unicode_subtype_new when the requested type is a str subclass. When type == &PyUnicode_Type, it returns the result of PyObject_Str(x) as-is, preserving any str subclass type that __str__ or __repr__ returned.

RustPython's PyStr::Constructor::py_new extracted the raw bytes via Self::from(s.as_wtf8().to_owned()), then slot_new re-materialized through into_ref_with_type(vm, cls). This stripped the subclass type even when cls is str.

Repro

class MyStr(str): pass
class Foo:
    def __repr__(self): return MyStr('hello')

print(type(str(Foo())))
# CPython 3.14:    <class '__main__.MyStr'>
# RustPython:      <class 'str'>     (before this PR)

Fix

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

Tests unmasked

test_str.StrTest.test_conversion (11 assertTypedEqual cases covering str(WithStr/StrWithStr/WithRepr(...)) × StrSubclass / OtherStrSubclass permutations)

Verification

CPython 3.14.4 byte-identical for 8 probed cases (plain str(), str('hi'), str(MyStr('x')), __str__/__repr__ returning subclass, MyStr('z'), OtherStr(MyStr('w')), str(b'abc', 'utf-8'))
No regressions across 14 modules (~3,349 tests): test_str, test_descr, test_class, test_typing, test_userstring, test_format, test_pickle, test_io, test_collections, test_dict, test_set, test_exceptions, test_fstring, test_genericalias

Summary by CodeRabbit

Bug Fixes
- Improved string constructor to correctly preserve behavior when using user-defined string subclasses.

Closes RustPython#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).

coderabbitai · 2026-04-27T14:20:41Z

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Reviewing files that changed from the base of the PR and between 1d42ee5 and f7870f6.

⛔ Files ignored due to path filters (1)

Lib/test/test_str.py is excluded by !Lib/**

📒 Files selected for processing (1)

crates/vm/src/builtins/str.rs

📝 Walkthrough

Walkthrough

This PR adds a short-circuit optimization to PyStr::slot_new that preserves str subclass types returned by __repr__. When constructing an exact str from a single object argument without encoding, the implementation now directly returns the result of input.str(vm) instead of converting it to a plain str, aligning with CPython's behavior.

Changes

Cohort / File(s)	Summary
Str constructor short-circuit `crates/vm/src/builtins/str.rs`	Added early return path in `PyStr::slot_new` for exact `str` type with single argument (no encoding), preserving subclass types from `__repr__` method returns.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related issues

#7451: Both changes address preserving str subclass objects by returning existing repr/str directly instead of always constructing a new plain str.

Suggested reviewers

ShaharNaveh
youknowone

Poem

🐰 A subclass preserved, oh what delight!
No more discarding types left and right,
When __repr__ speaks in custom voice,
The rabbit cheers—CPython's choice! ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: preserving str subclass types returned by str/repr methods in the constructor logic.
Linked Issues check	✅ Passed	The PR directly addresses issue `#7450` by adding a constructor short-circuit that returns input.str(vm) directly when cls is exactly str, preserving str subclass types as required.
Out of Scope Changes check	✅ Passed	All changes are localized to PyStr::slot_new constructor logic and directly address the linked issue requirements; no out-of-scope modifications detected.

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github-actions · 2026-04-27T14:21:15Z

📦 Library Dependencies

The following Lib/ modules were modified. Here are their dependencies:

[x] test: cpython/Lib/test/test_str.py (TODO: 14)
[x] test: cpython/Lib/test/test_fstring.py (TODO: 19)
[x] test: cpython/Lib/test/test_string_literals.py (TODO: 4)

dependencies:

dependent tests: (no tests depend on str)

Legend:

[+] path exists in CPython
[x] up-to-date, [ ] outdated

ShaharNaveh

Great!
ty for finding these:)

changjoon-park · 2026-04-27T14:38:01Z

ShaharNaveh approved these changes Apr 27, 2026

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youknowone approved these changes Apr 28, 2026

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youknowone merged commit 6c498fc into RustPython:main Apr 28, 2026
21 checks passed

coderabbitai Bot mentioned this pull request May 3, 2026

Tighten CPython parity for str format spec, %-format, and str() constructor #7769

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Preserve str subclass type returned by str / repr - #7701

Preserve str subclass type returned by str / repr#7701
youknowone merged 1 commit into
RustPython:mainfrom
changjoon-park:fix-str-subclass-preservation

changjoon-park commented Apr 27, 2026 •

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changjoon-park commented Apr 27, 2026 •

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