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Prevent __class__ reassignment across incompatible layouts#6521

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youknowone merged 6 commits intomainfrom
copilot/fix-class-attribute-panic
Dec 26, 2025
Merged

Prevent __class__ reassignment across incompatible layouts#6521
youknowone merged 6 commits intomainfrom
copilot/fix-class-attribute-panic

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Copilot AI commented Dec 25, 2025 •
edited by coderabbitai bot
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  • Guard class setter to reject incompatible layouts while still permitting module-style class swaps (e.g., lazy modules and request modules) when their layouts match, avoiding panics and keeping import/ensurepip flows working.
  • Move the regression coverage for layout-mismatch errors into extra_tests/snippets/builtin_type.py and remove the Rust-side unit test per review feedback.
  • Ran targeted rustpython-vm test to verify the build after the guard adjustment.
Original prompt

This section details on the original issue you should resolve

<issue_title>Panic when setting class with different layout</issue_title>
<issue_description>## Summary
Setting object's class attribute to a type with different layout cause panic

thread 'main' panicked at 'index out of bounds: the len is 0 but the index is 0', vm/src/object/core.rs:813:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

which is an error in cpython

TypeError: __class__ assignment: 'TypeB' object layout differs from 'TypeA'

Detailed Explanation

class TypeA:
    def __init__(self) -> None:
        self.a = 1

class TypeB:
    __slots__ = "b"
    def __init__(self) -> None:
        self.b = 2

obj = TypeA()
assert obj.a == 1

obj.__class__ = TypeB
print(obj.b)
```</issue_description>

<agent_instructions>payload type_id() must be cheked</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@youknowone</author><body>
maybe similar thing cause both this one and  RustPython/RustPython#1979</body></comment_new>
</comments>

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Summary by CodeRabbit

  • Bug Fixes

    • Enhanced validation for dynamic class reassignment to enforce object layout compatibility.
    • Now raises TypeError when attempting to reassign an instance to a class with incompatible layout, slot differences, or dict-slot presence mismatches.

  • Tests

    • Added test coverage for class layout compatibility constraints and dynamic class reassignment scenarios.

✏️ Tip: You can customize this high-level summary in your review settings.

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Panic when setting __class__ with different layout

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