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Fix isinstance by youknowone · Pull Request #5869 · RustPython/RustPython

youknowone

Walkthrough

Two new methods were introduced in the type system to provide low-level, exception-free attribute lookups directly in the MRO, closely matching CPython's behavior. Object protocol logic was refactored for safety and clarity, and a new method for special method lookup was added, ensuring correct handling of descriptors and error masking as in CPython.

Changes

File(s) Change Summary
vm/src/builtins/type.rs Added find_name_in_mro (private) and lookup_ref (public) methods to PyType for direct, exception-free attribute lookup in the MRO.
vm/src/protocol/object.rs Refactored abstract_issubclass for safer, clearer control flow; updated special method lookups to use new lookup_special method; added lookup_special to PyObject for CPython-like behavior.

Sequence Diagram(s)

sequenceDiagram
    participant Caller
    participant PyType
    participant PyStrInterned
    participant MRO

    Caller->>PyType: lookup_ref(name: &PyStr, vm)
    PyType->>PyStrInterned: Intern name
    PyType->>MRO: find_name_in_mro(interned_name)
    MRO-->>PyType: Return attribute value or None
    PyType-->>Caller: Return Option<PyObjectRef>
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sequenceDiagram
    participant Caller
    participant PyObject
    participant PyType
    participant MRO
    participant Descriptor

    Caller->>PyObject: lookup_special(attr: &PyStr, vm)
    PyObject->>PyType: Get type and MRO
    PyType->>MRO: Search for attr in MRO dicts
    MRO-->>PyType: Return value or None
    alt Found and is descriptor
        PyType->>Descriptor: Call __get__(self, instance, owner)
        Descriptor-->>PyType: Return result or None on exception
    end
    PyType-->>PyObject: Return Option<PyObjectRef>
    PyObject-->>Caller: Return Option<PyObjectRef>
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Poem

In the warren of types, a lookup anew,
With MRO magic, the attributes flew.
No exceptions to catch, just a None if you must,
And special methods found with descriptor trust.
With safety and clarity, the code now aligns—
A hop and a skip through Pythonic designs! 🐇

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📥 Commits

Reviewing files that changed from the base of the PR and between b6ff541 and 899abbb.

📒 Files selected for processing (2)
  • vm/src/builtins/type.rs (1 hunks)
  • vm/src/protocol/object.rs (6 hunks)
🧰 Additional context used
🧠 Learnings (1)
vm/src/builtins/type.rs (3) 
Learnt from: CR
PR: RustPython/RustPython#0
File: .github/copilot-instructions.md:0-0
Timestamp: 2025-06-26T12:52:11.138Z
Learning: When implementing a Python module in Rust for RustPython, use the #[pymodule] attribute and expose functions and classes using #[pyfunction], #[pyclass], and #[pymethod] as appropriate.

Learnt from: CR
PR: RustPython/RustPython#0
File: .github/copilot-instructions.md:0-0
Timestamp: 2025-06-26T12:52:11.138Z
Learning: To find unimplemented methods and contribution opportunities, run './whats_left.py' in the RustPython repository.

Learnt from: CR
PR: RustPython/RustPython#0
File: .github/copilot-instructions.md:0-0
Timestamp: 2025-06-26T12:52:11.138Z
Learning: Use the macro system (such as 'pyclass', 'pymodule', 'pyfunction') when implementing Python functionality in Rust for RustPython.
🔇 Additional comments (6)
vm/src/builtins/type.rs (2)

386-402: LGTM! Correct implementation of CPython's find_name_in_mro.

The method correctly implements direct dictionary lookup in the MRO, bypassing descriptors and other attribute access machinery as intended. The logic matches CPython's behavior by first checking the type's own attributes before traversing the MRO.

404-413: Correct implementation of _PyType_LookupRef with proper error handling.

The method properly handles the case where the string might not be interned (returning None), which matches CPython's behavior. The use of find_name_in_mro ensures descriptor logic is bypassed as expected.

vm/src/protocol/object.rs (4)

412-442: Excellent refactoring for safety and clarity!

The removal of unsafe pointer casts and the clearer control flow significantly improves code maintainability. The single inheritance optimization in the loop is a nice touch that avoids unnecessary recursion while maintaining the same behavior.

527-531: Correct alignment with CPython's special method lookup behavior.

Using lookup_special instead of vm.get_special_method properly implements CPython's semantics where special methods are looked up in the type's MRO without checking the instance dictionary. The change from invoke to call is also appropriate.

Also applies to: 618-622

541-552: Consistent use of is_subtype for type checking.

The change from fast_isinstance/fast_issubclass to is_subtype provides more direct and consistent subtype checking throughout the codebase.

Also applies to: 570-570

749-767: Well-implemented special method lookup matching CPython's semantics.

The lookup_special method correctly implements CPython's _PyObject_LookupSpecial:

  • Uses the new lookup_ref for MRO lookup without instance dict
  • Properly handles descriptors by calling __get__
  • Correctly masks exceptions by returning None

This is crucial for proper special method resolution in isinstance/issubclass checks.

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