may_add_dict #6084

youknowone · 2025-08-08T14:53:04Z

Summary by CodeRabbit

Bug Fixes
- Improved handling of class attribute dictionaries to prevent redundant dictionary support when the base class already provides it. This ensures more accurate behavior when using __slots__ and inheritance.

coderabbitai · 2025-08-08T14:53:11Z

Walkthrough

The logic for adding dictionary-related feature flags to new types in the type creation process has been refined. The new condition ensures that dictionary support is only added when the base class does not already provide it, or when __dict__ is explicitly included in __slots__.

Changes

Cohort / File(s)	Change Summary
Type Feature Flag Logic Refinement `vm/src/builtins/type.rs`	Refined the condition for adding `HAS_DICT` and `MANAGED_DICT` flags to new types, considering base class dictionary presence and `__slots__` configuration.

Sequence Diagram(s)

sequenceDiagram
    participant TypeCreator
    participant BaseClass
    participant NewType

    TypeCreator->>BaseClass: Check HAS_DICT flag
    alt __dict__ in __slots__
        TypeCreator->>NewType: Add HAS_DICT and MANAGED_DICT flags
    else __slots__ not defined AND base lacks HAS_DICT
        TypeCreator->>NewType: Add HAS_DICT and MANAGED_DICT flags
    else
        TypeCreator->>NewType: Do not add dictionary flags
    end

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~7 minutes

Possibly related PRs

Fix __dict__ getset type #6010: Refactors and enhances dictionary descriptor handling and base type dictionary detection in the same file and context, directly related to the refined flag logic in this PR.

Poem

A rabbit hopped through type-land bright,
Tweaking flags with careful might.
No extra dicts where base ones dwell,
Only when slots or needs compel.
With logic crisp and checks anew,
The code runs lean—hoppity-hoo!

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Reviewing files that changed from the base of the PR and between 33ea50c and b269c48.

📒 Files selected for processing (1)

vm/src/builtins/type.rs (1 hunks)

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📓 Path-based instructions (1)

**/*.rs

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**/*.rs: Follow the default rustfmt code style (cargo fmt to format)
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Files:

vm/src/builtins/type.rs

🔇 Additional comments (1)

vm/src/builtins/type.rs (1)

1072-1081: may_add_dict guard is an improvement but still leaves a CPython-incompatibility

Nice catch introducing may_add_dict; it stops us from unconditionally slapping MANAGED_DICT onto every heap type whose primary base already owns a dict.

However, note the follow-up flow in new_heap_inner: that helper always OR-propagates PyTypeFlags::HAS_DICT from the base (Lines 246-248).
As a consequence, a subclass that defines __slots__ = () (i.e. intends to drop the instance dict) will nevertheless finish with HAS_DICT set, diverging from CPython where such a subclass has no __dict__.

Consider short-circuiting the propagation when the heap type defined its own __slots__ without "__dict__" (and may_add_dict is false), or deferring the decision entirely to this block so the later OR doesn’t re-introduce the flag.

If you agree, an outline fix would be:
// in new_heap_inner, before propagating
-if base.slots.flags.has_feature(PyTypeFlags::HAS_DICT) {
-    slots.flags |= PyTypeFlags::HAS_DICT
-}
+if base.slots.flags.has_feature(PyTypeFlags::HAS_DICT)
+    && slots.flags.has_feature(PyTypeFlags::HAS_DICT)
+{
+    // only inherit when the heap type decided it wants a dict
+    slots.flags |= PyTypeFlags::HAS_DICT
+}
or equivalent logic moved here.

Please double-check against CPython’s behaviour with:
class A: pass
class B(A): __slots__ = ()
to ensure parity.

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youknowone · 2025-08-08T22:40:36Z

Need test

may_add_dict

b269c48

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Walkthrough

Changes

Sequence Diagram(s)

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