[mypyc] Preserve inherited attribute defaults under separate=True#21547
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p-sawicki merged 7 commits intoJun 3, 2026
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Under `separate=True`, when a subclass is recompiled while its parent
is loaded from mypy's incremental cache, parent default-attribute
assignments are silently dropped from the subclass's
`__mypyc_defaults_setup`. The first read of an inherited default-attr
then raises:
AttributeError: attribute '<name>' of '<Parent>' undefined
`find_attr_initializers` walks `cdef.info.mro` and reads
`info.defn.defs.body` for `AssignmentStmt`s. `ClassDef.serialize`
(mypy/nodes.py) does not serialize `defs`, so a cache-loaded parent
has `defs = Block([])`; the MRO walk collects no parent assignments
and the subclass's emitted setup leaves inherited slots in the
undefined-sentinel state.
Fix: under `separate=True`, scope `find_attr_initializers` to the
subclass's own body and have `generate_attr_defaults_init` emit a
chained call to the nearest ancestor with `__mypyc_defaults_setup`
before setting own defaults. Each class's setup is responsible only
for its own attributes; the chain runs ancestors first, mirroring
the `__init__` chain. The ancestor's return value is propagated so
a parent default that raised still aborts instance creation. Without
`separate=True`, the MRO AST walk is unaffected (all modules parsed
in the same pass), preserving the existing inline-all behavior.
Adds `testIncrementalCrossModuleInheritedAttrDefaultsWithOverride`
to `run-multimodule.test`, which triggers the cache-load path: clean
build of a two-module Parent/Child hierarchy, then a `.2` revision of
the subclass module to force an incremental rebuild while the parent
is served from the cache. The child overrides one inherited default
so it emits its own `__mypyc_defaults_setup` (the case the chain
composition is required for). The test fails on master without this
patch.
p-sawicki
reviewed
May 28, 2026
p-sawicki
reviewed
Jun 2, 2026
p-sawicki
approved these changes
Jun 3, 2026
georgesittas
added a commit
to VaggelisD/sqlglot-mypy
that referenced
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Jun 4, 2026
…thon#21547) Fixes python#21542 Under `separate=True`, when a subclass is recompiled while its parent is loaded from mypy's incremental cache, parent default-attribute assignments are silently dropped from the subclass's `__mypyc_defaults_setup`. The first read of an inherited default-attr then raises: ``` AttributeError: attribute '<name>' of '<Parent>' undefined ``` `find_attr_initializers` walks `cdef.info.mro` and reads `info.defn.defs.body` for `AssignmentStmt`s. `ClassDef.serialize` (mypy/nodes.py) does not serialize `defs`, so a cache-loaded parent has `defs = Block([])`; the MRO walk collects no parent assignments and the subclass's emitted setup leaves inherited slots in the undefined-sentinel state. This PR implements the fix discussed in the linked issue.
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Fixes #21542
Under
separate=True, when a subclass is recompiled while its parent is loaded from mypy's incremental cache, parent default-attribute assignments are silently dropped from the subclass's__mypyc_defaults_setup. The first read of an inherited default-attr then raises:find_attr_initializerswalkscdef.info.mroand readsinfo.defn.defs.bodyforAssignmentStmts.ClassDef.serialize(mypy/nodes.py) does not serializedefs, so a cache-loaded parent hasdefs = Block([]); the MRO walk collects no parent assignments and the subclass's emitted setup leaves inherited slots in the undefined-sentinel state.This PR implements the fix discussed in the linked issue.