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[mypyc] Keep Generic bases of non-native generic classes - #22183
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A non-native class declared with Generic[T] lost its Generic base when compiled, because mypyc used mypy's resolved base types, and mypy removes Generic[...] and erases type arguments there. The compiled class couldn't be subscripted at runtime and had no `__parameters__`. Non-native PEP 695 classes such as class G[T] had similar problems. Both cases now work similar to interpreted Python. Generic NamedTuple and TypedDict classes are not covered here. Since subscripted bases are now evaluated, the bases match interpreted code. For example, class C(Dict[str, int]) now gets (dict, Generic) as its bases. A base that is generic only in stubs (e.g. email.message.Message[str, str]) now fails at import, like it does in interpreted code.
p-sawicki
approved these changes
Oct 12, 2026
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A non-native class declared with Generic[T] lost its Generic base when compiled, because mypyc used mypy's resolved base types, and mypy removes Generic[...] and erases type arguments there. The compiled class couldn't be subscripted at runtime and had no
__parameters__. Non-native PEP 695 classes such as class G[T] had similar problems. Both cases now work similar to interpreted Python.Generic NamedTuple and TypedDict classes are not covered here.
Since subscripted bases are now evaluated, the bases match interpreted code. For example, class C(Dict[str, int]) now gets (dict, Generic) as its bases. A base that is generic only in stubs (e.g. email.message.Message[str, str]) now fails at import, like it does in interpreted code.