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gh-104050: Argument clinic: more misc typing improvements - #107264

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AlexWaygood merged 1 commit into
python:mainfrom
AlexWaygood:misc-clinic-typing
Jul 25, 2023
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gh-104050: Argument clinic: more misc typing improvements#107264
AlexWaygood merged 1 commit into
python:mainfrom
AlexWaygood:misc-clinic-typing

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@AlexWaygood

@AlexWaygoodAlexWaygood commented Jul 25, 2023

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Comment on lines +2698 to +2703
def __init__(
cls, name: str, bases: tuple[type, ...], classdict: dict[str, Any]
) -> None:
converter_cls = cast(type["CConverter"], cls)
add_c_converter(converter_cls)
add_default_legacy_c_converter(converter_cls)

@AlexWaygoodAlexWaygoodJul 25, 2023

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Mypy was complaining because our type annotation for add_c_converter says (correctly) that the argument passed in has to be a subclass of CConverter. But that isn't necessarily true here, since this metaclass could theoretically be used with classes other than CConverter (it isn't, and it won't be, but mypy doesn't know that).

Ideally we could do this:

def__init__(
cls, name: str, bases: tuple[type, ...], classdict: dict[str, Any]
) ->None:
assertissubclass(cls, CConverter)
add_c_converter(cls)
add_default_legacy_c_converter(cls)

But we can't... because if cls is CConverter itself, then at this point, CConverter doesn't exist in the global namespace yet! So that would fail with NameError.

I therefore elected to use typing.cast() to just tell mypy to go away here.

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Thanks for writing these excellent explanations!

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I'm glad they're all making sense! :D

@AlexWaygood
AlexWaygood merged commit ee90107 into python:mainJul 25, 2023
@AlexWaygood
AlexWaygood deleted the misc-clinic-typing branch July 25, 2023 21:08
jtcave pushed a commit to jtcave/cpython that referenced this pull request Jul 27, 2023
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@AlexWaygood@erlend-aasland@bedevere-bot