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Disallow Mapping as argument type? #3576
Description
This came up in #3569. See also the later comments in python/typing#11 for background. Mapping is not a protocol (as opposed to most other typing ABCs). This mean code like the following will not work:
from typing import Iterator, Mapping
class MyMap:
def __getitem__(self, key: str) -> int:
return 0
def __iter__(self) -> Iterator[str]:
return iter([])
def __len__(self) -> int:
return 0
def __contains__(self, key: object) -> bool:
return False
def foo(x: Mapping[str, int]) -> None:
pass
foo(MyMap()) # error: Argument 1 to "foo" has incompatible type "MyMap"; expected "Mapping[str, int]"Deriving MyMap from Mapping[str, int] works, though. I suggest we don't allow new submissions to use Mapping as argument type and work towards removing existing uses. Instead they should use a custom protocol, which can also be narrower than Mapping.
Activity
djb4ai commentedon Mar 26, 2020
@srittau I am trying to create a Protocol in PR#3877 but when I use
Protocol[_KT, _VT_co]it saysInvariant type variable '_KT' used in protocol where contravariant one is expected, can't understand what could be causing this.Introduce Mapping-like protocols
Introduce Mapping-like protocols (#4325)
jab commentedon Aug 12, 2020
Rather than disallowing
Mappingas an argument type becauseMappingis not a protocol, is it better to instead makeMappinga protocol? That seems like it would result in better usability than the alternative of forbiddingMappingarguments. Worth writing down the pros and cons of both options before picking which one? Related protocols (SupportsGetItem, etc.) were recently added in #4325 since this issue was created. Does that change things?Relatedly, if we had python/mypy#2922, I guess
Mapping.register(MyMap)could be added to the example in the issue description to resolve the error, but mypy does not yet supportABCMeta.register.srittau commentedon Aug 12, 2020
I think it's hard to make
Mappinga protocol, since it's an abstract base class incollections.abc, and since it's so widely used. But maybe it would make sense to introduce a separateMappingProto(or similar) protocol and use that going forward. Although, using more specific protocols is still more useful, but harder to implement.gvanrossum commentedon Aug 12, 2020
Indeed. We looked into this and found it would be very difficult. I strongly recommend just inheriting from Mapping over coming up with yet another duck typing protocol. FWIW mypy probably will never support .register().
AlexWaygood commentedon Nov 1, 2023
We have
_typeshed.SupportsKeysAndGetItemnow, and use it quite widely. It's a very useful protocol.Possibly we could look into adding lints to detect bad uses of
Mappingin argument annotations. The issue tracker at flake8-pyi would probably be a better place to discuss that, though.