Require argument names to match in Protocol

[sid-kap]

Feature

Currently mypy does not check that argument names in functions that implement protocols match the argument names in the protocol. This fails when arguments are passed as keywords.

For example, this passes mypy:

from typing import Protocol

class MyProtocol(Protocol):
    def foo(self, a: int) -> None: ...

class MyObject:
    def foo(self, b: int) -> None:
        pass

def call_foo(obj: MyProtocol) -> None:
    obj.foo(a=1)

call_foo(MyObject())

but fails at runtime:

Traceback (most recent call last):
  File "/Users/sidharthkapur/protocol_test.py", line 13, in <module>
    call_foo(MyObject())
  File "/Users/sidharthkapur/protocol_test.py", line 11, in call_foo
    obj.foo(a=1)
TypeError: MyObject.foo() got an unexpected keyword argument 'a'

Pitch

If we checked this in mypy, we could prevent runtime errors like the one above.

[sterliakov]

This seems to be a deliberate decision: only __call__ arg names are checked, other are explicitly ignored. But that dates to 2018, perhaps it's fine to get stricter now?

[erictraut]

FWIW, pyright has enforced this for many years. Plus, this is required by the Python typing spec and its conformance tests.

[A5rocks]

This seems to be a deliberate decision: only __call__ arg names are checked, other are explicitly ignored. But that dates to 2018, perhaps it's fine to get stricter now?

Maybe we could implement it and see fallout in mypy primer.