Type hint for Callable that takes kwargs

Question

I want to do something like

from typing import Callable


def a(foo: Callable[[int], None]):
    foo(b=5)

This code works, but gives a warning Unexpected argument.

Defining as

def a(foo: Callable[[int], None]):
    foo(5)

works with no warnings as expected.

---

How can I pass in an expected argument as a kwarg into a function with the type checker not being angry at me?

Answer 1

You can use a "callable protocol" here, so:

import typing

class MyCallableType(typing.Protocol):
    def __call__(self, bar:int) -> None:
        ...

def a(foo: MyCallableType):
    foo(32)
    foo(bar=32)

Now, testing the above with mypy:

jarrivillaga$ mypy --version
mypy 0.910
jarrivillaga$ mypy test.py
Success: no issues found in 1 source file

Note, this allows mypy to catch all kinds of errors, e.g. a function with the wrong argument name, or if we want b to be a function that specifies a keyword-only bar argument:

import typing

class MyCallableType(typing.Protocol):
    def __call__(self, b:int) -> None:
        ...

def a(foo: MyCallableType):
    foo(32)
    foo(b=32)

def bar(b: int) -> None:
    pass

def baz(*, b: int) -> None:
    pass

def bing(x: int) -> None:
    pass

a(bar)
a(baz)
a(bing)

And mypy will complain with the following:

jarrivillaga$ mypy test.py
test.py:21: error: Argument 1 to "a" has incompatible type "Callable[[NamedArg(int, 'b')], None]"; expected "MyCallableType"
test.py:22: error: Argument 1 to "a" has incompatible type "Callable[[int], None]"; expected "MyCallableType"
Found 2 errors in 1 file (checked 1 source file)