Avoid type warnings when mocking objects in unit tests?

Question

Assuming I have a function that takes a complex object and does something with it:

def foo(bar: SomeComplexObject):
    ...

In unit tests bar will be replaced by a mock object, but that of courses now raises type warnings. Should I simply ignore or suppress these or is there a proper way to deal with them (without changing the original function signature of course)?

Update: I've seen now that this is an open issue on mypy, but its been in that state for over two years. Has any consensus emerged on how to work around this?

Answer 1

I'm going to put my 2¢ in and say that you should type-check your testsuite. Its still code and static type checking will help you write better code faster.

That leaves the question of how. Ideally, if your function expects SomeComplexObject then you also pass in an instance thereof. Either by building one in your test fixtures, or by subclassing and overriding what you don't need. The best unit test is the one that operates on proper input.

That still leaves the case where this is impractical or we explicitly want to test how invalid input is handled. In that case just explicitly cast your mock to the type that mypy requires:

from typing import cast

def test_foo():
    mock_bar = cast(SomeComplexObject, MockBar())
    foo(mock_bar)