Fun times with unittest.mock

In foo.py:

from qux.bar import Baz

def some_fn(arg=Baz):
    # Something involving arg()
    pass

In test_foo.py (running under pytest, with pytest-mock):

from .foo import some_fn

def test_some_fn(mocker):
    mocker.patch('foo.Baz')
    # Oh no, tests don't mock it out right!

It took me a tick to realize that the function definition was being evaluated at import time, including the values for the kwargs. Even though the body, of course, wasn’t being evaluated. I solved it with this change to foo.py:

from qux.bar import Baz

def some_fn(arg=None):
    if arg is None:
        arg = Baz
    # Something involving arg()
    pass

This is related to the thing where you never write a function signature with {} or [] in it, because you’ll create an instance at evaluation time that’s shared among all the calls to that function.