Mock a function which is called when a module is imported

I want to test a module A which uses decorators with arguments. The arguments get evaluated when the module A is loaded. For some of the decorator args, I set the value by calling a function foo in module B.

# A.py
import B

@deco(arg1=B.foo())
def bar():
  ...

When I want to test A, I want to mock B.foo so that the decorator argument is set for my test cases. I think that B.foo must be mocked before A loads B.

In the unit test, as a caller of A, how do I mock B.foo to ensure the mocked version is used when evaluating the decorator arguments in A?

If you want to ensure that the mock is really used, you have to reload module A after patching foo, as bar already had been evaluated with the original foo. You could wrap that in a fixture like this (untested):

import importlib
import pytest
from unittest import mock
import A

@pytest.fixture
def mocked_foo():
    with mock.patch('B.foo') as mocked:
        importlib.reload(A)
        yield mocked

def test_bar(mocked_foo):
    ...