Python testing: How do I mock outside of the testing scope?

Question

Currently dealing with some legacy python code. I need to test one of the functions, function foo . Now to do that I need to mock the return value given by another function that's called by foo which is function bar.

However both of these are outside of my testing package and when I try to mock out bar, foo is unaffected. Is there a way of doing this without moving everything into a class? (which is preferred, but it's legacy code).

Answer 1

A simple example:

foo_and_bar_package/foo_and_bar_module.py

def bar():
    return 5


def foo():
    return 2 * bar()

test_foo_and_bar_module.py

from unittest import TestCase
from unittest.mock import patch

from foo_and_bar_package.foo_and_bar_module import foo


class TestFoo(TestCase):
    @patch('foo_and_bar_package.foo_and_bar_module.bar')
    def test_foo(self, mock_bar):
        mock_bar.return_value = 2
        self.assertEqual(foo.foo(), 2 * mock_bar.return_value)