Recording and asserting an empty mock list in python unittest

Question

I'm using unittest and mock (Michael Foord's module) to test some python code.

I have something like this (this is a proof of concept that could be rewritten more cleanly, but my real code needs to behave like that foo function):

import unittest
from mock import patch

def foo():
    my_list = []

class Test(unittest.TestCase):
    def test_foo(self):
        with patch('__main__.my_list', new=[], create=True) as baz:
            baz.extend(['foo', 'bar'])

            self.assertEqual(foo(), None)
            self.assertListEqual([], baz)

if __name__ == '__main__':
    unittest.main()

So the problem is that my baz mock object doesn't change accordingly after the foo() call and the last assertion fails.

If I use my_list.remove(x) in foo() then I can see the changes in my test case, but I just want to empty that list, I don't want to pass through every element of the list then remove it, no, I want a fast empty operation.

How can I check if my mock object is emptied without using .remove(x) , but using the current implementation of function foo ?

Answer 1

So, I end up answering my own question...

The solution was to use my_list[:] = [] in foo .

But I also realized that passing create=True is bad because if that list doesn't exist (exactly the case of this POC) it will be created and I'll possibly test broken code that passes the tests.