monkey patch not working properly

So I'm running py.test and trying to use monkeypatch. I understand that monkeypatch's intended purpose is to replace attributes in a module so that they can be tested. And I get that we can substitute in mock functions in order to do this.

Currently I am trying to run essentially the following block of code.

from src.module.submodule import *

def mockIfunction(parameter = None):
    return 0

def test_function_works(monkeypatch):
    monkeypatch.setattr("src.module.submodule.function",mock_function ]
   assert function(parameter = None) == 0

When the test runs, instead of swapping in mock_function, it just runs function . Could there be a reason why monkeypatch isn't activating

I have got monkey patch running succesfully with other code before. So I don't see why this isn't working.