How to mock a method used in method call

I would like to write a unit test for a function which uses os.path methods like listdir, os.path.join, etc.

# file my_function.py
import os
my_function():
    //do something with os, e.g call os.listdir
    return os.listdir(os.getcwd())

# file test_my_function.py
test_my_function():
    os = MagickMock()
    os.listdir = MagickMock(side_effect=["a","b"])
    self.assertEqual("a",my_function())

I assume os does it's job and want to restrict my unittest to test only my code and mock therefore the return values of the os method calls. Is this possible?