foo.py

from os import makedirs

from unittest import TestCase

def foo(_default_folder='foo', _os_makedirs=makedirs):
    """
    The purpose of this function is to create a folder called 'foo'
    on the file system.
    """
    try:
        _os_makedirs(_default_folder)
    except EnvironmentError as err:
        return 1

class Test_foo(TestCase):

    def _callFUT(*args, **kwargs):
        return foo(*args, **kwargs)

    def test_foo():
        
        test_folder = 'test_folder'
        
        dummy_dir_entry = []
        def dummy_os_makedirs(path):
            dummy_dir_entry = path
        
        desired_created_folder = test_folder
        
        self._callFUT(_default_folder = test_folder, _os_makedirs = dummy_os_makedirs)
        self.assertEqual(dummy_dir_entry, desired_created_folder)