Mocking modules

my_module.py

import sublime

def better_os():
    x = sublime.get_os()
    if x:
        return x.upper()
    else:
        return None

tests.py

import sys
from mock import patch

class FakeSublime():
    pass

sys.modules['sublime'] = FakeSublime
import sublime
import my_module

def test_stuff():
    with patch('sublime.get_os', new=lambda: 'windows', create=True):
        assert my_module.better_os() == 'WINDOWS'
    with patch('sublime.get_os', new=lambda: False, create=True):
        assert my_module.better_os() is None