You never re-assign `saved_sign` back to `requests.auth.Client.sign` at the end of this test, which means that other tests that run after this test are going to get this mocked version of the function. The [`mock`](http://www.voidspace.org.uk/python/mock/) library already has utilities designed for this purpose. You want to do something like:

```python
def setUp(self):
    def mocked_sign(self, *args, *kwargs):
        # function definition goes here

    patcher = mock.patch.object(requests.auth.Client, "sign", mocked_sign)
    patcher.start()
    self.addCleanup(patcher.stop)
```

The [`addCleanup` function](http://docs.python.org/2/library/unittest#unittest.TestCase.addCleanup) ensures that its argument gets called even if the test raises an exception, and [`patcher.stop()` undoes the patch](http://www.voidspace.org.uk/python/mock/patch.html#patch-methods-start-and-stop).