Mock allows you to replace parts of your system under test with mock objects and make assertions about how they have been used.
Mock provides a core 'MagicMock' class removing the need to create a host of stubs throughout your test suite. After performing an action, you can make assertions about which methods / attributes were used and arguments they were called with. You can also specify return values and set needed attributes in the normal way.
The mock module also provides utility functions / objects to assist with testing, particularly monkey patching.