def test_child_command_rolls_back_self_on_exception(self):
# Assemble command chain with mocked __do__ functions
# Initial command triggered will throw exception
c1 = Command()
c2 = Command(previous=c1)
c2.__do__ = Mock(side_effect=Exception('Oh No!'))
c2.__undo__ = Mock()
# Action
c1.run()
# Assert command 2 was rolled back
c2.__undo__.assert_called_once()
