Mocking __init__() for unittesting

Instead of mocking, you could simply subclass the database class and test against that:

class TestingDatabaseThing(DatabaseThing):
     def __init__(self, connection):
          self.connection = connection

and instantiate that class instead of DatabaseThing for your tests. The methods are still the same, the behaviour will still be the same, but now all methods using self.connection use your test-supplied connection instead.