Make _setup_next_sequence usable with DictFactory subclasses

[danjer]

Add new option class for DictFactory that checks for the implementation of _setup_next_sequence to determine counter reference.

Fixes #993

[francoisfreitag]

Hi! Thanks for taking the time to contribute a PR, with tests. 😁

Sadly, the suggested approach introduces a difference in behavior between DictFactory and the other factories.

DictFactory sets a model in its options, which results in all DictFactory subclasses sharing the same base factory (and reference_counter): DictFactory. Hence, only the _setup_next_sequence() from DictFactory is executed, and it returns 0. That is undesirable, but that’s the current behavior.

With the suggested patch, each factory in the dict hierarchy can reset its sequence, so you can make a subclass of Dog and have it define a _setup_next_sequence() to start from 0. Whereas, for other factories, all subclasses use the same sequence as the top-most factory that defines a model in the inheritance. To illustrate, the following test does not pass:

    def test_sequence_resets_on_subclass(self):
        class Base42Factory(factory.Factory):
            class Meta:
                model = TestObject

            @classmethod
            def _setup_next_sequence(cls):
                return 42

            one = factory.Sequence(lambda n: f'{n}')

        class Base9000Factory(Base42Factory):
            @classmethod
            def _setup_next_sequence(cls):
                return 9000

        base9000 = Base9000Factory.build()
        self.assertEqual('9000', base9000.one)  # Is '42'.

        base42 = Base42Factory.build()
        self.assertEqual('42', base42.one)

Possible ideas:

1. follow the existing practice, maybe special-casing the DictFactory so that the reference_counter points to the top-most factory beneath DictFactory in the inheritance.
2. allow resetting sequences for all factories.

I think the second would make for a nicer behavior, but that requires heavier changes to how the library works (and updating the docs to clearly define the behavior).

[francoisfreitag]

Not sure why this list is necessary? Why not directly assert the cat_result['pet_id'] and dog_result['pet_id'] ?