Conditional kwargs not working for nested factories

[fdegiuli]

Description

This seems to be a continuation of #466

It seems like defining a trait will let you pass kwargs, but only one level deep.
Here is an example:

To Reproduce

import factory


class Thing(object):
    def __init__(self, nested=None):
        self.nested = nested or []


class ThingFactory(factory.Factory):
    class Meta:
        model = Thing

    class Params:
        with_nested = factory.Trait(
            nested__count=1,
            nested__nested__count=1
        )

    @factory.post_generation
    def nested(obj, create, extracted, count=0, **kwargs):
        if not create:
            return

        if count > 0:
            obj.nested = ThingFactory.create_batch(count, **kwargs)


if __name__ == '__main__':
    value = ThingFactory()

This (naturally) fails:

Traceback (most recent call last):
  File "test.py", line 29, in <module>
    value = ThingFactory()
  File "/usr/local/lib/python3.6/site-packages/factory/base.py", line 46, in __call__
    return cls.create(**kwargs)
  File "/usr/local/lib/python3.6/site-packages/factory/base.py", line 563, in create
    return cls._generate(enums.CREATE_STRATEGY, kwargs)
  File "/usr/local/lib/python3.6/site-packages/factory/base.py", line 500, in _generate
    return step.build()
  File "/usr/local/lib/python3.6/site-packages/factory/builder.py", line 288, in build
    context=declaration.context,
  File "/usr/local/lib/python3.6/site-packages/factory/declarations.py", line 41, in unroll_context
    return step.recurse(subfactory, context, force_sequence=step.sequence)
  File "/usr/local/lib/python3.6/site-packages/factory/builder.py", line 233, in recurse
    return builder.build(parent_step=self, force_sequence=force_sequence)
  File "/usr/local/lib/python3.6/site-packages/factory/builder.py", line 257, in build
    base_post=self.factory_meta.post_declarations,
  File "/usr/local/lib/python3.6/site-packages/factory/builder.py", line 198, in parse_declarations
    for k, v in extra_maybenonpost.items()
  File "/usr/local/lib/python3.6/site-packages/factory/builder.py", line 92, in update
    sorted(self.declarations),
factory.errors.InvalidDeclarationError: Received deep context for unknown fields: 
{'nested__count': Maybe(<SelfAttribute('with_nested', default=False)>, yes=1, no=<factory.declarations.Skip object at 0x1068cdf28>)} 
(known=['count'])

It seems like the second param isn't being extracted properly. I don't understand the code well enough to diagnose exactly why this happens, so I won't put any half-baked theories in here.

The issue

I think this issue is a symptom of a deeper problem: there's no standard way to override the arguments passed to a post_generation hook. (I don't know if this is also the case for subfactories.)
Doing it in a Trait seems a little dirty, since (at least according to the docs) it's meant to override fields.
It seems like it could belong in _alter_kwargs(), but it that method doesn't actually alter the raw kwargs.
If it's meant to stay in a Trait, updating the docs to reflect that would be very helpful.

[fdegiuli]

Possibly related to #550 as well.

[roman-karpovich]

same issue for me. after migration from 2.8.0 to 2.11.1 all traits seems broken. i widely used deep arguments in code to provide additional context in dependency of object status.

for example

    class Params:
            reported = Trait(
                report_attachments__count=1,
                report_attachments__file_type__name='report',
            )

Does any way exists to refactor code without bloody mess?