Use introspection for improved ergonomics

[rbarrois]

The problem

If a developer wants to use various helpers in a given declaration (for instance, having a unique company name), they have to go through unnecessary complicated hoops:

class ContractFactory(factory.Factory):
    class Meta:
        model = Contract

    class Params:
        company_base = factory.Faker('company')
    company = factory.LazyAttributeSequence(lambda o, n: "{c} {n}".format(c=o.company, n=n))

There is no way to pass the result of a factory.Faker() call to a factory.Sequence declaration, for instance.

Proposed solution

One option would be to add an extra declaration, for instance:

company = factory.Transform(
    lambda c, n: "{c} {n}".format(c=o.company, n=n),
    c=factory.Faker('company'),
    n=factory.Sequence(str),
)

However, this requires us to add more and more building blocks as user needs evolve.

Another option would be to play with method introspection: if the default values of a function's arguments are factory.Declaration subclasses, we could automatically detect it:

def company(self, base=factory.Faker('company'), n=factory.Sequence()):
    return "{base} {n}".format(base=base, n=n)

Summary

Pros:

• Simpler API
• Avoids the need for more and more declarations for advanced use cases

Cons:

• Somewhat magic: nothing explains why those attributes are detected
• Doesn't work with complex structures (def foo(x=SomeClass(y=factory.Faker('name'))))

Alternative

A possible compromise would be to only inspect the parameters of factory.Declaration subclasses:

@factory.lazy_attribute
def company(self, base=factory.Faker('company'), n=factory.Sequence()):
    return "{base} {n}".format(base=base, n=n)

Here, the reader would have an idea that something specific is happening thanks to the @lazy_attribute decorator.

[timorthi]

Chiming in here from #767 . From an API perspective, the alternative proposal looks the most intuitive to me. Anectodally, I expected RelatedFactoryList(SomeFactory, size=factory.SelfAttribute('foo')) to work because the base object had already been instantiated by the time RelatedFactoryList is called.