SubFactory with a None value by default

[aleehedl]

The problem

Occasionally I want to have a SubFactory (or a RelatedFactory) whose value is set to None unless explicitly stated. That is, I would want to FooFactory().bar to be None but still be able to use FooFactory(bar__name='test) or FooFactory(bar=BarFactory()).

class BarFactory(DjangoModelFactory):
    name = factory.Faker("first_name")

class FooFactory(DjangoModelFactory):
    bar = factory.SubFactory(BarFactory)

Is this achievable somehow? Obviously I could use FooFactory(bar=None) but this is not ideal.

[rbarrois]

It's quite hard to support directly: in the current engine, declarations will be grouped by their "root", e.g all declarations starting with bar__ are considered to be parameters for the bar= declaration.

If the default is bar=None, any value provided to bar__name would either cause a failure or be ignored.

The simplest option would be to use a factory.Trait:

class FooFactory(factory.Factory):
  class Params:
    with_bar = factory.Trait(bar=factory.SubFactory(BarFactory))

With that, you could use:

>>> FooFactory()  # Nothing requested
<Foo, bar=None>
>>> FooFactory(with_bar=True)  # Default call to `BarFactory`
<Foo, bar=<Bar, name="John">>
>>> FooFactory(with_bar=True, bar__name="test")  # Include `bar=`, and provide a value
<Foo, bar=<Bar, name="test">>
>>> FooFactory(bar__name="test")  # Slightly weird behaviour: the `BarFactory` is fully ignored.
<Foo, bar=None>

I hope this helps?

[aleehedl]

Thanks for the reply. Traits will have to suffice :-)