Reduce boilerplate for setting defaults on a nested model

[antdking]

There are some limitations, and potentially unnecessary boilerplate, when setting default values on child models.

Given the models:

from pydantic import BaseModel


class Address(BaseModel):
  city: str
  country: str


class Person(BaseModel):
  name: str
  address: Address

We can already override the child values with .build(...):

>>> ModelFactory.create_factory(Person).build(address={"city": "Paris", "country": "France"})
Person(name='rjneZiujBVyPFCnAOkrq', address=Address(city='Paris', country='France'))

For a PersonFactory to set a default country on their address, Use(...) is used in tests and the docs:

from polyfactory.factories.pydantic_factory import ModelFactory
from polyfactory import Use

class PersonFactory(ModelFactory[Person]):
  address = Use(ModelFactory.create_factory(Address).build, country="England")

The first limitation here is that this prevents you from specifying values through the .build call:

>>> person = PersonFactory.build(address={"city": "Paris", "country": "France"})
>>> person.address
Address(city='tFwrUHwNvWypjRWMSySF', country='England')

The workaround here is to use PostGenerated, example included for other people's benefit:

from polyfactory import PostGenerated


def use_post(fn, *args, **kwargs):
  def inner(name: str, values: dict, *args, **kwargs):
    build_kwargs = values.get(name, {})
    # TODO: merge args + kwargs properly with `inspect.signature`
    return fn(*args, **(kwargs | build_kwargs))
  return PostGenerated(inner, *args, **kwargs)

class PersonFactory(ModelFactory[Person]):
  address = use_post(ModelFactory.create_factory(Address).build, country="England")


>>> PersonFactory.build(address={"city": "Paris", "country": "France"})
Person(name='npmmgyjKwSlzfwBxiYxv', address=Address(city='Paris', country='France'))

This seems like a fair amount of boilerplate just to be able to set default values (or potentially more constrainted Fakers) on a child factory.

What could be done to simplify this usecase?

Perhaps something like this?:

class PersonFactory(ModelFactory[Person]):
  address = Defaults(country="England")

[adhtruong]

Does using factory as field work for this use case?

[antdking]

Does this allow setting a default field from the parent class?

[adhtruong]

This could be inlined with create_factory. This does duplicate the type a bit

class MyFactory(ModelFactory[Person]):
      address = ModelFactory.create_factory(Address, country="England")

PRs welcome to add this functionality. Probably generalised as Subfactory to allow other settings too

[antdking]

this does work, yes, thank you.
Additionally it works if you already have a factory for the child model:

class AddressFactory(ModelFactory[Address]):
  ...

class PersonFactory(ModelFactory[Person]):
  address = AddressFactory.create_factory(country="England")