Chained Factories pattern

This example demonstrates “Chained Factories” pattern.

The idea of the pattern is in wrapping Factory into another Factory that adds additional arguments.

base_factory = providers.Factory(
    SomeClass,
    base_argument=1,
)

concrete_factory = providers.Factory(
    base_factory,
    extra_argument=2,
)


if __name__ == '__main__':
    instance = concrete_factory()
    # Same as: # instance = SomeClass(base_argument=1, extra_argument=2)

Sample code

Listing of the pattern example:

"""`Chained Factories` pattern."""

from dependency_injector import containers, providers


class SqlAlchemyDatabaseService:

    def __init__(self, session, base_class):
        self.session = session
        self.base_class = base_class


class TokensService:

    def __init__(self, id_generator, database):
        self.id_generator = id_generator
        self.database = database


class Token:
    ...


class UsersService:

    def __init__(self, id_generator, database):
        self.id_generator = id_generator
        self.database = database


class User:
    ...


# Sample objects
session = object()
id_generator = object()


class Container(containers.DeclarativeContainer):

    database = providers.Factory(
        SqlAlchemyDatabaseService,
        session=session,
    )

    token_service = providers.Factory(
        TokensService,
        id_generator=id_generator,
        database=providers.Factory(
            database,
            base_class=Token,
        ),
    )

    user_service = providers.Factory(
        UsersService,
        id_generator=id_generator,
        database=providers.Factory(
            database,
            base_class=User,
        ),
    )


if __name__ == '__main__':
    container = Container()

    token_service = container.token_service()
    assert token_service.database.base_class is Token

    user_service = container.user_service()
    assert user_service.database.base_class is User

Arguments priority

Passing of the arguments works the same way like for any other Factory provider.

# 1. Keyword arguments of upper level factory are added to lower level factory
chained_dict_factory = providers.Factory(
    providers.Factory(dict, arg1=1),
    arg2=2,
)
print(chained_dict_factory())  # prints: {'arg1': 1, 'arg2': 2}

# 2. Keyword arguments of upper level factory have priority
chained_dict_factory = providers.Factory(
    providers.Factory(dict, arg1=1),
    arg1=2,
)
print(chained_dict_factory())  # prints: {'arg1': 2}

# 3. Keyword arguments provided from context have the most priority
chained_dict_factory = providers.Factory(
    providers.Factory(dict, arg1=1),
    arg1=2,
)
print(chained_dict_factory(arg1=3))  # prints: {'arg1': 3}

Credits

The “Chained Factories” pattern was suggested by the Dependency Injector users.