Dynamically accessing attributes of provided objects.

Hi, I recently needed to access an attribute of a provided object for constructing a new object. A minimal example would look like this

import random

from dependency_injector import containers, providers


class B:
    def __init__(self):
        self.val = random.randint(0, 10)

    def __repr__(self):
        return f"B({self.val})"


class A:
    def __init__(self, b_val: int):
        self.b_val = b_val

    def __repr__(self):
        return f"A({self.b_val})"

naively I tried to do

class MyContainer(containers.DeclarativeContainer):
    b = providers.Singleton(B)
    a = providers.Singleton(A, b.val)


def main() -> None:
    container = MyContainer()
    print(container.b())
    print(container.a())


if __name__ == "__main__":
    main()

which does not work, because of AttributeError: 'dependency_injector.providers.Singleton' object has no attribute 'val'.

I was able to work around this, by using

class MyContainer(containers.DeclarativeContainer):
    b = providers.Singleton(B)
    b_val = providers.Callable(getattr, b, "val")
    a = providers.Singleton(A, b_val)

However, I thought that it would be nice if dependency_injector would provide this behavior out of the box.

My attempts at implementing a provider that has this behavior look like

class AttributeFactory(providers.Provider):

    __slots__ = ('_factory',)

    def __init__(self, *args, **kwargs):
        self._factory = providers.Factory(*args, **kwargs)
        super().__init__()

    def __getattr__(self, item):
        return providers.Callable(getattr, self._factory, item)

class MyContainer(containers.DeclarativeContainer):
    b = AttributeFactory(B)
    a = providers.Singleton(A, b.val)

However, this fails with TypeError: __init__() takes at least 1 positional argument (0 given) because of some copying, that I can not really interpret.

I would be interested in hearing thoughts on this or if there are other ways to achieve this kind of attribute access.

[JarnoRFB]

@rmk135 Glad I could contribute some idea. I never heard about TRIZ, but it sounds like an interesting read!

I have it working. Productizing it will take some time. I would like to get all this well covered with tests.

Great, let me know if I can contribute something. I believe the feature will make dependency injector extremely flexible, lowering the barrier for people adopting it.

[JarnoRFB]

@rmk135 I am amazed by the speed you implemented this. Thanks a lot!