Dependency Injection problem with FastAPI on Python

Good day! Please tell me how you can solve the following problem in Python + FastAPI.

There is a test project:

app / main.py - main file
app / routes / users.py -set of api methods
app / repos / factory.py - repository factory
app / repos / user_repository.py - repositories
app / handlers / factory.py - handler factory
app / handlers / users.py - handlers
app / domain / user.py - data class

The main and routes structure is the same as in the example https://fastapi.tiangolo.com/tutorial/bigger-applications/

In the routes/users.py file:

from fastapi import APIRouter, Depends
from ..handlers import factory

router = APIRouter()

@router.get("/users/", tags=["users"])
def read_users(handler=Depends(factory.get_handler)):
    return handler.get_all()

In the handlers/factory.py:

from fastapi import Depends
from .users import UserHandler1

def get_handler(handler=Depends(UserHandler1)):
    return handler

In the handlers/users.py:

from fastapi import Depends
from ..repos import factory

class UserHandler1:
    def __init__(self):
        pass

    def get_all(self, repo=Depends(factory.get_repo)):
        return repo.get_all()

repos/factory.py:

from fastapi import Depends
from ..repos.user_repository import UserRepository

def get_repo(repo=Depends(UserRepository)):
    return repo

repos/user_repository.py:

from ..domain.user import User

class UserRepository:
    def __init__(self):
        pass

    def get_all(self):
        return [User(1, 'A'), User(2, 'B'), User(3, 'C')]

domain/user.py:

class User:
    id: int
    name: str

    def __init__(self, id, name):
        self.id = id
        self.name = name

Then I run hypercorn server: app.main:app --reload Try call api method: http://127.0.0.1:8000/users/ And get the error AttributeError: 'Depends' object has no attribute 'get_all'

If you remove the handlers layer and do this, then everything will work.

routes/users.py:

from fastapi import APIRouter, Depends
from ..repos import factory

router = APIRouter()

@router.get("/users/", tags=["users"])
def read_users(repo=Depends(factory.get_repo)):
    return repo.get_all()
It also works if you completely remove all Depends and create
UserRepository and UserHandler1 directly in factories.

Question 1: How do I use "Depends" in this case and why doesn't it work?

In general, the factory does not look like a good solution to this problem. I saw an example of DI implementation using multiple inheritance but as for me it is the same as factory method. I also tried to use the Pinject library, but it requires the initial construction of a graph, which needs to be saved somewhere in order to access it in api handlers.

Question 2 (more important): How Dependency Injection can be applied in this case ?

As noted in the comments, a dependency can be anything that is a callable and thus a class as well. The only caveat in the latter case is that the class will only be initialized (i.e. only the __init__(...) function will be called).

So, in order to have a class as dependency, as in the example of https://fastapi.tiangolo.com/tutorial/dependencies/classes-as-dependencies/#shortcut you just need to call the target functions within the init and set the values as attributes of the class.

from ..domain.user import User

class UserRepository:
    def __init__(self):
        self.get_all()

    def get_all(self):
        self.users = [User(1, 'A'), User(2, 'B'), User(3, 'C')]


from fastapi import Depends
from ..repos.user_repository import UserRepository

def get_repo(repo=Depends(UserRepository)):
    print(repo.users) # This will print the list of users
    return repo

QUESTION 2

NB

This is a modelling question. Here I propose what I believe is suitable from my point of view. It does not necessarily have to be best or simplest approach.

Answering your second question, I would not advice for such complex dependencies. If the dependencies are at the router level, you can simply add them to the router, using the parameter depends=[...] and providing a list of dependency classes/functions.

Alternatively, you could declare all of the dependencies as function parameters of the endpoint, as you did for the factory. This method may lead to big chunks of code getting copied and pasted, so I advise for the above approach.

If you need to process the data parameters, then you add them to the request and access them from within the endpoint. See FastAPI get user ID from API key for a minimal example.

The __call__ method must be implemented in the class.