How to handle circular dependency [non-string keys for Dict provider]

[ijalalfrz]

so i have handler class to be injected by container to get repositry object

service/handler.py

from ... import Container
from dependency_injector.wiring import inject, Provide
from .rating_repository import RatingRepository
from ..domain.commands import SaveRating
from ..domain.models import RunnerRating


class CommandHandler:
    # Dep Injector
    @inject
    def __init__(self, rating_repo: RatingRepository =
                 Provide[Container.repo.rating]):
        self.rating_repo = rating_repo

this handler is included in messagebus module like this

service/messagebus.py

from .handler import CommandHandler

Message = Union[Command, Event]

logger = logging.getLogger(__name__)
command_handler = CommandHandler()


COMMAND_HANDLERS = {
    SaveRating: command_handler.save_rating,
}

class MessageBus:

    def __init__(self):
        pass

    # rest methods...

but i also add MessageBus class on container that make me include service/messagebus.py to container.py

container.py

from .runner_rating.service import messagebus

'''
    Dependency injector for application
'''


class MessageBusContainer(containers.DeclarativeContainer):

    bus = providers.Container(
        messagebus.MessageBus
    )

when i run the app, it shows ImportError on container.py because i imported messagebus and inside it there is a handler object that import container.py

what should i do to solve this? because message bus will be injected to other module in future

[rmk135]

Hi @ijalalfrz ,

I think that it is a typical circular dependency problem. In your case it seems like the loop is: handler -> container -> messagebus -> handler.

The quick solution is to move CommandHandler in the runtime. Here is some sample code:

File service/messagebus.py:

from .commands import Command, SaveRating, DoSomethingElse


class MessageBus:

    COMMAND_HANDLERS = {}

    def __init__(self):
        from .handler import CommandHandler
        command_handler = CommandHandler()

        self.COMMAND_HANDLERS = {
            SaveRating: command_handler.save_rating,
            DoSomethingElse: command_handler.something_else,
        }

    def handle(self, command: Command):
        self.COMMAND_HANDLERS[command]()

File service/containers.py:

from dependency_injector import containers, providers

from . import repositories, messagebus


class Container(containers.DeclarativeContainer):

    rating_repository = providers.Singleton(repositories.RatingRepository)

    message_bus = providers.Factory(messagebus.MessageBus)

And a sample code to run it:

from .containers import Container
from .commands import SaveRating, DoSomethingElse


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

    message_bus.handle(SaveRating)
    message_bus.handle(DoSomethingElse)

---

The other way

The other way is to pull CommandHandler into the container.

File service/handler.py:

from .repositories import RatingRepository


class CommandHandler:
    def __init__(self, rating_repo: RatingRepository):
        self.rating_repo = rating_repo

    def save_rating(self):
        print('Saving rating')

    def something_else(self):
        print('Doing something else')

File service/messagebus.py:

from typing import Dict, Callable, Any

from .commands import Command


class MessageBus:

    def __init__(self, command_handlers: Dict[str, Callable[..., Any]]):
        self.command_handlers = command_handlers

    def handle(self, command: Command):
        self.command_handlers[command]()

File service/containers.py:

from dependency_injector import containers, providers

from . import repositories, handler, messagebus, commands


class Container(containers.DeclarativeContainer):

    rating_repository = providers.Singleton(repositories.RatingRepository)

    command_handler = providers.Singleton(
        handler.CommandHandler,
        rating_repo=rating_repository,
    )

    message_bus = providers.Factory(
        messagebus.MessageBus,
        command_handlers=providers.Dict({
            commands.SaveRating: command_handler.provided.save_rating,
            commands.DoSomethingElse: command_handler.provided.something_else,
        }),
    )

Sample code for running is the same.

Note: this option can not be implemented cause providers.Dict support only string keys in master. I will make a quick release to fix that.

[rmk135]

I have released version 4.5.0. It supports non-string keys for Dict provider.

I've also included this example in the mini application examples: https://github.com/ets-labs/python-dependency-injector/tree/master/examples/miniapps/commands-and-handlers

[ijalalfrz]

I have released version 4.5.0. It supports non-string keys for Dict provider.

I've also included this example in the mini application examples: https://github.com/ets-labs/python-dependency-injector/tree/master/examples/miniapps/commands-and-handlers

Wow thank you @rmk135 for your fast reply and solution, i'm glad to contribute for this feature in this project 👏

[rmk135]

Thank you for bringing up the issue @ijalalfrz . Appreciate you contribution.

Let me know if you face other issues or need any other help.