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AsyncCreatorError

Symptom

modern_di.exceptions.registration.AsyncCreatorError: Factory creator create_http_client is async, but resolution is synchronous, so resolve() would return an un-awaited coroutine or async generator. Build the resource in your framework's lifespan and hand it to the container with set_context() and a ContextProvider: https://modern-di.modern-python.org/recipes/async-lifespan/
See: https://modern-di.modern-python.org/troubleshooting/async-creator-error/

The Factory(...) call raises it before any container exists, with or without skip_creator_parsing=True. The exception carries .creator.

Cause

The creator passed to Factory returns a coroutine or an async generator when called. modern-di resolves synchronously, so it can never await or iterate it, and resolve() would hand back that object instead of the resource.

The check catches three shapes, each with or without yield in the async def:

  • An async def function or method.
  • A functools.partial around an async def function.
  • An instance of a class whose __call__ is async def.

A class is accepted even when its __call__ is async, because calling the class builds an instance synchronously. A sync function that returns an awaitable is also accepted, since nothing short of calling it shows what it returns.

Fix

Await the resource outside the container, in the framework's lifespan, and register the live object with container.set_context() behind a ContextProvider. The async lifespan recipe shows the full pattern with FastAPI.

# Broken:
providers.Factory(create_http_client, scope=Scope.APP)

# Works:
class Dependencies(Group):
    http_client = providers.ContextProvider(HttpClient, scope=Scope.APP)


container = Container(groups=[Dependencies])
container.set_context(HttpClient, await create_http_client())
assert isinstance(container.resolve(HttpClient), HttpClient)

If the client builds without an await and only its cleanup is async, keep a sync creator and give it an async finalizer instead; see When a sync creator works instead.

See also