Dependency defined on startup event

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Example Code

@app.on_event("startup")
async def startup_event():
    something = await func()
    myobj = MyObj(something)

###

@router.get("")
async def index():
  # need myobj instance here

Description

I need to execute some async initialization on startup creating an instance of myobj, and then I need this instance as a dependency in routers endpoint.

Operating System

Windows

Operating System Details

No response

FastAPI Version

0.68.1

Python Version

3.8.8

Additional Context

I need to create an instance of a “master” object asynchronously during the startup of FastApi, and then to get this exact instance in routers endpoint, but I cannot figure out how to pass it as a dependency.

(I’m aware that if I have multiple workers each worker will get a different “master” object but that is ok, the main point here is that within a single worker an instance is created after async operation on startup, and then reused in routers endpoint.)

[adriangb]

Another alternative is to store it in the app state:

from fastapi import FastAPI, Depends, Request


class MyObj:
    def __init__(self, something: int) -> None:
        self.something = something


async def on_startup() -> None:
    app.state.myobj = MyObj(1)


app = FastAPI(on_startup=[on_startup])


def get_myobj(request: Request) -> MyObj:
    return request.app.state.myobj


@app.get("/")
async def index(myobj: MyObj = Depends(get_myobj)) -> None:
    assert isinstance(myobj, MyObj)
    assert myobj.something == 1


from fastapi.testclient import TestClient


with TestClient(app) as client:
    resp = client.get("/")
    assert resp.status_code == 200

Or, more in line with traditional dependency injection if you imagine that dependency_overrides is a “container”:

from fastapi import FastAPI, Depends


class MyObj:
    def __init__(self, something: int) -> None:
        self.something = something


async def on_startup() -> None:
    myobj = MyObj(1)
    app.dependency_overrides[MyObj] = lambda: myobj


app = FastAPI(on_startup=[on_startup])


@app.get("/")
async def index(myobj: MyObj = Depends()) -> None:
    assert isinstance(myobj, MyObj)
    assert myobj.something == 1


from fastapi.testclient import TestClient


with TestClient(app) as client:
    resp = client.get("/")
    assert resp.status_code == 200

If this was a more realistic situation, I’d hope this looked like:

from fastapi import FastAPI, Depends


class MyObj:
    def __init__(self, something: int) -> None:
        self.something = something


async def on_startup(container: Container) -> None:
    # get the container or app injected, no need for a reference
    container.bind(MyObj, value=MyObj(1))   # or something like this


app = FastAPI(on_startup=[on_startup])


@app.get("/")
async def index(myobj: MyObj = Depends()) -> None:
    ...

[desilinguist]

@adriangb thank you so much for the idea about using dependency overrides. It really helped me out!