How to use FastAPI Depends for endpoint/route in separate file?

Question

I have an Websocket endpoint defined in separate file, like:

from starlette.endpoints import WebSocketEndpoint
from connection_service import ConnectionService


class WSEndpoint(WebSocketEndpoint):
    """Handles Websocket connections"""

    async def on_connect(self,
            websocket: WebSocket,
            connectionService: ConnectionService = Depends(ConnectionService)):
        """Handles new connection"""
        self.connectionService = connectionService
        ...

and in the main.py I register endpoint as:

from fastapi import FastAPI
from starlette.routing import WebSocketRoute
from ws_endpoint import WSEndpoint

app = FastAPI(routes=[ WebSocketRoute("/ws", WSEndpoint) ])

But Depends for my endpoint is never resolved. Is there a way to make it work?

Plus, what is even the purpose of this mechanism in FastAPI? Cannot we just use local/global variables?

Answer 1

TL;DR

The documents seem to hint that you can only use Depends for request functions.

Explanation

I found a related issue #2057 in the FastAPI repo and it seems the Depends(...) only works with the requests and not anything else.

I confirmed this by,

from fastapi import Depends, FastAPI

app = FastAPI()


async def foo_func():
    return "This is from foo"


async def test_depends(foo: str = Depends(foo_func)):
    return foo


@app.get("/")
async def read_items():
    depends_result = await test_depends()
    return depends_result

In this case, the dependency didn't get resolved.

---

Coming to your case, you can resolve the dependency something like this,

from starlette.endpoints import WebSocketEndpoint
from connection_service import ConnectionService


class WSEndpoint(WebSocketEndpoint):
    async def on_connect(
            self,
            websocket: WebSocket,
            connectionService=None
    ):
        if connectionService is None:
            connectionService = ConnectionService()  # calling the depend function

        self.connectionService = connectionService