FastAPI dependencies and websocket

[gjanezic]

First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google "How to X in FastAPI" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Commit to Help

• I commit to help with one of those options 👆

Example Code

from fastapi import FastAPI, WebSocket, Request, Depends

async def my_context_dependency(request: Request):
    # Do something with request...
    pass

app = FastAPI(dependencies=[Depends(my_context_dependency)])

@app.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
    pass

Description

% websocat ws://127.0.0.1:8000/ws
websocat: WebSocketError: WebSocketError: Received unexpected status code (500 Internal Server Error)

Error:
TypeError: my_context_dependency() missing 1 required positional argument: 'request'

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.100.0

Python Version

3.11.4

Additional Context

No response

[gjanezic]

def my_dependency(request: Request | WebSocket): as suggested in #8503 does not work (anymore?)

fastapi.exceptions.FastAPIError: Invalid args for response field! Hint: check that starlette.requests.Request | starlette.websockets.WebSocket is a valid Pydantic field type. If you are using a return type annotation that is not a valid Pydantic field (e.g. Union[Response, dict, None]) you can disable generating the response model from the type annotation with the path operation decorator parameter response_model=None. Read more: https://fastapi.tiangolo.com/tutorial/response-model/

[monometa]

I think I asked the same question recently - intility/fastapi-azure-auth#155. Perhaps someone can look at it?

[YuriiMotov]

FastAPI relies on type annotations to inject instances of Response or WebSocket.
Since WebSocket is not a subclass of Response and there is no Request in websocket paths, it fails to inject it.

To fix this you can change the type annotation to HTTPConnection that is a common ancestor of Response and WebSocket:

async def my_context_dependency(request: HTTPConnection):
    if isinstance(request, Request):
        # Do something with request...
        pass