request body get empty when using middleware to save request to contextvar

[LoadingZhang]

I want to save request to ContextVar so I can use it anywhere. Here is code sample:

from contextvars import ContextVar

import uvicorn
from fastapi import FastAPI
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.requests import Request

ctx: ContextVar[Request] = ContextVar("request")
app = FastAPI()


class RequestContextMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        token = ctx.set(request)
        try:
            response = await call_next(request)
        finally:
            ctx.reset(token)
        return response

app.add_middleware(RequestContextMiddleware)

async def root(request: Request, body: str):
    print(1, request._stream_consumed)
    print(2, request._body)
    ctx_request = ctx.get()
    print(3, ctx_request._body)


app.add_api_route("/", root, methods=["POST"])

if __name__ == '__main__':
    uvicorn.run(app)

Output:

1 True
2 "the request data"
...
    print(ctx_request._body)
          ^^^^^^^^^^^^^^^^^
AttributeError: '_CachedRequest' object has no attribute '_body'. Did you mean: 'body'?

so, my question is

• Why middleware will read body?
• Why the ctx_request._body get empty, but the request._body in param is worked

[adriangb]

I'm not sure I understood your question / problem but _body is a private attribute that you should not be accessing or interacting with. Maybe try await request.body()?

[LoadingZhang]

I'm not sure I understood your question / problem but _body is a private attribute that you should not be accessing or interacting with. Maybe try await request.body()?

@adriangb I used await request.body() before, but it raised an error:

raise RuntimeError("Stream consumed")
RuntimeError: Stream consumed

After investigation, I found out that _body was missing and _stream_consumed is True after middleware processed.

[LoadingZhang]

And I discovered similar topic in the discussion, which might be related.
#2556