"receive" in middleware doesn't work

[dmitrykaramin]

starlette 0.45.3
fastapi 0.115.8

Using middleware from documentation

class LoggedRequestBodySizeMiddleware:
    def __init__(self, app):
        self.app = app

    async def __call__(self, scope, receive, send):
    if scope["type"] != "http":
        await self.app(scope, receive, send)
        return

    body_size = 0
    async def receive_logging_request_body_size():
        nonlocal body_size

        message = await receive()
        assert message["type"] == "http.request"

        body_size += len(message.get("body", b""))

        if not message.get("more_body", False):
            print(f"Size of request body was: {body_size} bytes")

        return message

    await self.app(scope, receive_logging_request_body_size, send)

doesn't work as "receive" (receive_logging_request_body_size) never called during request.
"receive" called only once on startup FastAPI application.
middleware added also as documented

application.add_middleware(LoggedRequestBodySizeMiddleware)

[adriangb]

Did you send an actual request with data?

from fastapi import FastAPI
from fastapi.middleware import Middleware
from fastapi.testclient import TestClient
from pydantic import BaseModel
from starlette.types import ASGIApp, Message, Receive, Scope, Send


class LoggedRequestBodySizeMiddleware:
    def __init__(self, app: ASGIApp) -> None:
        self.app = app

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        if scope['type'] != 'http':
            await self.app(scope, receive, send)
            return

        body_size = 0

        async def receive_logging_request_body_size() -> Message:
            nonlocal body_size

            message = await receive()
            assert message['type'] == 'http.request'

            body_size += len(message.get('body', b''))

            if not message.get('more_body', False):
                print(f'Size of request body was: {body_size} bytes')

            return message

        await self.app(scope, receive_logging_request_body_size, send)


app = FastAPI(middleware=[Middleware(LoggedRequestBodySizeMiddleware)])


class Salute(BaseModel):
    name: str


class SaluteResponse(BaseModel):
    message: str


@app.post('/echo')
async def upload(salute: Salute) -> SaluteResponse:
    return SaluteResponse(message=f'Hello {salute.name}')


client = TestClient(app)

response = client.post('/echo', json={'name': 'World'})
assert response.status_code == 200, response.text

❯ python test.py
Size of request body was: 17 bytes

[dmitrykaramin]

"Size of request body was: 17 bytes" will be written only if you have request scheme in method argument: salute: Salute, in your example. But if make your method like

@app.get('/echo')
async def upload() -> SaluteResponse:
    return SaluteResponse(message=f'Hello')

receive in middleware wont work.

But according to documentation 'receive' method gives me a point of some manipulation with request - not if only body exists.

[adriangb]

receive only gets called if you read a body

[dmitrykaramin]

My experiments show than presence of scheme in method arguments makes logic execute "receive" method

@app.get('/echo')
async def upload(salute: Salute) -> SaluteResponse:
    return SaluteResponse(message=f'Hello ')

even if there is no data in body

Size of request body was: 0 bytes

[adriangb]

that reads a body, it's just an empty body. to be clearer: receive gets called whenever the server (your application) tries to read data from the client (the thing that sent the request). you can still try to read data even if there is no data to read. putting salute: Salute in there makes FastAPI try to load Salute from the data the client sent. In this case I expect you get an error because you can't load a Salute from b''.

[dmitrykaramin]

Yes FastAPI raises error, as it doesn't find anything in body.

But receive was called much earlier than exception raised.

Anyway, i think i have answer on my question.