FastAPI middleware peeking into responses

Question

I try to write a simple middleware for FastAPI peeking into response bodies.

In this example I just log the body content:

app = FastAPI()

@app.middleware("http")
async def log_request(request, call_next):
    logger.info(f'{request.method} {request.url}')
    response = await call_next(request)
    logger.info(f'Status code: {response.status_code}')
    async for line in response.body_iterator:
        logger.info(f'    {line}')
    return response

However it looks like I "consume" the body this way, resulting in this exception:

  ...
  File ".../python3.7/site-packages/starlette/middleware/base.py", line 26, in __call__
    await response(scope, receive, send)
  File ".../python3.7/site-packages/starlette/responses.py", line 201, in __call__
    await send({"type": "http.response.body", "body": b"", "more_body": False})
  File ".../python3.7/site-packages/starlette/middleware/errors.py", line 156, in _send
    await send(message)
  File ".../python3.7/site-packages/uvicorn/protocols/http/httptools_impl.py", line 515, in send
    raise RuntimeError("Response content shorter than Content-Length")
RuntimeError: Response content shorter than Content-Length

Trying to look into the response object I couldn't see any other way to read its content. What is the correct way to do it?

Answer 1

I had a similar need in a FastAPI middleware and although not ideal here's what we ended up with:

app = FastAPI()

@app.middleware("http")
async def log_request(request, call_next):
    logger.info(f'{request.method} {request.url}')
    response = await call_next(request)
    logger.info(f'Status code: {response.status_code}')
    body = b""
    async for chunk in response.body_iterator:
        body += chunk
    # do something with body ...
    return Response(
        content=body,
        status_code=response.status_code,
        headers=dict(response.headers),
        media_type=response.media_type
    )

Be warned that such an implementation is problematic with responses streaming a body that would not fit in your server RAM (imagine a response of 100GB).

Depending on what your application does, you will rule if it is an issue or not.

---

In the case where some of your endpoints produce large responses, you might want to avoid using a middleware and instead implement a custom ApiRoute. This custom ApiRoute would have the same issue with consuming the body, but you can limit it's usage to a particular endpoints.

Learn more athttps://fastapi.tiangolo.com/advanced/custom-request-and-route/

Answer 2

I know this is a relatively old post now, but I recently ran into this problem and came up with a solution:

Middleware Code

from starlette.middleware.base import BaseHTTPMiddleware
from starlette.requests import Request
import json
from .async_iterator_wrapper import async_iterator_wrapper as aiwrap

class some_middleware(BaseHTTPMiddleware):
   async def dispatch(self, request:Request, call_next:RequestResponseEndpoint):
      # --------------------------
      # DO WHATEVER YOU TO DO HERE
      #---------------------------
      
      response = await call_next(request)

      # Consuming FastAPI response and grabbing body here
      resp_body = [section async for section in response.__dict__['body_iterator']]
      # Repairing FastAPI response
      response.__setattr__('body_iterator', aiwrap(resp_body)

      # Formatting response body for logging
      try:
         resp_body = json.loads(resp_body[0].decode())
      except:
         resp_body = str(resp_body)

async_iterator_wrapper Code from TypeError from Python 3 async for loop

class async_iterator_wrapper:
    def __init__(self, obj):
        self._it = iter(obj)
    def __aiter__(self):
        return self
    async def __anext__(self):
        try:
            value = next(self._it)
        except StopIteration:
            raise StopAsyncIteration
        return value

I really hope this can help someone! I found this very helpful for logging.

Big thanks to @Eddified for the aiwrap class

Answer 3

Or if you using APIRouter, we can also do like this:

class CustomAPIRoute(APIRoute):
    def get_route_handler(self):
        app = super().get_route_handler()
        return wrapper(app)

def wrapper(func):
    async def _app(request):
        response = await func(request)

        print(vars(request), vars(response))

        return response
    return _app

router = APIRouter(route_class=CustomAPIRoute)

you can directly see or access response and request's body, and other attirbutes.
If you want to capture the httpexcpetion, you should wrap response = await func(request) with try: except HTTPException as e .

references:
get_request_handler() - get_route_handler call get_request_handler
get_route_handler()
APIRoute class

Answer 4

This can be done easily with BackgroundTasks ( https://fastapi.tiangolo.com/tutorial/background-tasks/ )

Non blocking, the code executes after the response is sent to client, pretty easy to add.

The downside is that those have to be added to each endpoint.