middleware: assert message["type"] == "http.response.start"

Describe the bug

I’ve tried to add a middleware to my app and then I started hitting a assertation error with the test client.

To Reproduce

Smallest reproducer I could find. It needs jinja2+middlware+testclient to hit the error.

import os
import tempfile
from typing import Callable
from fastapi import FastAPI

from fastapi import Request, Response
from fastapi.testclient import TestClient
from fastapi.templating import Jinja2Templates

dir_ = tempfile.mkdtemp()
with open(os.path.join(dir_, "index.html"), "w"):
    pass

app = FastAPI()
templates = Jinja2Templates(directory=dir_)


@app.get("/")
async def index(request: Request) -> Response:
    return templates.TemplateResponse("index.html", {"request": request})


@app.middleware("http")
async def some_middleware(request: Request, call_next: Callable) -> Response:
    response: Response = await call_next(request)
    return response


with TestClient(app) as client:
    client.get("/")

Error (partial)

  File "/home/lazka/.cache/pypoetry/virtualenvs/msys2-web-tQnlPJcR-py3.8/lib/python3.8/site-packages/starlette/middleware/errors.py", line 159, in __call__
    await self.app(scope, receive, _send)
  File "/home/lazka/.cache/pypoetry/virtualenvs/msys2-web-tQnlPJcR-py3.8/lib/python3.8/site-packages/starlette/middleware/base.py", line 25, in __call__
    response = await self.dispatch_func(request, self.call_next)
  File "a.py", line 25, in some_middleware
    response: Response = await call_next(request)
  File "/home/lazka/.cache/pypoetry/virtualenvs/msys2-web-tQnlPJcR-py3.8/lib/python3.8/site-packages/starlette/middleware/base.py", line 47, in call_next
    assert message["type"] == "http.response.start"
AssertionError

Expected behavior

Adding a no-op middleware shouldn’t make the TestClient error out.

[lazka]

Thanks, good to know that it’s not just me.

[tiangolo]

Thanks! The issue would not be removed, only closed. So if they for that they will find it, just closed, and it will help me (try to) keep things a bit cleaner.

Given that, I’ll then close it. Thanks!