Error when raising exception in FastAPI: UnboundLocalError: local variable 'status_code' referenced before assignment

[kbakk]

Hi,

I'm seeing an issue with FastAPI, where I am raising an exception in a route handler. I've created a small reproducer:

from fastapi import FastAPI
from starlette.middleware import Middleware
from starlette_prometheus import PrometheusMiddleware


middleware = [
    Middleware(PrometheusMiddleware)
]

app = FastAPI(middleware=middleware)

@app.get("/")
def read_root():
    raise ValueError("Test error")
    # return {"Hello": "World"}

Here's the output from running the reproducer and calling it with curl localhost:8000/:

output

$ uvicorn example:app                             
INFO:     Started server process [5099]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/uvicorn/protocols/http/h11_impl.py", line 373, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/uvicorn/middleware/proxy_headers.py", line 75, in __call__
    return await self.app(scope, receive, send)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/fastapi/applications.py", line 208, in __call__
    await super().__call__(scope, receive, send)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette/applications.py", line 112, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette/middleware/errors.py", line 159, in __call__
    await self.app(scope, receive, _send)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette/middleware/base.py", line 57, in __call__
    task_group.cancel_scope.cancel()
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/anyio/_backends/_asyncio.py", line 572, in __aexit__
    raise ExceptionGroup(exceptions)
anyio._backends._asyncio.ExceptionGroup: 2 exceptions were raised in the task group:
----------------------------
Traceback (most recent call last):
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette/middleware/base.py", line 30, in coro
    await self.app(scope, request.receive, send_stream.send)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette/exceptions.py", line 82, in __call__
    raise exc
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette/exceptions.py", line 71, in __call__
    await self.app(scope, receive, sender)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette/routing.py", line 656, in __call__
    await route.handle(scope, receive, send)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette/routing.py", line 259, in handle
    await self.app(scope, receive, send)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette/routing.py", line 61, in app
    response = await func(request)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/fastapi/routing.py", line 226, in app
    raw_response = await run_endpoint_function(
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/fastapi/routing.py", line 161, in run_endpoint_function
    return await run_in_threadpool(dependant.call, **values)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette/concurrency.py", line 39, in run_in_threadpool
    return await anyio.to_thread.run_sync(func, *args)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/anyio/to_thread.py", line 28, in run_sync
    return await get_asynclib().run_sync_in_worker_thread(func, *args, cancellable=cancellable,
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/anyio/_backends/_asyncio.py", line 818, in run_sync_in_worker_thread
    return await future
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/anyio/_backends/_asyncio.py", line 754, in run
    result = context.run(func, *args)
  File "./example.py", line 14, in read_root
    raise ValueError("Test error")
ValueError: Test error
----------------------------
Traceback (most recent call last):
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette_prometheus/middleware.py", line 53, in dispatch
    response = await call_next(request)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette/middleware/base.py", line 35, in call_next
    message = await recv_stream.receive()
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/anyio/streams/memory.py", line 89, in receive
    await receive_event.wait()
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/anyio/_backends/_asyncio.py", line 1655, in wait
    await checkpoint()
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/anyio/_backends/_asyncio.py", line 440, in checkpoint
    await sleep(0)
  File "/Users/krisb/.pyenv/versions/3.8.9/lib/python3.8/asyncio/tasks.py", line 644, in sleep
    await __sleep0()
  File "/Users/krisb/.pyenv/versions/3.8.9/lib/python3.8/asyncio/tasks.py", line 638, in __sleep0
    yield
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette/middleware/base.py", line 55, in __call__
    response = await self.dispatch_func(request, call_next)
  File "/Users/krisb/Code/temp/starlette-prometheus-repro/.venv/lib/python3.8/site-packages/starlette_prometheus/middleware.py", line 65, in dispatch
    RESPONSES.labels(method=method, path_template=path_template, status_code=status_code).inc()
UnboundLocalError: local variable 'status_code' referenced before assignment

`pip freeze` output

anyio==3.4.0
asgiref==3.4.1
click==8.0.3
fastapi==0.70.1
h11==0.12.0
idna==3.3
prometheus-client==0.11.0
pydantic==1.9.0
sniffio==1.2.0
starlette==0.16.0
starlette-prometheus==0.8.0
typing-extensions==4.0.1
uvicorn==0.16.0

It seems like starlette has started raising asyncio.exceptions.CancelledError, which is not based on Exception caught here

starlette-prometheus/starlette_prometheus/middleware.py

Line 54 in 672ffc3

except Exception as e:

but rather BaseException.

I believe this was introduced in version 0.15.0 of Starlette, in PR encode/starlette#1157.

I've tried to change the exception catching to include both – i.e. except (Exception, asyncio.exceptions.CancelledError), this seems to revert the behavior to the expected.

[kbakk]

This appears to have been fixed in Starlette some time ago (encode/starlette#1262, in https://github.com/encode/starlette/releases/tag/0.17.0), however FastAPI wasn't updated to use >0.17 before now: https://github.com/tiangolo/fastapi/releases/tag/0.71.0

When testing the reproducer after having upgraded to version 0.71.0, it raises the expected exception (ValueError: Test error).