I get TypeError: StopIteration interacts badly with generators and cannot be raised into a Future

Hi,

lately I’ve started to see in the logs this error:

[2020-02-10 12:34:40 +0000] [1] [INFO] Starting gunicorn 20.0.4
[2020-02-10 12:34:40 +0000] [1] [INFO] Listening at: http://0.0.0.0:8000 (1)
[2020-02-10 12:34:40 +0000] [1] [INFO] Using worker: uvicorn.workers.UvicornWorker
[2020-02-10 12:34:40 +0000] [13] [INFO] Booting worker with pid: 13
[2020-02-10 12:34:41 +0000] [13] [INFO] Started server process [13]
[2020-02-10 12:34:41 +0000] [13] [INFO] Waiting for application startup.
[2020-02-10 12:34:41 +0000] [13] [INFO] Application startup complete.
Exception in callback <function _chain_future.<locals>._set_state at 0x7fdd9e719f70>
handle: <Handle _chain_future.<locals>._set_state>
Traceback (most recent call last):
  File "uvloop/cbhandles.pyx", line 70, in uvloop.loop.Handle._run
  File "/usr/local/lib/python3.8/asyncio/futures.py", line 356, in _set_state
    _copy_future_state(other, future)
  File "/usr/local/lib/python3.8/asyncio/futures.py", line 332, in _copy_future_state
    dest.set_exception(_convert_future_exc(exception))
TypeError: StopIteration interacts badly with generators and cannot be raised into a Future

It doesn’t show up straight away, but after a few queries. After a few more queries I get:

[2020-02-10 12:12:49 +0000] [13] [ERROR] Exception in ASGI application
....
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/orm/query.py", line 3355, in scalar
    ret = self.one()
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/orm/query.py", line 3325, in one
    ret = self.one_or_none()
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/orm/query.py", line 3294, in one_or_none
    ret = list(self)
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/orm/query.py", line 3367, in __iter__
    return self._execute_and_instances(context)
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/orm/query.py", line 3388, in _execute_and_instances
    conn = self._get_bind_args(
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/orm/query.py", line 3403, in _get_bind_args
    return fn(
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/orm/query.py", line 3382, in _connection_from_session
    conn = self.session.connection(**kw)
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/orm/session.py", line 1130, in connection
    return self._connection_for_bind(
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/orm/session.py", line 1138, in _connection_for_bind
    return self.transaction._connection_for_bind(
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/orm/session.py", line 432, in _connection_for_bind
    conn = bind._contextual_connect()
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 2242, in _contextual_connect
    self._wrap_pool_connect(self.pool.connect, None),
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/engine/base.py", line 2276, in _wrap_pool_connect
    return fn()
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/pool/base.py", line 363, in connect
    return _ConnectionFairy._checkout(self)
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/pool/base.py", line 760, in _checkout
    fairy = _ConnectionRecord.checkout(pool)
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/pool/base.py", line 492, in checkout
    rec = pool._do_get()
  File "/usr/local/lib/python3.8/site-packages/sqlalchemy/pool/impl.py", line 127, in _do_get
    raise exc.TimeoutError(
sqlalchemy.exc.TimeoutError: QueuePool limit of size 5 overflow 10 reached, connection timed out, timeout 30 (Background on this error at: http://sqlalche.me/e/3o7r)

Any idea what could be happening?

[maximmarakov]

I got the same traceback, and found that any StopIteration exception will kill your request handling. For example, this server:

from fastapi import FastAPI
app = FastAPI()

@app.get("/")
def root():
    next(a for a in [])
    return {"message": "Hello World"}

will raise

Exception in callback <function _chain_future.<locals>._set_state at 0x7fb9a5ab9c10>
handle: <Handle _chain_future.<locals>._set_state>
Traceback (most recent call last):
  File "uvloop/cbhandles.pyx", line 70, in uvloop.loop.Handle._run
  File "/usr/lib/python3.8/asyncio/futures.py", line 353, in _set_state
    _copy_future_state(other, future)
  File "/usr/lib/python3.8/asyncio/futures.py", line 329, in _copy_future_state
    dest.set_exception(_convert_future_exc(exception))
TypeError: StopIteration interacts badly with generators and cannot be raised into a Future

and request will look like “infinity loading”.

So, if you had this exception “into Future” , check if there was an unhandled StopIteration exception in your code. Wish you luck 😄

[tiangolo]

This actually goes deep down to asyncio. It was recently fixed in AnyIO: https://github.com/agronholm/anyio/pull/477

The fix will be available in AnyIO 4.0.0 (not yet released).

Meanwhile, you could install from the git repo. But you would have to monkey patch a removed attribute used in Starlette.