__call__() missing 1 required positional argument: 'send' FastAPI on App Engine

Question

When trying to host an API on App Engine, the following error keeps coming up. The program used to run on Flask which was working but very slow.

Error:

"Traceback (most recent call last):
  File "/env/lib/python3.7/site-packages/gunicorn/workers/sync.py", line 134, in handle
    self.handle_request(listener, req, client, addr)
  File "/env/lib/python3.7/site-packages/gunicorn/workers/sync.py", line 175, in handle_request
    respiter = self.wsgi(environ, resp.start_response)
TypeError: __call__() missing 1 required positional argument: 'send'
"

Docker File:

FROM gcr.io/google_appengine/python

RUN apt-get update && apt-get install -y ffmpeg

# Create a virtualenv for dependencies. This isolates these packages from
# system-level packages.

RUN virtualenv /env -p python3.7

# Setting these environment variables are the same as running
# source /env/bin/activate.

ENV VIRTUAL_ENV /env
ENV PATH /env/bin:$PATH

ADD requirements.txt /app/requirements.txt
RUN pip install -r /app/requirements.txt

# Add the application source code.

ADD . /app

CMD gunicorn -w 4 -k uvicorn.workers.UvicornWorker main:app

app.yaml

runtime: custom
env: flex
entrypoint: gunicorn -w 4 -k uvicorn.workers.UvicornWorker main:app
service: encoder

runtime_config:
  python_version: 3

handlers:

- url: /.*
  script: auto

Septuplicate

The error basically says you are trying to run FastAPI as WSGI which is not right and acceptable because FastAPI is only compatible with ASGI. See this – Walkthrough

Thanks @YagizcanDegirmenci I'll try running it with just Uvicorn – Septuplicate

So it runs, but now I get this error, "[error] 33#33: *92500 connect() failed (111: Connection refused) while connecting to upstream, client: 216.58.212.244, server: – Septuplicate

Answer 1

As Dustin said I found out that worker class need to be changed. Try the below one.

gunicorn -k uvicorn.workers.UvicornWorker main:app

Found this on github issues

Housemaster

That actually is the most direct answer. Thank you for the link. – Tombouctou

Answer 2

App Engine requires your main.py file to declare an app variable which corresponds to a WSGI Application.

Since FastAPI is an asynchronous web framework, it is not compatible with WSGI (which is synchronous).

Your best option would be to use a service like Cloud Run, which would allow you to define your own runtime and use an asynchronous HTTP server compatible with FastAPI.

Occam

I am trying to find any info on how to configure Cloud Run to use an asynchronous HTTP server. Can anyone please guide me to the appropriate docs? Thanks. – Crosswise

@BellaveJayaram Since Cloud Run allows you to use any container image, there is nothing specific required to use an async HTTP server. If there was a particular server you wanted to use and weren't sure how to get started, I'd recommend asking a new question instead of leaving a comment here. – Occam

This is the best answer since it also explains why FastAPI requires a uvicorn worker. Thank you. – Boeotian

Answer 3

I ran into the same issue when I want to deploy a FastAPI app to Heroku. Indeed, you can't use uvicorn (which is the ASGI framework that FastAPI is using) with Heroku that uses gunicorn.

However, by adding a uvicorn worker to gunicorn then it works!:

gunicorn api:app --bind 0.0.0.0:$PORT --worker-class uvicorn.workers.UvicornWorker

Mutualism

Answer 4

In the Starlette code, I found a reference to a2wsgi. And it looks like it is going in both directions (wsgi<>asgi).

I gave it a try and it worked well for me:

from a2wsgi import ASGIMiddleware
wsgi_app = ASGIMiddleware(app)
# use this wsgi instead of your app

Goins

You might also add mount call fastapi.tiangolo.com/advanced/wsgi/?h=wsgi – Corvine

Answer 5

I solved it by putting python main.py as the build command and setting my uvicorn command in the main.py as uvicorn.run("main:app", host="0.0.0.0", port=8080)

Tamtama

It might works for Google Cloud, but might not work in others settings like pythonanywhere.com – Corvine