Flask-SocketIO: Flask apps not using Flask-SocketIO get exception/ValueError

Flask apps without use of Flask-SocketIO seem to get a ValueError after just typing flask run in terminal.

Example application code:

import os

from flask import Flask

app = Flask(__name__)
app.config["SECRET_KEY"] = 'secret!'

@app.route("/")
def index():
	print('hello world')
	return 'hello!'

if __name__ == '__main__':
    app.run(debug=True)

Terminal log:

flask run
 * Serving Flask-SocketIO app "application.py"
 * Forcing debug mode on
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 289-305-673
Exception in thread Thread-1:
Traceback (most recent call last):
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\threading.py", line 917, in _bootstrap_inner
    self.run()
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages\flask_socketio\cli.py", line 59, in run_server
    return run_command()
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages\click\core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages\click\core.py", line 697, in main
    rv = self.invoke(ctx)
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages\click\core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages\click\core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages\click\decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args[1:], **kwargs)
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages\click\core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages\flask\cli.py", line 771, in run_command
    threaded=with_threads, ssl_context=cert)
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages\werkzeug\serving.py", line 812, in run_simple
    reloader_type)
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages\werkzeug\_reloader.py", line 267, in run_with_reloader
    signal.signal(signal.SIGTERM, lambda *args: sys.exit(0))
  File "c:\users\daniel\appdata\local\programs\python\python37-32\lib\signal.py", line 47, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
ValueError: signal only works in main thread

It seems like with Flask-SocketIO installed, a Flask-SocketIO app is always being run even if the application doesn’t import Flask-SocketIO. So this issue doesn’t occur with flask run with Flask-SocketIO applications, but now occurs with all other Flask applications I’ve written without Flask-SocketIO. I’m not sure if this is an issue with Flask or Flask-SocketIO.

The one workaround I have is to just type python application.py in command line. Any other suggestions for running the non-SocketIO applications would be appreciated. Thanks!

About this issue

Original URL
State: closed
Created 6 years ago
Comments: 22 (10 by maintainers)

Commits related to this issue

Removed eventlet (https://github.com/miguelgrinberg/Flask-SocketIO/issues/817) — committed to swiewiora/python-webshell by deleted user 5 years ago

Most upvoted comments

@dcsan you can use debug mode to enable the reloader:

if __name__ == "__main__":
    socketio.run(app, debug=True)

miguelgrinberg on Jan 1, 2020

Starting with Flask-SocketIO 3.2.0 (just released), the flask run command can only be used with the threading mode. For all other async modes, the socketio.run(app) method should be used to start the server.

Hello, I work with VS Code and don’t know how to make it launch with socket.run(app) ?

I’m struggling integrate flask-socketio to my current project, I always have this screen : capture

EDIT

Seems like add --no-reload like you suggest in an other issue works.

Senseikaii on Jan 29, 2019

The lines from my last comment were displayed after running pip install --upgrade flask-socketio. I assumed this would show you the versions of all relevant packages and would be up to date. Sorry for the confusion.

However, after checking pip list --outdated I did find that python-engineio was outdated. Here is what I now see after upgrading all outdated packages. Running pip install --upgrade flask-socketio now shows the following:

λ pip install --upgrade flask-socketio
Requirement already up-to-date: flask-socketio in c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages (3.0.2)
Requirement already satisfied, skipping upgrade: Flask>=0.9 in c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages (from flask-socketio) (1.0.2)
Requirement already satisfied, skipping upgrade: python-socketio>=1.6.1 in c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages (from flask-socketio) (2.0.0)
Requirement already satisfied, skipping upgrade: itsdangerous>=0.24 in c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages (from Flask>=0.9->flask-socketio) (0.24)
Requirement already satisfied, skipping upgrade: Werkzeug>=0.14 in c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages (from Flask>=0.9->flask-socketio) (0.14.1)
Requirement already satisfied, skipping upgrade: click>=5.1 in c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages (from Flask>=0.9->flask-socketio) (7.0)
Requirement already satisfied, skipping upgrade: Jinja2>=2.10 in c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages (from Flask>=0.9->flask-socketio) (2.10)
Requirement already satisfied, skipping upgrade: six>=1.9.0 in c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages (from python-socketio>=1.6.1->flask-socketio) (1.11.0)
Requirement already satisfied, skipping upgrade: python-engineio>=2.2.0 in c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages (from python-socketio>=1.6.1->flask-socketio) (2.3.2)
Requirement already satisfied, skipping upgrade: MarkupSafe>=0.23 in c:\users\daniel\appdata\local\programs\python\python37-32\lib\site-packages (from Jinja2>=2.10->Flask>=0.9->flask-socketio) (1.0)

However, I still get the error. Is there something else that’s outdated?

Just to make sure, here is my full pip list

Package          Version
---------------- ----------
certifi          2018.10.15
cffi             1.11.5
chardet          3.0.4
Click            7.0
Django           2.1.2
dnspython        1.15.0
eventlet         0.24.1
Flask            1.0.2
Flask-Session    0.3.1
Flask-SocketIO   3.0.2
gevent           1.3.7
gevent-websocket 0.10.1
greenlet         0.4.15
idna             2.7
itsdangerous     0.24
Jinja2           2.10
MarkupSafe       1.0
monotonic        1.5
pip              18.1
psycopg2         2.7.5
pycparser        2.19
python-engineio  2.3.2
python-socketio  2.0.0
pytz             2018.5
requests         2.20.0
setuptools       40.4.3
six              1.11.0
SQLAlchemy       1.2.12
urllib3          1.24
Werkzeug         0.14.1

holistudio on Oct 18, 2018

Hey guys, I was also facing the same problem. So to summarize, if you’re using socket-io, don’t do flask run. First, add

if __name__ == "__main__":
    socketio.run(app)

at the end of your application. To run it just do

python3 __init__.py

Hope it helped.

AkshayKumar007 on Aug 12, 2019

@Senseikaii you may also want to add "gevent": true to your vscode configuration if you are using gevent or eventlet. Yes, even if you use eventlet the option is called gevent.

miguelgrinberg on Jan 29, 2019

The list is incomplete. Run pip install --upgrade flask-socketio to get all the latest and report back if there are still issues.

miguelgrinberg on Oct 18, 2018