WebSocket transport is not available, you must install a WebSocket server that is compatible with your async mode to enable it

[Sebastian-Nielsen]

Describe the bug

Unable to get rid of "WebSocket transport is not available, you must install a WebSocket server that is compatible with your async mode to enable it"

To Reproduce

from flask import Flask, render_template, session
from flask import send_from_directory, jsonify, request
from flask_cors import CORS, cross_origin

app = Flask("someName", static_folder='frontend/build')
app.config['SECRET_KEY'] = "something"
app.config['TEMPLATES_AUTO_RELOAD'] = True
app.config['SESSION_PERMANENT'] = True
app.config['FLASK_ENV'] = "development"
CORS(app, supports_credentials=True) #comment this on deployment

from flask_socketio import SocketIO, send
socketIo = SocketIO(app,
    logger=True,
    engineio_logger=True,
    cors_allowed_origins="*"
)

# Serve React App
@app.route('/', defaults={'path': ''})
@app.route('/<path:path>')
def serve(path):
    if path != "" and os.path.exists(app.static_folder + '/' + path):
        return send_from_directory(app.static_folder, path)
    else:
        return send_from_directory(app.static_folder, 'index.html')

$ cat requirements.txt
aniso8601==9.0.1
argon2-cffi==21.1.0
attrs==21.2.0
backcall==0.2.0
beautifulsoup4==4.9.3
bidict==0.21.4
bleach==4.1.0
bs4==0.0.1
certifi==2020.12.5
cffi==1.14.6
chardet==4.0.0
click==7.1.2
colorama==0.4.4
configparser==5.0.2
crayons==0.4.0
cssselect2==0.4.1
cycler==0.11.0
debugpy==1.4.1
decorator==5.0.9
defusedxml==0.7.1
dnspython==1.16.0
entrypoints==0.3
eventlet==0.33.0             <--------------
fake-useragent==0.1.11
Flask==1.1.2
Flask-Cors==3.0.10
Flask-RESTful==0.3.8
Flask-SocketIO==5.1.1
gevent==21.12.0            <-------------
gevent-websocket==0.10.1   <-------------
greenlet==1.1.2
gunicorn==20.1.0
idna==2.10
ipykernel==6.3.1
ipython==7.27.0
ipython-genutils==0.2.0
ipywidgets==7.6.4
itsdangerous==1.1.0
jedi==0.18.0
Jinja2==2.11.3
joblib==1.1.0
jsonschema==3.2.0
jupyter==1.0.0
jupyter-client==7.0.2
jupyter-console==6.4.0
jupyter-core==4.7.1
jupyterlab-pygments==0.1.2
jupyterlab-widgets==1.0.1
kiwisolver==1.3.2
lxml==4.6.2
MarkupSafe==1.1.1
matplotlib==3.4.3
matplotlib-inline==0.1.2
mistune==0.8.4
nbclient==0.5.4
nbconvert==6.1.0
nbformat==5.1.3
nest-asyncio==1.5.1
notebook==6.4.3
numpy==1.20.1
packaging==21.0
pandas==1.3.4
pandocfilters==1.4.3
parso==0.8.2
passlib==1.7.4
pickleshare==0.7.5
Pillow==8.1.2
prometheus-client==0.11.0
prompt-toolkit==3.0.20
pycparser==2.20
Pygments==2.10.0
pyimgur==0.6.0
pymongo==3.11.3
pyparsing==2.4.7
PyPDF2==1.26.0
pyperclip==1.8.2
pyrsistent==0.18.0
python-dateutil==2.8.2
python-engineio==4.3.1
python-placeholder==0.2
python-snappy==0.6.0
python-socketio==5.5.1
pytz==2021.1
pyzmq==22.2.1
qtconsole==5.1.1
QtPy==1.11.0
reportlab==3.5.66
requests==2.25.1
scikit-learn==1.0.1
scipy==1.7.2
selenium==3.141.0
Send2Trash==1.8.0
six==1.16.0
sklearn==0.0
soupsieve==2.2.1
svglib==1.0.1
terminado==0.11.1
testpath==0.5.0
threadpoolctl==3.0.0
tinycss2==1.1.0
tornado==6.1
traitlets==5.1.0
undetected-chromedriver==2.2.1
urllib3==1.26.4
wcwidth==0.2.5
webdriver-manager==3.3.0
webencodings==0.5.1
Werkzeug==1.0.1
widgetsnbextension==3.5.1
zope.event==4.5.0
zope.interface==5.4.0

Expected behavior
I expected "WebSocket transport to be available" after having "pip installed": "gevent", "gevent-websocket", and "eventlet".

Logs
flask run

 * Environment: production
   WARNING: This is a development server. Do not use it in a production deployment.
   Use a production WSGI server instead.
 * Debug mode: off
Server initialized for threading.
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
The WebSocket transport is not available, you must install a WebSocket server that is compatible
with your async mode to enable it. See the documentation for details. (further occurrences of thi
s error will be logged with level INFO)
AoldfF99VYHS5xn-AAAA: Sending packet OPEN data {'sid': 'AoldfF99VYHS5xn-AAAA', 'upgrades': [], 'p
ingTimeout': 20000, 'pingInterval': 25000}
127.0.0.1 - - [28/Jan/2022 21:13:18] "←[37mGET /socket.io/?EIO=4&transport=polling&t=NwYKXIa HTTP
/1.1←[0m" 200 -
The WebSocket transport is not available, you must install a WebSocket server that is compatible
with your async mode to enable it. See the documentation for details.
LvOyT7_uJ9IFJpaOAAAB: Sending packet OPEN data {'sid': 'LvOyT7_uJ9IFJpaOAAAB', 'upgrades': [], 'p
ingTimeout': 20000, 'pingInterval': 25000}
127.0.0.1 - - [28/Jan/2022 21:13:18] "←[37mGET /socket.io/?EIO=4&transport=polling&t=NwYKY1M HTTP
/1.1←[0m" 200 -
AoldfF99VYHS5xn-AAAA: Received packet MESSAGE data 0
AoldfF99VYHS5xn-AAAA: Sending packet MESSAGE data 0{"sid":"EYkYCqbiP2dmj5TbAAAC"}
127.0.0.1 - - [28/Jan/2022 21:13:18] "←[37mGET /socket.io/?EIO=4&transport=polling&t=NwYKY2A.0&si
d=AoldfF99VYHS5xn-AAAA HTTP/1.1←[0m" 200 -
127.0.0.1 - - [28/Jan/2022 21:13:18] "←[37mPOST /socket.io/?EIO=4&transport=polling&t=NwYKY2A&sid
=AoldfF99VYHS5xn-AAAA HTTP/1.1←[0m" 200 -
LvOyT7_uJ9IFJpaOAAAB: Received packet MESSAGE data 0
LvOyT7_uJ9IFJpaOAAAB: Sending packet MESSAGE data 0{"sid":"kSMV_A7MHElyDLV9AAAD"}
127.0.0.1 - - [28/Jan/2022 21:13:18] "←[37mPOST /socket.io/?EIO=4&transport=polling&t=NwYKY6I&sid
=LvOyT7_uJ9IFJpaOAAAB HTTP/1.1←[0m" 200 -
127.0.0.1 - - [28/Jan/2022 21:13:18] "←[37mGET /socket.io/?EIO=4&transport=polling&t=NwYKY6J&sid=
LvOyT7_uJ9IFJpaOAAAB HTTP/1.1←[0m" 200 -
The WebSocket transport is not available, you must install a WebSocket server that is compatible
with your async mode to enable it. See the documentation for details.
ao2rkEiDmH-MwBrsAAAE: Sending packet OPEN data {'sid': 'ao2rkEiDmH-MwBrsAAAE', 'upgrades': [], 'p
ingTimeout': 20000, 'pingInterval': 25000}
127.0.0.1 - - [28/Jan/2022 21:13:19] "←[37mGET /socket.io/?EIO=4&transport=polling&t=NwYKYGz HTTP
/1.1←[0m" 200 -
ao2rkEiDmH-MwBrsAAAE: Received packet MESSAGE data 0
ao2rkEiDmH-MwBrsAAAE: Sending packet MESSAGE data 0{"sid":"q9LQ1Dt-7s384dy-AAAF"}
127.0.0.1 - - [28/Jan/2022 21:13:19] "←[37mGET /socket.io/?EIO=4&transport=polling&t=NwYKYLv.0&si
d=ao2rkEiDmH-MwBrsAAAE HTTP/1.1←[0m" 200 -
127.0.0.1 - - [28/Jan/2022 21:13:19] "←[37mPOST /socket.io/?EIO=4&transport=polling&t=NwYKYLv&sid
=ao2rkEiDmH-MwBrsAAAE HTTP/1.1←[0m" 200 -
The WebSocket transport is not available, you must install a WebSocket server that is compatible
with your async mode to enable it. See the documentation for details.
DiXZmTVz9rXCRWqCAAAG: Sending packet OPEN data {'sid': 'DiXZmTVz9rXCRWqCAAAG', 'upgrades': [], 'p
ingTimeout': 20000, 'pingInterval': 25000}
127.0.0.1 - - [28/Jan/2022 21:13:20] "←[37mGET /socket.io/?EIO=4&transport=polling&t=NwYKYWf HTTP
/1.1←[0m" 200 -
DiXZmTVz9rXCRWqCAAAG: Received packet MESSAGE data 0

... there is a lot more here, but it's just more of the above ...

---

FYI, I posted this issue on stackoverflow too.

[miguelgrinberg]

Please don't write bugs on this project when you need help figuring out why your application doesn't work as expected. The discussion board is for troubleshooting. Thank you.

Your problem is that you are using flask run to run the application. When you start that way, only the threading mode can be used, as I'm limited to use Werkzeug as web server. Use the socketio.run(app) method to run on a different web server. See the documentation for some example code.

[miguelgrinberg]

Are you reading the documentation? This is all explained there.