Uvicorn: Error during WebSocket handshake: Unexpected response code: 400 (anonymous) @ (index):16

[jstoebel]

Checklist

• [x] The bug is reproducible against the latest release and/or master.
• [x] There are no similar issues or pull requests to fix it yet.

Describe the bug

I am following the Fastapi websockets tutorial (https://fastapi.tiangolo.com/advanced/websockets/#await-for-messages-and-send-messages). With the exact code given (see link) I start uvicorn with uvicorn main:app --reload visit http://127.0.0.1:8000/. I get this error in my chrome console:

(index):16 WebSocket connection to 'ws://localhost:8000/ws' failed: Error during WebSocket handshake: Unexpected response code: 400

To reproduce

Expected behavior

When I click "send" I expect to see the message I typed appear on the screen

Actual behavior

When I click "send" nothing happens

Debugging material

Environment

• OS / Python / Uvicorn version: just run uvicorn --version
  macos 10.15.5
  python 3.8.2
  uvicorn 0.12.1

• The exact command you're running uvicorn with, all flags you passed included. If you run it with gunicorn please do the same. If there is a reverse-proxy involved and you cannot reproduce without it please give the minimal config of it to reproduce.
  uvicorn main:app --reload

  Additional context

[euri10]

I just tested the code you linked and it's working fine for me, you got no logs ?

❯ uvicorn fapiws:app --reload
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [8960] using watchgod
INFO:     Started server process [8970]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     127.0.0.1:43636 - "GET / HTTP/1.1" 200 OK
INFO:     ('127.0.0.1', 43640) - "WebSocket /ws" [accepted]

[jstoebel]

Not when running with the above command. Here it is with log level trace

(.venv) ➜  websocket_test uvicorn main:app --reload --log-level trace
INFO:     Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO:     Started reloader process [4573] using statreload
INFO:     Started server process [4575]
INFO:     Waiting for application startup.
TRACE:    ASGI [1] Started scope={'type': 'lifespan', 'asgi': {'version': '3.0', 'spec_version': '2.0'}}
TRACE:    ASGI [1] Receive {'type': 'lifespan.startup'}
TRACE:    ASGI [1] Send {'type': 'lifespan.startup.complete'}
INFO:     Application startup complete.
TRACE:    127.0.0.1:50372 - Connection made
TRACE:    127.0.0.1:50373 - Connection made
TRACE:    127.0.0.1:50372 - ASGI [2] Started scope={'type': 'http', 'asgi': {'version': '3.0', 'spec_version': '2.1'}, 'http_version': '1.1', 'server': ('127.0.0.1', 8000), 'client': ('127.0.0.1', 50372), 'scheme': 'http', 'method': 'GET', 'root_path': '', 'path': '/', 'raw_path': b'/', 'query_string': b'', 'headers': '<...>'}
TRACE:    127.0.0.1:50372 - ASGI [2] Send {'type': 'http.response.start', 'status': 200, 'headers': '<...>'}
INFO:     127.0.0.1:50372 - "GET / HTTP/1.1" 200 OK
TRACE:    127.0.0.1:50372 - ASGI [2] Send {'type': 'http.response.body', 'body': '<1025 bytes>'}
TRACE:    127.0.0.1:50372 - ASGI [2] Completed
TRACE:    127.0.0.1:50375 - Connection made
WARNING:  Unsupported upgrade request.
TRACE:    127.0.0.1:50375 - Connection lost
TRACE:    127.0.0.1:50372 - ASGI [3] Started scope={'type': 'http', 'asgi': {'version': '3.0', 'spec_version': '2.1'}, 'http_version': '1.1', 'server': ('127.0.0.1', 8000), 'client': ('127.0.0.1', 50372), 'scheme': 'http', 'method': 'GET', 'root_path': '', 'path': '/', 'raw_path': b'/', 'query_string': b'', 'headers': '<...>'}
TRACE:    127.0.0.1:50372 - ASGI [3] Send {'type': 'http.response.start', 'status': 200, 'headers': '<...>'}
INFO:     127.0.0.1:50372 - "GET / HTTP/1.1" 200 OK
TRACE:    127.0.0.1:50372 - ASGI [3] Send {'type': 'http.response.body', 'body': '<1025 bytes>'}
TRACE:    127.0.0.1:50372 - ASGI [3] Completed
TRACE:    127.0.0.1:50372 - Connection lost

[euri10]

I fail to see what could cause this, do you use h11 or websockets ? Any particular browser used ?

[KoehlerClem]

I get the same error with exactly the same setup. Tested with Crome and Safari.

[KoehlerClem]

Fixed it with:
pip3 install websockets

[euri10]

ok the new packaging does not install websocket or h11 by default so with a simple pip install uvicorn no ws will be handled.

to fix this. install with pip install uvicorn[standard].

we need to either provide a gentle message on ws usage if the latter installation was used or update docs, or both

[KoehlerClem]

In zsh on macos 10.15.5 i got the error
zsh: no matches found: uvicorn[standard]

I had to escape the square brackets:
pip3 install uvicorn\[standard\]

[euri10]

In zsh on macos 10.15.5 i got the error
zsh: no matches found: uvicorn[standard]

I had to escape the square brackets:
pip3 install uvicorn\[standard\]

this is a zsh feature yes http://zsh.sourceforge.net/Guide/zshguide05.html#l137
usually you'd better quoting the brackets rather than escaping them

[jakabk]

@euri10 yes, you should provide a gentle message...

[konoufo]

Definitely should provide a gentle message...