FAQ

Many questions asked in websockets’ issue tracker are really about asyncio.

Python’s documentation about developing with asyncio is a good complement.

Server side

Why does my server close the connection prematurely?

Your connection handler exits prematurely. Wait for the work to be finished before returning.

For example, if your handler has a structure similar to:

async def handler(websocket):
    asyncio.create_task(do_some_work())

change it to:

async def handler(websocket):
    await do_some_work()

Why does the server close the connection after processing one message?

Your connection handler exits after processing one message. Write a loop to process multiple messages.

For example, if your handler looks like this:

async def handler(websocket):
    print(websocket.recv())

change it like this:

async def handler(websocket):
    async for message in websocket:
        print(message)

Don’t feel bad if this happens to you — it’s the most common question in websockets’ issue tracker :-)

Why can only one client connect at a time?

Your connection handler blocks the event loop. Look for blocking calls. Any call that may take some time must be asynchronous.

For example, if you have:

async def handler(websocket):
    time.sleep(1)

change it to:

async def handler(websocket):
    await asyncio.sleep(1)

This is part of learning asyncio. It isn’t specific to websockets.

See also Python’s documentation about running blocking code.

How can I pass additional arguments to the connection handler?

You can bind additional arguments to the connection handler with functools.partial():

import asyncio
import functools
import websockets

async def handler(websocket, extra_argument):
    ...

bound_handler = functools.partial(handler, extra_argument='spam')
start_server = websockets.serve(bound_handler, ...)

Another way to achieve this result is to define the handler coroutine in a scope where the extra_argument variable exists instead of injecting it through an argument.

How do I get access HTTP headers, for example cookies?

To access HTTP headers during the WebSocket handshake, you can override process_request:

async def process_request(self, path, request_headers):
    cookies = request_header["Cookie"]

Once the connection is established, they’re available in request_headers:

async def handler(websocket):
    cookies = websocket.request_headers["Cookie"]

How do I get the IP address of the client connecting to my server?

It’s available in remote_address:

async def handler(websocket):
    remote_ip = websocket.remote_address[0]

How do I set which IP addresses my server listens to?

Look at the host argument of create_server().

serve() accepts the same arguments as create_server().

How do I close a connection properly?

websockets takes care of closing the connection when the handler exits.

How do I run a HTTP server and WebSocket server on the same port?

You don’t.

HTTP and WebSockets have widely different operational characteristics. Running them with the same server becomes inconvenient when you scale.

Providing a HTTP server is out of scope for websockets. It only aims at providing a WebSocket server.

There’s limited support for returning HTTP responses with the process_request hook.

If you need more, pick a HTTP server and run it separately.

Alternatively, pick a HTTP framework that builds on top of websockets to support WebSocket connections, like Sanic.

Client side

Why does my client close the connection prematurely?

You’re exiting the context manager prematurely. Wait for the work to be finished before exiting.

For example, if your code has a structure similar to:

async with connect(...) as websocket:
    asyncio.create_task(do_some_work())

change it to:

async with connect(...) as websocket:
    await do_some_work()

How do I close a connection properly?

The easiest is to use connect() as a context manager:

async with connect(...) as websocket:
    ...

How do I reconnect automatically when the connection drops?

See issue 414.

How do I stop a client that is continuously processing messages?

You can close the connection.

Here’s an example that terminates cleanly when it receives SIGTERM on Unix:

#!/usr/bin/env python

import asyncio
import signal
import websockets

async def client():
    uri = "ws://localhost:8765"
    async with websockets.connect(uri) as websocket:
        # Close the connection when receiving SIGTERM.
        loop = asyncio.get_running_loop()
        loop.add_signal_handler(
            signal.SIGTERM, loop.create_task, websocket.close())

        # Process messages received on the connection.
        async for message in websocket:
            ...

asyncio.run(client())

How do I disable TLS/SSL certificate verification?

Look at the ssl argument of create_connection().

connect() accepts the same arguments as create_connection().

asyncio usage

How do I do two things in parallel? How do I integrate with another coroutine?

You must start two tasks, which the event loop will run concurrently. You can achieve this with asyncio.gather() or asyncio.create_task().

Keep track of the tasks and make sure they terminate or you cancel them when the connection terminates.

Why does my program never receive any messages?

Your program runs a coroutine that never yields control to the event loop. The coroutine that receives messages never gets a chance to run.

Putting an await statement in a for or a while loop isn’t enough to yield control. Awaiting a coroutine may yield control, but there’s no guarantee that it will.

For example, send() only yields control when send buffers are full, which never happens in most practical cases.

If you run a loop that contains only synchronous operations and a send() call, you must yield control explicitly with asyncio.sleep():

async def producer(websocket):
    message = generate_next_message()
    await websocket.send(message)
    await asyncio.sleep(0)  # yield control to the event loop

asyncio.sleep() always suspends the current task, allowing other tasks to run. This behavior is documented precisely because it isn’t expected from every coroutine.

See issue 867.

Why does my very simple program misbehave mysteriously?

You are using time.sleep() instead of asyncio.sleep(), which blocks the event loop and prevents asyncio from operating normally.

This may lead to messages getting send but not received, to connection timeouts, and to unexpected results of shotgun debugging e.g. adding an unnecessary call to send() makes the program functional.

Both sides

What does ConnectionClosedError: no close frame received or sent mean?

If you’re seeing this traceback in the logs of a server:

connection handler failed
Traceback (most recent call last):
  ...
asyncio.exceptions.IncompleteReadError: 0 bytes read on a total of 2 expected bytes

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  ...
websockets.exceptions.ConnectionClosedError: no close frame received or sent

or if a client crashes with this traceback:

Traceback (most recent call last):
  ...
ConnectionResetError: [Errno 54] Connection reset by peer

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  ...
websockets.exceptions.ConnectionClosedError: no close frame received or sent

it means that the TCP connection was lost. As a consequence, the WebSocket connection was closed without receiving and sending a close frame, which is abnormal.

You can catch and handle ConnectionClosed to prevent it from being logged.

There are several reasons why long-lived connections may be lost:

  • End-user devices tend to lose network connectivity often and unpredictably because they can move out of wireless network coverage, get unplugged from a wired network, enter airplane mode, be put to sleep, etc.

  • HTTP load balancers or proxies that aren’t configured for long-lived connections may terminate connections after a short amount of time, usually 30 seconds, despite websockets’ keepalive mechanism.

If you’re facing a reproducible issue, enable debug logs to see when and how connections are closed.

What does ConnectionClosedError: sent 1011 (unexpected error) keepalive ping timeout; no close frame received mean?

If you’re seeing this traceback in the logs of a server:

connection handler failed
Traceback (most recent call last):
  ...
asyncio.exceptions.CancelledError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  ...
websockets.exceptions.ConnectionClosedError: sent 1011 (unexpected error) keepalive ping timeout; no close frame received

or if a client crashes with this traceback:

Traceback (most recent call last):
  ...
asyncio.exceptions.CancelledError

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  ...
websockets.exceptions.ConnectionClosedError: sent 1011 (unexpected error) keepalive ping timeout; no close frame received

it means that the WebSocket connection suffered from excessive latency and was closed after reaching the timeout of websockets’ keepalive mechanism.

You can catch and handle ConnectionClosed to prevent it from being logged.

There are two main reasons why latency may increase:

  • Poor network connectivity.

  • More traffic than the recipient can handle.

See the discussion of timeouts for details.

If websockets’ default timeout of 20 seconds is too short for your use case, you can adjust it with the ping_timeout argument.

How do I set a timeout on recv()?

Use wait_for():

await asyncio.wait_for(websocket.recv(), timeout=10)

This technique works for most APIs, except for asynchronous context managers. See issue 574.

How can I pass additional arguments to a custom protocol subclass?

You can bind additional arguments to the protocol factory with functools.partial():

import asyncio
import functools
import websockets

class MyServerProtocol(websockets.WebSocketServerProtocol):
    def __init__(self, extra_argument, *args, **kwargs):
        super().__init__(*args, **kwargs)
        # do something with extra_argument

create_protocol = functools.partial(MyServerProtocol, extra_argument='spam')
start_server = websockets.serve(..., create_protocol=create_protocol)

This example was for a server. The same pattern applies on a client.

How do I keep idle connections open?

websockets sends pings at 20 seconds intervals to keep the connection open.

It closes the connection if it doesn’t get a pong within 20 seconds.

You can adjust this behavior with ping_interval and ping_timeout.

How do I respond to pings?

websockets takes care of responding to pings with pongs.

Miscellaneous

How do I create channels or topics?

websockets doesn’t have built-in publish / subscribe for these use cases.

Depending on the scale of your service, a simple in-memory implementation may do the job or you may need an external publish / subscribe component.

Can I use websockets synchronously, without async / await?

You can convert every asynchronous call to a synchronous call by wrapping it in asyncio.get_event_loop().run_until_complete(...). Unfortunately, this is deprecated as of Python 3.10.

If this turns out to be impractical, you should use another library.

Are there onopen, onmessage, onerror, and onclose callbacks?

No, there aren’t.

websockets provides high-level, coroutine-based APIs. Compared to callbacks, coroutines make it easier to manage control flow in concurrent code.

If you prefer callback-based APIs, you should use another library.

Why do I get the error: module 'websockets' has no attribute '...'?

Often, this is because you created a script called websockets.py in your current working directory. Then import websockets imports this module instead of the websockets library.

I’m having problems with threads

You shouldn’t use threads. Use tasks instead.

call_soon_threadsafe() may help.