Server (asyncio)#

Starting a server#

await websockets.server.serve(ws_handler, host=None, port=None, *, create_protocol=None, logger=None, compression='deflate', origins=None, extensions=None, subprotocols=None, extra_headers=None, server_header='Python/x.y.z websockets/X.Y', process_request=None, select_subprotocol=None, ping_interval=20, ping_timeout=20, close_timeout=10, max_size=2**20, max_queue=2**5, read_limit=2**16, write_limit=2**16, **kwds)[source]#

Start a WebSocket server listening on host and port.

Whenever a client connects, the server creates a WebSocketServerProtocol, performs the opening handshake, and delegates to the connection handler, ws_handler.

The handler receives the WebSocketServerProtocol and uses it to send and receive messages.

Once the handler completes, either normally or with an exception, the server performs the closing handshake and closes the connection.

Awaiting serve() yields a WebSocketServer. This object provides a close() method to shut down the server:

# set this future to exit the server
stop = asyncio.get_running_loop().create_future()

server = await serve(...)
await stop
await server.close()

serve() can be used as an asynchronous context manager. Then, the server is shut down automatically when exiting the context:

# set this future to exit the server
stop = asyncio.get_running_loop().create_future()

async with serve(...):
    await stop
Parameters:

  • ws_handler (Union[Callable[[WebSocketServerProtocol], Awaitable[Any]], Callable[[WebSocketServerProtocol, str], Awaitable[Any]]]) – Connection handler. It receives the WebSocket connection, which is a WebSocketServerProtocol, in argument.

  • host (Optional[Union[str, Sequence[str]]]) – Network interfaces the server binds to. See create_server() for details.

  • port (Optional[int]) – TCP port the server listens on. See create_server() for details.

  • create_protocol (Optional[Callable[..., WebSocketServerProtocol]]) – Factory for the asyncio.Protocol managing the connection. It defaults to WebSocketServerProtocol. Set it to a wrapper or a subclass to customize connection handling.

  • logger (Optional[LoggerLike]) – Logger for this server. It defaults to logging.getLogger("websockets.server"). See the logging guide for details.

  • compression (Optional[str]) – The “permessage-deflate” extension is enabled by default. Set compression to None to disable it. See the compression guide for details.

  • origins (Optional[Sequence[Optional[Origin]]]) – Acceptable values of the Origin header, for defending against Cross-Site WebSocket Hijacking attacks. Include None in the list if the lack of an origin is acceptable.

  • extensions (Optional[Sequence[ServerExtensionFactory]]) – List of supported extensions, in order in which they should be negotiated and run.

  • subprotocols (Optional[Sequence[Subprotocol]]) – List of supported subprotocols, in order of decreasing preference.

  • extra_headers (Union[HeadersLike, Callable[[str, Headers], HeadersLike]]) – Arbitrary HTTP headers to add to the response. This can be a HeadersLike or a callable taking the request path and headers in arguments and returning a HeadersLike.

  • server_header (Optional[str]) – Value of the Server response header. It defaults to "Python/x.y.z websockets/X.Y". Setting it to None removes the header.

  • process_request (Optional[Callable[[str, Headers], Awaitable[Optional[Tuple[StatusLike, HeadersLike, bytes]]]]]) – Intercept HTTP request before the opening handshake. See process_request() for details.

  • select_subprotocol (Optional[Callable[[Sequence[Subprotocol], Sequence[Subprotocol]], Subprotocol]]) – Select a subprotocol supported by the client. See select_subprotocol() for details.

  • open_timeout (Optional[float]) – Timeout for opening connections in seconds. None disables the timeout.

See WebSocketCommonProtocol for the documentation of ping_interval, ping_timeout, close_timeout, max_size, max_queue, read_limit, and write_limit.

Any other keyword arguments are passed the event loop’s create_server() method.

For example:

  • You can set ssl to a SSLContext to enable TLS.

  • You can set sock to a socket that you created outside of websockets.

Returns:

WebSocket server.

Return type:

None

await websockets.server.unix_serve(ws_handler, path=None, *, create_protocol=None, logger=None, compression='deflate', origins=None, extensions=None, subprotocols=None, extra_headers=None, server_header='Python/x.y.z websockets/X.Y', process_request=None, select_subprotocol=None, ping_interval=20, ping_timeout=20, close_timeout=10, max_size=2**20, max_queue=2**5, read_limit=2**16, write_limit=2**16, **kwds)[source]#

Start a WebSocket server listening on a Unix socket.

This function is identical to serve(), except the host and port arguments are replaced by path. It is only available on Unix.

Unrecognized keyword arguments are passed the event loop’s create_unix_server() method.

It’s useful for deploying a server behind a reverse proxy such as nginx.

Parameters:

path (str | None) – File system path to the Unix socket.

Stopping a server#

class websockets.server.WebSocketServer(logger=None)[source]#

WebSocket server returned by serve().

This class mirrors the API of Server.

It keeps track of WebSocket connections in order to close them properly when shutting down.

Parameters:

logger (Optional[LoggerLike]) – Logger for this server. It defaults to logging.getLogger("websockets.server"). See the logging guide for details.

close(close_connections=True)[source]#

Close the server.

  • Close the underlying Server.

  • When close_connections is True, which is the default, close existing connections. Specifically:

    • Reject opening WebSocket connections with an HTTP 503 (service unavailable) error. This happens when the server accepted the TCP connection but didn’t complete the opening handshake before closing.

    • Close open WebSocket connections with close code 1001 (going away).

  • Wait until all connection handlers terminate.

close() is idempotent.

await wait_closed()[source]#

Wait until the server is closed.

When wait_closed() returns, all TCP connections are closed and all connection handlers have returned.

To ensure a fast shutdown, a connection handler should always be awaiting at least one of:

Then the connection handler is immediately notified of the shutdown; it can clean up and exit.

get_loop()[source]#

See asyncio.Server.get_loop().

is_serving()[source]#

See asyncio.Server.is_serving().

await start_serving()[source]#

See asyncio.Server.start_serving().

Typical use:

server = await serve(..., start_serving=False)
# perform additional setup here...
# ... then start the server
await server.start_serving()
await serve_forever()[source]#

See asyncio.Server.serve_forever().

Typical use:

server = await serve(...)
# this coroutine doesn't return
# canceling it stops the server
await server.serve_forever()

This is an alternative to using serve() as an asynchronous context manager. Shutdown is triggered by canceling serve_forever() instead of exiting a serve() context.

sockets#

See asyncio.Server.sockets.

Using a connection#

class websockets.server.WebSocketServerProtocol(ws_handler, ws_server, *, logger=None, origins=None, extensions=None, subprotocols=None, extra_headers=None, server_header='Python/x.y.z websockets/X.Y', process_request=None, select_subprotocol=None, ping_interval=20, ping_timeout=20, close_timeout=10, max_size=2**20, max_queue=2**5, read_limit=2**16, write_limit=2**16)[source]#

WebSocket server connection.

WebSocketServerProtocol provides recv() and send() coroutines for receiving and sending messages.

It supports asynchronous iteration to receive messages:

async for message in websocket:
    await process(message)

The iterator exits normally when the connection is closed with close code 1000 (OK) or 1001 (going away) or without a close code. It raises a ConnectionClosedError when the connection is closed with any other code.

You may customize the opening handshake in a subclass by overriding process_request() or select_subprotocol().

Parameters:

ws_server (WebSocketServer) – WebSocket server that created this connection.

See serve() for the documentation of ws_handler, logger, origins, extensions, subprotocols, extra_headers, and server_header.

See WebSocketCommonProtocol for the documentation of ping_interval, ping_timeout, close_timeout, max_size, max_queue, read_limit, and write_limit.

await recv()[source]#

Receive the next message.

When the connection is closed, recv() raises ConnectionClosed. Specifically, it raises ConnectionClosedOK after a normal connection closure and ConnectionClosedError after a protocol error or a network failure. This is how you detect the end of the message stream.

Canceling recv() is safe. There’s no risk of losing the next message. The next invocation of recv() will return it.

This makes it possible to enforce a timeout by wrapping recv() in timeout() or wait_for().

Returns:

A string (str) for a Text frame. A bytestring (bytes) for a Binary frame.

Raises:
Return type:

str | bytes

await send(message)[source]#

Send a message.

A string (str) is sent as a Text frame. A bytestring or bytes-like object (bytes, bytearray, or memoryview) is sent as a Binary frame.

send() also accepts an iterable or an asynchronous iterable of strings, bytestrings, or bytes-like objects to enable fragmentation. Each item is treated as a message fragment and sent in its own frame. All items must be of the same type, or else send() will raise a TypeError and the connection will be closed.

send() rejects dict-like objects because this is often an error. (If you want to send the keys of a dict-like object as fragments, call its keys() method and pass the result to send().)

Canceling send() is discouraged. Instead, you should close the connection with close(). Indeed, there are only two situations where send() may yield control to the event loop and then get canceled; in both cases, close() has the same effect and is more clear:

  1. The write buffer is full. If you don’t want to wait until enough data is sent, your only alternative is to close the connection. close() will likely time out then abort the TCP connection.

  2. message is an asynchronous iterator that yields control. Stopping in the middle of a fragmented message will cause a protocol error and the connection will be closed.

When the connection is closed, send() raises ConnectionClosed. Specifically, it raises ConnectionClosedOK after a normal connection closure and ConnectionClosedError after a protocol error or a network failure.

Parameters:

message (str | bytes | Iterable[str | bytes] | AsyncIterable[str | bytes]) – Message to send.

Raises:
await close(code=CloseCode.NORMAL_CLOSURE, reason='')[source]#

Perform the closing handshake.

close() waits for the other end to complete the handshake and for the TCP connection to terminate. As a consequence, there’s no need to await wait_closed() after close().

close() is idempotent: it doesn’t do anything once the connection is closed.

Wrapping close() in create_task() is safe, given that errors during connection termination aren’t particularly useful.

Canceling close() is discouraged. If it takes too long, you can set a shorter close_timeout. If you don’t want to wait, let the Python process exit, then the OS will take care of closing the TCP connection.

Parameters:

  • code (int) – WebSocket close code.

  • reason (str) – WebSocket close reason.

await wait_closed()[source]#

Wait until the connection is closed.

This coroutine is identical to the closed attribute, except it can be awaited.

This can make it easier to detect connection termination, regardless of its cause, in tasks that interact with the WebSocket connection.

await ping(data=None)[source]#

Send a Ping.

A ping may serve as a keepalive, as a check that the remote endpoint received all messages up to this point, or to measure latency.

Canceling ping() is discouraged. If ping() doesn’t return immediately, it means the write buffer is full. If you don’t want to wait, you should close the connection.

Canceling the Future returned by ping() has no effect.

Parameters:

data (str | bytes | None) – Payload of the ping. A string will be encoded to UTF-8. If data is None, the payload is four random bytes.

Returns:

A future that will be completed when the corresponding pong is received. You can ignore it if you don’t intend to wait. The result of the future is the latency of the connection in seconds.

pong_waiter = await ws.ping()
# only if you want to wait for the corresponding pong
latency = await pong_waiter

Raises:

  • ConnectionClosed – When the connection is closed.

  • RuntimeError – If another ping was sent with the same data and the corresponding pong wasn’t received yet.

Return type:

Awaitable[float]

await pong(data=b'')[source]#

Send a Pong.

An unsolicited pong may serve as a unidirectional heartbeat.

Canceling pong() is discouraged. If pong() doesn’t return immediately, it means the write buffer is full. If you don’t want to wait, you should close the connection.

Parameters:

data (str | bytes) – Payload of the pong. A string will be encoded to UTF-8.

Raises:

ConnectionClosed – When the connection is closed.

You can customize the opening handshake in a subclass by overriding these methods:

await process_request(path, request_headers)[source]#

Intercept the HTTP request and return an HTTP response if appropriate.

You may override this method in a WebSocketServerProtocol subclass, for example:

  • to return an HTTP 200 OK response on a given path; then a load balancer can use this path for a health check;

  • to authenticate the request and return an HTTP 401 Unauthorized or an HTTP 403 Forbidden when authentication fails.

You may also override this method with the process_request argument of serve() and WebSocketServerProtocol. This is equivalent, except process_request won’t have access to the protocol instance, so it can’t store information for later use.

process_request() is expected to complete quickly. If it may run for a long time, then it should await wait_closed() and exit if wait_closed() completes, or else it could prevent the server from shutting down.

Parameters:

  • path (str) – Request path, including optional query string.

  • request_headers (Headers) – Request headers.

Returns:

None to continue the WebSocket handshake normally.

An HTTP response, represented by a 3-uple of the response status, headers, and body, to abort the WebSocket handshake and return that HTTP response instead.

Return type:

Tuple[StatusLike, HeadersLike, bytes] | None

select_subprotocol(client_subprotocols, server_subprotocols)[source]#

Pick a subprotocol among those supported by the client and the server.

If several subprotocols are available, select the preferred subprotocol by giving equal weight to the preferences of the client and the server.

If no subprotocol is available, proceed without a subprotocol.

You may provide a select_subprotocol argument to serve() or WebSocketServerProtocol to override this logic. For example, you could reject the handshake if the client doesn’t support a particular subprotocol, rather than accept the handshake without that subprotocol.

Parameters:
Returns:

Selected subprotocol, if a common subprotocol was found.

None to continue without a subprotocol.

Return type:

Subprotocol | None

WebSocket connection objects also provide these attributes:

id: uuid.UUID#

Unique identifier of the connection. Useful in logs.

logger: LoggerLike#

Logger for this connection.

property local_address: Any#

Local address of the connection.

For IPv4 connections, this is a (host, port) tuple.

The format of the address depends on the address family; see getsockname().

None if the TCP connection isn’t established yet.

property remote_address: Any#

Remote address of the connection.

For IPv4 connections, this is a (host, port) tuple.

The format of the address depends on the address family; see getpeername().

None if the TCP connection isn’t established yet.

property open: bool#

True when the connection is open; False otherwise.

This attribute may be used to detect disconnections. However, this approach is discouraged per the EAFP principle. Instead, you should handle ConnectionClosed exceptions.

property closed: bool#

True when the connection is closed; False otherwise.

Be aware that both open and closed are False during the opening and closing sequences.

latency: float#

Latency of the connection, in seconds.

This value is updated after sending a ping frame and receiving a matching pong frame. Before the first ping, latency is 0.

By default, websockets enables a keepalive mechanism that sends ping frames automatically at regular intervals. You can also send ping frames and measure latency with ping().

The following attributes are available after the opening handshake, once the WebSocket connection is open:

path: str#

Path of the opening handshake request.

request_headers: Headers#

Opening handshake request headers.

response_headers: Headers#

Opening handshake response headers.

subprotocol: Subprotocol | None#

Subprotocol, if one was negotiated.

The following attributes are available after the closing handshake, once the WebSocket connection is closed:

property close_code: int | None#

WebSocket close code, defined in section 7.1.5 of RFC 6455.

None if the connection isn’t closed yet.

property close_reason: str | None#

WebSocket close reason, defined in section 7.1.6 of RFC 6455.

None if the connection isn’t closed yet.

Basic authentication#

websockets supports HTTP Basic Authentication according to RFC 7235 and RFC 7617.

websockets.auth.basic_auth_protocol_factory(realm=None, credentials=None, check_credentials=None, create_protocol=None)[source]#

Protocol factory that enforces HTTP Basic Auth.

basic_auth_protocol_factory() is designed to integrate with serve() like this:

websockets.serve(
    ...,
    create_protocol=websockets.basic_auth_protocol_factory(
        realm="my dev server",
        credentials=("hello", "iloveyou"),
    )
)
Parameters:

  • realm (str | None) – Scope of protection. It should contain only ASCII characters because the encoding of non-ASCII characters is undefined. Refer to section 2.2 of RFC 7235 for details.

  • credentials (Tuple[str, str] | Iterable[Tuple[str, str]] | None) – Hard coded authorized credentials. It can be a (username, password) pair or a list of such pairs.

  • check_credentials (Callable[[str, str], Awaitable[bool]] | None) – Coroutine that verifies credentials. It receives username and password arguments and returns a bool. One of credentials or check_credentials must be provided but not both.

  • create_protocol (Callable[[...], BasicAuthWebSocketServerProtocol] | None) – Factory that creates the protocol. By default, this is BasicAuthWebSocketServerProtocol. It can be replaced by a subclass.

Raises:

TypeError – If the credentials or check_credentials argument is wrong.

class websockets.auth.BasicAuthWebSocketServerProtocol(*args, realm=None, check_credentials=None, **kwargs)[source]#

WebSocket server protocol that enforces HTTP Basic Auth.

realm: str = ''#

Scope of protection.

If provided, it should contain only ASCII characters because the encoding of non-ASCII characters is undefined.

username: str | None = None#

Username of the authenticated user.

await check_credentials(username, password)[source]#

Check whether credentials are authorized.

This coroutine may be overridden in a subclass, for example to authenticate against a database or an external service.

Parameters:

  • username (str) – HTTP Basic Auth username.

  • password (str) – HTTP Basic Auth password.

Returns:

True if the handshake should continue; False if it should fail with an HTTP 401 error.

Return type:

bool

Broadcast#

websockets.broadcast(websockets, message, raise_exceptions=False)[source]#

Broadcast a message to several WebSocket connections.

A string (str) is sent as a Text frame. A bytestring or bytes-like object (bytes, bytearray, or memoryview) is sent as a Binary frame.

broadcast() pushes the message synchronously to all connections even if their write buffers are overflowing. There’s no backpressure.

If you broadcast messages faster than a connection can handle them, messages will pile up in its write buffer until the connection times out. Keep ping_interval and ping_timeout low to prevent excessive memory usage from slow connections.

Unlike send(), broadcast() doesn’t support sending fragmented messages. Indeed, fragmentation is useful for sending large messages without buffering them in memory, while broadcast() buffers one copy per connection as fast as possible.

broadcast() skips connections that aren’t open in order to avoid errors on connections where the closing handshake is in progress.

broadcast() ignores failures to write the message on some connections. It continues writing to other connections. On Python 3.11 and above, you may set raise_exceptions to True to record failures and raise all exceptions in a PEP 654 ExceptionGroup.

Parameters:

  • websockets (Iterable[WebSocketCommonProtocol]) – WebSocket connections to which the message will be sent.

  • message (str | bytes) – Message to send.

  • raise_exceptions (bool) – Whether to raise an exception in case of failures.

Raises:

TypeError – If message doesn’t have a supported type.