miguelgrinberg.python-socketio/docs/client.rst

The Socket.IO Clients
=====================

This package contains two Socket.IO clients:

- a "simple" client, which provides a straightforward API that is sufficient
  for most applications
- an "event-driven" client, which provides access to all the features of the
  Socket.IO protocol

Each of these clients comes in two variants: one for the standard Python
library, and another for asynchronous applications built with the ``asyncio``
package.

Installation
------------

To install the standard Python client along with its dependencies, use the
following command::

    pip install "python-socketio[client]"

If instead you plan on using the ``asyncio`` client, then use this::

    pip install "python-socketio[asyncio_client]"

Using the Simple Client
-----------------------

The advantage of the simple client is that it abstracts away the logic required
to maintain a Socket.IO connection. This client handles disconnections and
reconnections in a completely transparent way, without adding any complexity to
the application.

Creating a Client Instance
~~~~~~~~~~~~~~~~~~~~~~~~~~

The easiest way to create a Socket.IO client is to use the context manager
interface::

    import socketio

    # standard Python
    with socketio.SimpleClient() as sio:
        # ... connect to a server and use the client
        # ... no need to manually disconnect!

    # asyncio
    async with socketio.AsyncSimpleClient() as sio:
        # ... connect to a server and use the client
        # ... no need to manually disconnect!


With this usage the context manager will ensure that the client is properly
disconnected before exiting the ``with`` or ``async with`` block.

If preferred, a client can be manually instantiated::

    import socketio

    # standard Python
    sio = socketio.SimpleClient()

    # asyncio
    sio = socketio.AsyncSimpleClient()

Connecting to a Server
~~~~~~~~~~~~~~~~~~~~~~

The connection to a server is established by calling the ``connect()``
method::

    sio.connect('http://localhost:5000')

In the case of the ``asyncio`` client, the method is a coroutine::

    await sio.connect('http://localhost:5000')

By default the client first connects to the server using the long-polling
transport, and then attempts to upgrade the connection to use WebSocket. To
connect directly using WebSocket, use the ``transports`` argument::

    sio.connect('http://localhost:5000', transports=['websocket'])

Upon connection, the server assigns the client a unique session identifier.
The application can find this identifier in the ``sid`` attribute::

    print('my sid is', sio.sid)

The Socket.IO transport that is used in the connection can be obtained from the
``transport`` attribute::

    print('my transport is', sio.transport)

The transport is given as a string, and can be either ``'websocket'`` or
``'polling'``.

TLS/SSL Support
^^^^^^^^^^^^^^^

The client supports TLS/SSL connections. To enable it, use a ``https://``
connection URL::

    sio.connect('https://example.com')

Or when using ``asyncio``::

    await sio.connect('https://example.com')

The client verifies server certificates by default. Consult the documentation
for the event-driven client for information on how to customize this behavior.

Emitting Events
~~~~~~~~~~~~~~~

The client can emit an event to the server using the ``emit()`` method::

    sio.emit('my message', {'foo': 'bar'})

Or in the case of ``asyncio``, as a coroutine::

    await sio.emit('my message', {'foo': 'bar'})

The arguments provided to the method are the name of the event to emit and the
optional data that is passed on to the server. The data can be of type ``str``,
``bytes``, ``dict``, ``list`` or ``tuple``. When sending a ``list`` or a
``tuple``, the elements in it need to be of any allowed types except ``tuple``.
When a tuple is used, the elements of the tuple will be passed as individual
arguments to the server-side event handler function.

Receiving Events
~~~~~~~~~~~~~~~~

The client can wait for the server to emit an event with the ``receive()``
method::

    event = sio.receive()
    print(f'received event: "{event[0]}" with arguments {event[1:]}')

When using ``asyncio``, this method needs to be awaited::

    event = await sio.receive()
    print(f'received event: "{event[0]}" with arguments {event[1:]}')

The return value of ``receive()`` is a list. The first element of this list is
the event name, while the remaining elements are the arguments passed by the
server.

With the usage shown above, the ``receive()`` method will return only when an
event is received from the server. An optional timeout in seconds can be passed
to prevent the client from waiting forever::

    from socketio.exceptions import TimeoutError

    try:
        event = sio.receive(timeout=5)
    except TimeoutError:
        print('timed out waiting for event')
    else:
        print('received event:', event)

Or with ``asyncio``::

    from socketio.exceptions import TimeoutError

    try:
        event = await sio.receive(timeout=5)
    except TimeoutError:
        print('timed out waiting for event')
    else:
        print('received event:', event)

Disconnecting from the Server
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

At any time the client can request to be disconnected from the server by
invoking the ``disconnect()`` method::

    sio.disconnect()

For the ``asyncio`` client this is a coroutine::

    await sio.disconnect()

Debugging and Troubleshooting
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To help you debug issues, the client can be configured to output logs to the
terminal::

    import socketio

    # standard Python
    sio = socketio.Client(logger=True, engineio_logger=True)

    # asyncio
    sio = socketio.AsyncClient(logger=True, engineio_logger=True)

The ``logger`` argument controls logging related to the Socket.IO protocol,
while ``engineio_logger`` controls logs that originate in the low-level
Engine.IO transport. These arguments can be set to ``True`` to output logs to
``stderr``, or to an object compatible with Python's ``logging`` package
where the logs should be emitted to. A value of ``False`` disables logging.

Logging can help identify the cause of connection problems, unexpected
disconnections and other issues.

Using the Event-Driven Client
-----------------------------

Creating a Client Instance
~~~~~~~~~~~~~~~~~~~~~~~~~~

To instantiate an Socket.IO client, simply create an instance of the
appropriate client class::

    import socketio

    # standard Python
    sio = socketio.Client()

    # asyncio
    sio = socketio.AsyncClient()

Defining Event Handlers
~~~~~~~~~~~~~~~~~~~~~~~

The Socket.IO protocol is event based. When a server wants to communicate with
a client it *emits* an event. Each event has a name, and a list of
arguments. The client registers event handler functions with the
:func:`socketio.Client.event` or :func:`socketio.Client.on` decorators::

    @sio.event
    def message(data):
        print('I received a message!')

    @sio.on('my message')
    def on_message(data):
        print('I received a message!')

In the first example the event name is obtained from the name of the
handler function. The second example is slightly more verbose, but it
allows the event name to be different than the function name or to include
characters that are illegal in function names, such as spaces.

For the ``asyncio`` client, event handlers can be regular functions as above,
or can also be coroutines::

    @sio.event
    async def message(data):
        print('I received a message!')

If the server includes arguments with an event, those are passed to the
handler function as arguments.

Catch-All Event and Namespace Handlers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A "catch-all" event handler is invoked for any events that do not have an
event handler. You can define a catch-all handler using ``'*'`` as event name::

   @sio.on('*')
   def any_event(event, sid, data):
        pass

Asyncio servers can also use a coroutine::

   @sio.on('*')
   async def any_event(event, sid, data):
       pass

A catch-all event handler receives the event name as a first argument. The
remaining arguments are the same as for a regular event handler.

The ``connect`` and ``disconnect`` events have to be defined explicitly and are
not invoked on a catch-all event handler.

Similarily, a "catch-all" namespace handler is invoked for any connected
namespaces that do not have an explicitly defined event handler. As with
catch-all events, ``'*'`` is used in place of a namespace::

   @sio.on('my_event', namespace='*')
   def my_event_any_namespace(namespace, sid, data):
       pass

For these events, the namespace is passed as first argument, followed by the
regular arguments of the event.

Lastly, it is also possible to define a "catch-all" handler for all events on
all namespaces::

   @sio.on('*', namespace='*')
   def any_event_any_namespace(event, namespace, sid, data):
       pass

Event handlers with catch-all events and namespaces receive the event name and
the namespace as first and second arguments.

Connect, Connect Error and Disconnect Event Handlers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``connect``, ``connect_error`` and ``disconnect`` events are special; they
are invoked automatically when a client connects or disconnects from the
server::

    @sio.event
    def connect():
        print("I'm connected!")

    @sio.event
    def connect_error(data):
        print("The connection failed!")

    @sio.event
    def disconnect(reason):
        print("I'm disconnected! reason:", reason)

The ``connect_error`` handler is invoked when a connection attempt fails. If
the server provides arguments, these are passed on to the handler. The server
can use an argument to provide information to the client regarding the
connection failure.

The ``disconnect`` handler is invoked for application initiated disconnects,
server initiated disconnects, or accidental disconnects, for example due to
networking failures. In the case of an accidental disconnection, the client is
going to attempt to reconnect immediately after invoking the disconnect
handler. As soon as the connection is re-established the connect handler will
be invoked once again. The handler receives a ``reason`` argument which
provides the cause of the disconnection::

    @sio.event
    def disconnect(reason):
        if reason == sio.reason.CLIENT_DISCONNECT:
            print('the client disconnected')
        elif reason == sio.reason.SERVER_DISCONNECT:
            print('the server disconnected the client')
        else:
            print('disconnect reason:', reason)

See the The :attr:`socketio.Client.reason` attribute for a list of possible
disconnection reasons.

The ``connect``, ``connect_error`` and ``disconnect`` events have to be
defined explicitly and are not invoked on a catch-all event handler.

Connecting to a Server
~~~~~~~~~~~~~~~~~~~~~~

The connection to a server is established by calling the ``connect()``
method::

    sio.connect('http://localhost:5000')

In the case of the ``asyncio`` client, the method is a coroutine::

    await sio.connect('http://localhost:5000')

Upon connection, the server assigns the client a unique session identifier.
The application can find this identifier in the ``sid`` attribute::

    print('my sid is', sio.sid)

The Socket.IO transport that is used in the connection can be obtained from the
``transport`` attribute::

    print('my transport is', sio.transport)

The transport is given as a string, and can be either ``'websocket'`` or
``'polling'``.

TLS/SSL Support
^^^^^^^^^^^^^^^

The client supports TLS/SSL connections. To enable it, use a ``https://``
connection URL::

    sio.connect('https://example.com')

Or when using ``asyncio``::

    await sio.connect('https://example.com')

The client will verify the server certificate by default. To disable
certificate verification, or to use other less common options such as client
certificates, the client must be initialized with a custom HTTP session object
that is configured with the desired TLS/SSL options.

The following example disables server certificate verification, which can be
useful when connecting to a server that uses a self-signed certificate::

    http_session = requests.Session()
    http_session.verify = False
    sio = socketio.Client(http_session=http_session)
    sio.connect('https://example.com')

And when using ``asyncio``::

    connector = aiohttp.TCPConnector(ssl=False)
    http_session = aiohttp.ClientSession(connector=connector)
    sio = socketio.AsyncClient(http_session=http_session)
    await sio.connect('https://example.com')

Instead of disabling certificate verification, you can provide a custom
certificate authority bundle to verify the certificate against::

    http_session = requests.Session()
    http_session.verify = '/path/to/ca.pem'
    sio = socketio.Client(http_session=http_session)
    sio.connect('https://example.com')

And for ``asyncio``::

    ssl_context = ssl.create_default_context()
    ssl_context.load_verify_locations('/path/to/ca.pem')
    connector = aiohttp.TCPConnector(ssl=ssl_context)
    http_session = aiohttp.ClientSession(connector=connector)
    sio = socketio.AsyncClient(http_session=http_session)
    await sio.connect('https://example.com')

Below you can see how to use a client certificate to authenticate against the
server::

    http_session = requests.Session()
    http_session.cert = ('/path/to/client/cert.pem', '/path/to/client/key.pem')
    sio = socketio.Client(http_session=http_session)
    sio.connect('https://example.com')

And for ``asyncio``::

    ssl_context = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
    ssl_context.load_cert_chain('/path/to/client/cert.pem',
                                '/path/to/client/key.pem')
    connector = aiohttp.TCPConnector(ssl=ssl_context)
    http_session = aiohttp.ClientSession(connector=connector)
    sio = socketio.AsyncClient(http_session=http_session)
    await sio.connect('https://example.com')

Emitting Events
~~~~~~~~~~~~~~~

The client can emit an event to the server using the ``emit()`` method::

    sio.emit('my message', {'foo': 'bar'})

Or in the case of ``asyncio``, as a coroutine::

    await sio.emit('my message', {'foo': 'bar'})

The arguments provided to the method are the name of the event to emit and the
optional data that is passed on to the server. The data can be of type ``str``,
``bytes``, ``dict``, ``list`` or ``tuple``. When sending a ``list`` or a
``tuple``, the elements in it need to be of any allowed types except ``tuple``.
When a tuple is used, the elements of the tuple will be passed as individual
arguments to the server-side event handler function.

The ``emit()`` method can be invoked inside an event handler as a response
to a server event, or in any other part of the application, including in
background tasks.

Event Callbacks
~~~~~~~~~~~~~~~

When a server emits an event to a client, it can optionally provide a
callback function, to be invoked as a way of acknowledgment that the server
has processed the event. While this is entirely managed by the server, the
client can provide a list of return values that are to be passed on to the
callback function set up by the server. This is achieved simply by returning
the desired values from the handler function::

    @sio.event
    def my_event(sid, data):
        # handle the message
        return "OK", 123

Likewise, the client can request a callback function to be invoked after the
server has processed an event. The :func:`socketio.Server.emit` method has an
optional ``callback`` argument that can be set to a callable. If this
argument is given, the callable will be invoked after the server has processed
the event, and any values returned by the server handler will be passed as
arguments to this function.

Namespaces
~~~~~~~~~~

The Socket.IO protocol supports multiple logical connections, all multiplexed
on the same physical connection. Clients can open multiple connections by
specifying a different *namespace* on each. Namespaces use a path syntax
starting with a forward slash. A list of namespaces can be given by the client
in the ``connect()`` call. For example, this example creates two logical
connections, the default one plus a second connection under the ``/chat``
namespace::

    sio.connect('http://localhost:5000', namespaces=['/chat'])

To define event handlers on a namespace, the ``namespace`` argument must be
added to the corresponding decorator::

    @sio.event(namespace='/chat')
    def my_custom_event(sid, data):
        pass

    @sio.on('connect', namespace='/chat')
    def on_connect():
        print("I'm connected to the /chat namespace!")

Likewise, the client can emit an event to the server on a namespace by
providing its in the ``emit()`` call::

    sio.emit('my message', {'foo': 'bar'}, namespace='/chat')

If the ``namespaces`` argument of the ``connect()`` call isn't given, any
namespaces used in event handlers are automatically connected.

Class-Based Namespaces
~~~~~~~~~~~~~~~~~~~~~~

As an alternative to the decorator-based event handlers, the event handlers
that belong to a namespace can be created as methods of a subclass of 
:class:`socketio.ClientNamespace`::

    class MyCustomNamespace(socketio.ClientNamespace):
        def on_connect(self):
            pass

        def on_disconnect(self, reason):
            pass

        def on_my_event(self, data):
            self.emit('my_response', data)

    sio.register_namespace(MyCustomNamespace('/chat'))

For asyncio based servers, namespaces must inherit from
:class:`socketio.AsyncClientNamespace`, and can define event handlers as
coroutines if desired::

    class MyCustomNamespace(socketio.AsyncClientNamespace):
        def on_connect(self):
            pass

        def on_disconnect(self, reason):
            pass

        async def on_my_event(self, data):
            await self.emit('my_response', data)

    sio.register_namespace(MyCustomNamespace('/chat'))

A catch-all class-based namespace handler can be defined by passing ``'*'`` as
the namespace during registration::

    sio.register_namespace(MyCustomNamespace('*'))

When class-based namespaces are used, any events received by the client are
dispatched to a method named as the event name with the ``on_`` prefix. For
example, event ``my_event`` will be handled by a method named ``on_my_event``.
If an event is received for which there is no corresponding method defined in
the namespace class, then the event is ignored. All event names used in
class-based namespaces must use characters that are legal in method names.

As a convenience to methods defined in a class-based namespace, the namespace
instance includes versions of several of the methods in the 
:class:`socketio.Client` and :class:`socketio.AsyncClient` classes that
default to the proper namespace when the ``namespace`` argument is not given.

In the case that an event has a handler in a class-based namespace, and also a
decorator-based function handler, only the standalone function handler is
invoked.

Disconnecting from the Server
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

At any time the client can request to be disconnected from the server by
invoking the ``disconnect()`` method::

    sio.disconnect()

For the ``asyncio`` client this is a coroutine::

    await sio.disconnect()

Managing Background Tasks
~~~~~~~~~~~~~~~~~~~~~~~~~

When a client connection to the server is established, a few background
tasks will be spawned to keep the connection alive and handle incoming
events. The application running on the main thread is free to do any
work, as this is not going to prevent the functioning of the Socket.IO
client.

If the application does not have anything to do in the main thread and
just wants to wait until the connection with the server ends, it can call
the ``wait()`` method::

    sio.wait()

Or in the ``asyncio`` version::

    await sio.wait()

For the convenience of the application, a helper function is provided to
start a custom background task::

    def my_background_task(my_argument):
        # do some background work here!
        pass

    task = sio.start_background_task(my_background_task, 123)

The arguments passed to this method are the background function and any
positional or keyword arguments to invoke the function with. 

Here is the ``asyncio`` version::

    async def my_background_task(my_argument):
        # do some background work here!
        pass

    task = sio.start_background_task(my_background_task, 123)

Note that this function is not a coroutine, since it does not wait for the
background function to end. The background function must be a coroutine.

The ``sleep()`` method is a second convenience function that is provided for
the benefit of applications working with background tasks of their own::

    sio.sleep(2)

Or for ``asyncio``::

    await sio.sleep(2)

The single argument passed to the method is the number of seconds to sleep
for.

Debugging and Troubleshooting
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To help you debug issues, the client can be configured to output logs to the
terminal::

    import socketio

    # standard Python
    sio = socketio.Client(logger=True, engineio_logger=True)

    # asyncio
    sio = socketio.AsyncClient(logger=True, engineio_logger=True)

The ``logger`` argument controls logging related to the Socket.IO protocol,
while ``engineio_logger`` controls logs that originate in the low-level
Engine.IO transport. These arguments can be set to ``True`` to output logs to
``stderr``, or to an object compatible with Python's ``logging`` package
where the logs should be emitted to. A value of ``False`` disables logging.

Logging can help identify the cause of connection problems, unexpected
disconnections and other issues.