Comms allow custom messages between the frontend and the kernel. They are used, for instance, in ipywidgets to update widget state.

A comm consists of a pair of objects, in the kernel and the frontend, with an automatically assigned unique ID. When one side sends a message, a callback on the other side is triggered with that message data. Either side, the frontend or kernel, can open or close the comm.

.. seealso::

    `Custom Messages <https://jupyter-client.readthedocs.io/en/latest/messaging.html#custom-messages>`__
      The messaging specification section on comms

First, the function to accept the comm must be available on the frontend. This can either be specified in a requirejs module, or registered in a registry, for example when an :doc:`extension <extending/frontend_extensions>` is loaded. This example shows a frontend comm target registered in a registry:

Jupyter.notebook.kernel.comm_manager.register_target('my_comm_target',
    function(comm, msg) {
        // comm is the frontend comm instance
        // msg is the comm_open message, which can carry data

        // Register handlers for later messages:
        comm.on_msg(function(msg) {...});
        comm.on_close(function(msg) {...});
        comm.send({'foo': 0});
    });

Now that the frontend comm is registered, you can open the comm from the kernel:

from ipykernel.comm import Comm

# Use comm to send a message from the kernel
my_comm = Comm(target_name='my_comm_target', data={'foo': 1})
my_comm.send({'foo': 2})

# Add a callback for received messages.
@my_comm.on_msg
def _recv(msg):
    # Use msg['content']['data'] for the data in the message

This example uses the IPython kernel; it's up to each language kernel what API, if any, it offers for using comms.

This is very similar to above, but in reverse. First, a comm target must be registered in the kernel. For instance, this may be done by code displaying output: it will register a target in the kernel, and then display output containing Javascript to connect to it.

def target_func(comm, open_msg):
    # comm is the kernel Comm instance
    # msg is the comm_open message

    # Register handler for later messages
    @comm.on_msg
    def _recv(msg):
        # Use msg['content']['data'] for the data in the message
        comm.send({'echo': msg['content']['data']})

    # Send data to the frontend on creation
    comm.send({'foo': 5})

get_ipython().kernel.comm_manager.register_target('my_comm_target', target_func)

This example uses the IPython kernel again; this example will be different in other kernels that support comms. Refer to the specific language kernel's documentation for comms support.

And then open the comm from the frontend:

const comm = Jupyter.notebook.kernel.comm_manager.new_comm('my_comm_target', {'foo': 6})
// Send data
comm.send({'foo': 7})

// Register a handler
comm.on_msg(function(msg) {
    console.log(msg.content.data.foo);
});