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.

See also

Custom Messages
The messaging specification section on comms

Opening a comm from the kernel

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 extension is loaded. This example shows a frontend comm target registered in a registry:

    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.
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.

Opening a comm from the frontend

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, msg):
    # comm is the kernel Comm instance
    # msg is the comm_open message

    # Register handler for later messages
    def _recv(msg):
        # Use msg['content']['data'] for the data in the message

    # Send data to the frontend
    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:

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) {