Add and register a comm to register *other* comms

ipykernel/comm/comms.py

@@ -0,0 +1,17 @@
+REGISTER_TARGET_COMM_TARGET_NAME = 'register_target'
+
+
+def configure_comm(comm, comm_id, msg_callback):
+    comm.comm_id = comm_id
+    comm.on_msg(msg_callback)
+
+
+def register_target_comm_open(comm, open_msg):
+    # Register a new comm target.
+    def msg_callback(msg):
+        data = msg['content']['data']
+        comm.kernel.comm_manager.register_target(
+            target_name=data['new_target_name'],
+            f=data['comm_open_callback']
+        )
+    configure_comm(comm, comm_id='', msg_callback=msg_callback)

ipykernel/ipkernel.py

@@ -8,7 +8,7 @@
 from IPython.utils.tokenutil import token_at_cursor, line_at_cursor
 from traitlets import Instance, Type, Any, List, Bool
 
-from .comm import CommManager
+from .comm import CommManager, comms
 from .kernelbase import Kernel as KernelBase
 from .zmqshell import ZMQInteractiveShell
 
@@ -70,6 +70,11 @@ def __init__(self, **kwargs):
         for msg_type in comm_msg_types:
             self.shell_handlers[msg_type] = getattr(self.comm_manager, msg_type)
 
+        self.comm_manager.register_target(
+            target_name=comms.REGISTER_TARGET_COMM_TARGET_NAME,
+            f=comms.register_target_comm_open
+        )
+
     help_links = List([
         {
             'text': "Python Reference",