Fix Comm interface for downstream users

.github/workflows/downstream.yml

@@ -25,6 +25,20 @@ jobs:
           package_name: nbclient
           env_values: IPYKERNEL_CELL_NAME=\<IPY-INPUT\>
 
+  ipywidgets:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Base Setup
+        uses: jupyterlab/maintainer-tools/.github/actions/base-setup@v1
+
+      - name: Run Test
+        uses: jupyterlab/maintainer-tools/.github/actions/downstream-test@v1
+        with:
+          package_name: ipywidgets
+
   jupyter_client:
     runs-on: ubuntu-latest
     steps:
@@ -69,3 +83,18 @@ jobs:
           cd jupyter_kernel_test
           pip install -e ".[test]"
           python test_ipykernel.py
+
+  downstream_check: # This job does nothing and is only used for the branch protection
+    if: always()
+    needs:
+      - nbclient
+      - ipywidgets
+      - jupyter_client
+      - ipyparallel
+      - jupyter_kernel_test
+    runs-on: ubuntu-latest
+    steps:
+      - name: Decide whether the needed jobs succeeded or failed
+        uses: re-actors/alls-green@release/v1
+        with:
+          jobs: ${{ toJSON(needs) }}

ipykernel/comm/comm.py

@@ -3,10 +3,12 @@
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
+import uuid
 from typing import Optional
 
 import comm.base_comm
 import traitlets.config
+from traitlets import Bool, Bytes, Instance, Unicode, default
 
 from ipykernel.jsonutil import json_clean
 from ipykernel.kernelbase import Kernel
@@ -43,8 +45,30 @@ def publish_msg(self, msg_type, data=None, metadata=None, buffers=None, **keys):
 class Comm(traitlets.config.LoggingConfigurable, BaseComm):
     """Class for communicating between a Frontend and a Kernel"""
 
+    kernel = Instance("ipykernel.kernelbase.Kernel", allow_none=True)  # type:ignore[assignment]
+    comm_id = Unicode()
+    primary = Bool(True, help="Am I the primary or secondary Comm?")
+
+    target_name = Unicode("comm")
+    target_module = Unicode(
+        None,
+        allow_none=True,
+        help="""requirejs module from
+        which to load comm target.""",
+    )
+
+    topic = Bytes()
+
+    @default("kernel")
+    def _default_kernel(self):
+        if Kernel.initialized():
+            return Kernel.instance()
+
+    @default("comm_id")
+    def _default_comm_id(self):
+        return uuid.uuid4().hex
+
     def __init__(self, *args, **kwargs):
-        self.kernel = None
         # Comm takes positional arguments, LoggingConfigurable does not, so we explicitly forward arguments
         traitlets.config.LoggingConfigurable.__init__(self, **kwargs)
         BaseComm.__init__(self, *args, **kwargs)

ipykernel/comm/manager.py

@@ -5,10 +5,16 @@
 
 
 import comm.base_comm
+import traitlets
 import traitlets.config
 
 
 class CommManager(traitlets.config.LoggingConfigurable, comm.base_comm.CommManager):
+
+    kernel = traitlets.Instance("ipykernel.kernelbase.Kernel")
+    comms = traitlets.Dict()
+    targets = traitlets.Dict()
+
     def __init__(self, **kwargs):
         # CommManager doesn't take arguments, so we explicitly forward arguments
         traitlets.config.LoggingConfigurable.__init__(self, **kwargs)