Prevent exception spam during interpreter teardown

[apmorton]

Minimal reproducer:

import sys

from ipykernel.comm import Comm

# simply construct a Comm object
Comm(show_warning=False)

# someone somewhere in some other module somehow takes a copy of the modules dict
MODULES_COPY = dict(sys.modules)

Slightly more real-world example (how we actually got here):

import sys

# someone somewhere is using dash
import dash

# someone somewhere in some other module somehow takes a copy of the modules dict
MODULES_COPY = dict(sys.modules)

These will both output:

Exception ignored in: <function BaseComm.__del__ at 0x7f6608e32480>
Traceback (most recent call last):
  File "comm/base_comm.py", line 68, in __del__
  File "comm/base_comm.py", line 104, in close
  File "ipykernel/comm/comm.py", line 26, in publish_msg
AttributeError: 'NoneType' object has no attribute 'initialized'

The relevant line is:

if not Kernel.initialized():
    return

where Kernel is:

from ipykernel.kernelbase import Kernel

Due to a series of unfortunate events, the __del__ for Comm is being called after the partial teardown of the ipykernel.kernelbase module, as can be seen from the output of python -v:

# cleanup[3] wiping ipykernel.kernelbase
...
# cleanup[3] wiping comm
Exception ignored in: <function BaseComm.__del__ at 0x7f6608e32480>
Traceback (most recent call last):
  File "comm/base_comm.py", line 68, in __del__
  File "comm/base_comm.py", line 104, in close
  File "ipykernel/comm/comm.py", line 26, in publish_msg
AttributeError: 'NoneType' object has no attribute 'initialized'

The above testing was done in a conda environment with:

comm-0.1.4-pyhd8ed1ab_0
dash-2.14.2-pyhd8ed1ab_0
ipykernel-6.26.0-pyhf8b6a83_0
python-3.11.6-hab00c5b_0_cpython

[blink1073]

Thank you!