Incompatibility with IPython >= 9.0.0

[ELC]

Metakernel has a "hidden" dependency on IPython, hidden because it is not explicitly stated in the pyproject.toml:

metakernel/pyproject.toml

Lines 17 to 22 in 2de5d37

	dependencies = [
	"ipykernel >=5.5.6,<7",
	"jupyter_core >=4.9.2",
	"pexpect >=4.8",
	"jedi >=0.18",
	]

However, there are parts of the code that explicitly import from IPython:

metakernel/metakernel/_metakernel.py

Lines 35 to 42 in 2de5d37

	try:
	from IPython.utils.PyColorize import NeutralColors
	RED = NeutralColors.colors["header"]
	NORMAL = NeutralColors.colors["normal"]
	except:
	from IPython.core.excolors import TermColors
	RED = TermColors.Red
	NORMAL = TermColors.Normal

This is relevant in this case because in the latest release of IPython, v9.0.0, there was a massive refactor on how colors are managed:
ipython/ipython@a796b95

This changes are backwards incompatible and make kernels derived from metakernel failed. I encounter this in the octave-kernel:

Traceback (most recent call last):
  File "<frozen runpy>", line 189, in _run_module_as_main
  File "<frozen runpy>", line 148, in _get_module_details
  File "<frozen runpy>", line 112, in _get_module_details
  File "/home/ubuntu/.pyenv/versions/3.11.9/lib/python3.11/site-packages/octave_kernel/__init__.py", line 4, in <module>
    from .kernel import OctaveKernel
  File "/home/ubuntu/.pyenv/versions/3.11.9/lib/python3.11/site-packages/octave_kernel/kernel.py", line 19, in <module>
    from metakernel import MetaKernel, ProcessMetaKernel, REPLWrapper, u, MetaKernelApp
  File "/home/ubuntu/.pyenv/versions/3.11.9/lib/python3.11/site-packages/metakernel/__init__.py", line 2, in <module>
    from ._metakernel import (
  File "/home/ubuntu/.pyenv/versions/3.11.9/lib/python3.11/site-packages/metakernel/_metakernel.py", line 40, in <module>
    from IPython.core.excolors import TermColors
ModuleNotFoundError: No module named 'IPython.core.excolors'
[I 2025-03-02 16:28:56.633 ServerApp] AsyncIOLoopKernelRestarter: restarting kernel (5/5), new random ports
Traceback (most recent call last):
  File "/home/ubuntu/.pyenv/versions/3.11.9/lib/python3.11/site-packages/metakernel/_metakernel.py", line 36, in <module>
    from IPython.utils.PyColorize import NeutralColors
ImportError: cannot import name 'NeutralColors' from 'IPython.utils.PyColorize' (/home/ubuntu/.pyenv/versions/3.11.9/lib/python3.11/site-packages/IPython/utils/PyColorize.py)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<frozen runpy>", line 189, in _run_module_as_main
  File "<frozen runpy>", line 148, in _get_module_details
  File "<frozen runpy>", line 112, in _get_module_details
  File "/home/ubuntu/.pyenv/versions/3.11.9/lib/python3.11/site-packages/octave_kernel/__init__.py", line 4, in <module>
    from .kernel import OctaveKernel
  File "/home/ubuntu/.pyenv/versions/3.11.9/lib/python3.11/site-packages/octave_kernel/kernel.py", line 19, in <module>
    from metakernel import MetaKernel, ProcessMetaKernel, REPLWrapper, u, MetaKernelApp
  File "/home/ubuntu/.pyenv/versions/3.11.9/lib/python3.11/site-packages/metakernel/__init__.py", line 2, in <module>
    from ._metakernel import (
  File "/home/ubuntu/.pyenv/versions/3.11.9/lib/python3.11/site-packages/metakernel/_metakernel.py", line 40, in <module>
    from IPython.core.excolors import TermColors
ModuleNotFoundError: No module named 'IPython.core.excolors'

There is a need for a patch in metakernel where the version is set strictly to IPython < 9.0.0 and a future fix where the code is refactored so that it is compatible with IPython ~= 9.0.0