allow notebooks to exit early (e.g. with sys.exit(0) or pytest.skip(reason, allow_module_level=True))

[tswast]

Is your feature request related to a problem? Please describe.

I have some notebooks that I test with multiple versions of Python, but they are not compatible with Python 3.13. I would love to be able to exit early from these notebooks without causing the test to be reported as a failure.

Describe the solution you'd like

I have tried the following cells:

import sys

# Python 3.13 is not yet a supported runtime.
if sys.version_info >= (3, 13, 0):
    sys.exit(0)

also

import sys

# Python 3.13 is not yet a supported runtime.
if sys.version_info >= (3, 13, 0):
    import pytest

    pytest.skip("Python 3.13 not supported", allow_module_level=True)

I would expect at least the latter to work, if not both. Note: one can disambiguate if SystemExit exception is "ok" / intended from a failure by looking at SystemExit.code == 0.

Describe alternatives you've considered

• I've also tried creating an autouse fixture in a conftext.py file that wraps the yield in a try / except block to capture these expected exceptions, but it seems conftest.py fixtures don't run. Perhaps an alternative to consider to allow such flexibility.

• I've tried the "metadata": { "tags": [ "raises-exception" ] } on the cell, but this actually continues execution after the failed cell. I suspect this is desirable to have code cells that demonstrate exceptions, but it wasn't the right solution to be able to exit early.

Additional context

N/A

[tswast]

I think the solution will end up being somewhere around here:

nbmake/src/nbmake/nb_run.py

Lines 117 to 138 in 5de6e18

	error = self._get_error(nb)
	except CellTimeoutError as err:
	trace = err.args[0]
	error = NotebookError(
	summary=trace.split("\n")[0],
	trace=trace,
	failing_cell_index=self._get_timeout_cell(nb),
	)
	except Exception as err:
	# if at top causes https://github.com/jupyter/nbclient/issues/128
	# from jupyter_client.kernelspec import KernelSpecManager, NoSuchKernel
	# trace=f"{summary}\n\nInstalled Kernels: {str(KernelSpecManager().find_kernel_specs())}", # noqa
	# https://github.com/treebeardtech/nbmake/runs/1536896858?check_suite_focus=true
	if str(type(err)) != "<class 'jupyter_client.kernelspec.NoSuchKernel'>":
	raise err
	summary = f"Error - No such kernel: '{err.name}'" # type: ignore
	error = NotebookError(
	summary=summary,
	trace=f"{summary}",
	failing_cell_index=0,
	)

We could explicitly catch SystemExit (if exc.code == 0: pass; else error = NotebookError(...);) / the pytest.skip.Skipped exception. Though, maybe those end up being wrapped by one of those CellExecutionErrors?