ValueError: No entry_points plugins could be loaded for 'nbcheck.exec' on Linux

[lbianchi-lbl]

This occurs when calling pytest --nbcheck=exec, resulting in the following error on Linux, but seemingly not on Windows (at least with GitHub Actions runners):

INTERNALERROR> Traceback (most recent call last):
INTERNALERROR>   File "/opt/hostedtoolcache/Python/3.8.17/x64/lib/python3.8/site-packages/_pytest/main.py", line 266, in wrap_session
INTERNALERROR>     config._do_configure()
INTERNALERROR>   File "/opt/hostedtoolcache/Python/3.8.17/x64/lib/python3.8/site-packages/_pytest/config/__init__.py", line 1054, in _do_configure
INTERNALERROR>     self.hook.pytest_configure.call_historic(kwargs=dict(config=self))
INTERNALERROR>   File "/opt/hostedtoolcache/Python/3.8.17/x64/lib/python3.8/site-packages/pluggy/_hooks.py", line 452, in call_historic
INTERNALERROR>     res = self._hookexec(self.name, self._hookimpls, kwargs, False)
INTERNALERROR>   File "/opt/hostedtoolcache/Python/3.8.17/x64/lib/python3.8/site-packages/pluggy/_manager.py", line 112, in _hookexec
INTERNALERROR>     return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
INTERNALERROR>   File "/opt/hostedtoolcache/Python/3.8.17/x64/lib/python3.8/site-packages/pluggy/_callers.py", line 116, in _multicall
INTERNALERROR>     raise exception.with_traceback(exception.__traceback__)
INTERNALERROR>   File "/opt/hostedtoolcache/Python/3.8.17/x64/lib/python3.8/site-packages/pluggy/_callers.py", line 80, in _multicall
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/opt/hostedtoolcache/Python/3.8.17/x64/lib/python3.8/site-packages/nbcheck/plugin.py", line 48, in pytest_configure
INTERNALERROR>     raise ValueError(f"No entry_points plugins could be loaded for {entry_point_group_name!r}")
INTERNALERROR> ValueError: No entry_points plugins could be loaded for 'nbcheck.exec'

After researching possible causes, including:

• pytest
• setuptools
• importlib-resources
• importlib-metadata
• pluggy

it seems the cause could be the updates between pluggy 1.0.0 and 1.2.0: pytest-dev/pluggy@1.0.0...1.2.0, in the sense that the same code in the same environment works when pluggy 1.0.0 is installed, and has the error above for 1.2.0.

The actual cause of the error is:

• The pytest_configure() hook in nbcheck.plugin is for some reason run twice, leading to the error being raised the second time since no setuptools (entry points) plugins are added via load_setuptools_plugins as they were already registered
• I'm not 100% sure why this only happens for nbcheck.exec and not nbcheck.static, but my guess is that it might have something to do with nbcheck.execution defining its own pytest_configure() hook with tryfirst=True
• As to why this happens on Linux but not on Windows: 🤷‍♂️

At any rate, just reworking the after-loading verification logic, and using a warning instead of raising an exception just to be on the safe side, seem to work to resolve this (b4e2f91/0.2.2).