Sphinx 8.1: numpydoc test failures related to _exception_on_warning

[rossbar]

Describe the bug

Numpydoc experienced test-suite failures in a mocked sphinx-app due to missing a private _exception_on_warning attribute. This started happening with sphinx 8.1 and is likely related to #12743.

Generally speaking, I don't think this is a problem with sphinx just a change in the underlying logic related to warningiserror and _exception_on_warning. It's likely the behavior change is intentional - in that case please close this. I just thought I'd raise it in case this is unexpected.

The fix in numpydoc is to explicitly add the _exception_on_warning attribute to the mocked sphinx app instance: numpy/numpydoc#586.

How to Reproduce

pip install numpydoc[test]==1.8.0 sphinx>=8.1
pytest --pyargs numpydoc

Environment Information

Package                       Version
----------------------------- -----------
alabaster                     1.0.0
babel                         2.16.0
certifi                       2024.8.30
charset-normalizer            3.4.0
contourpy                     1.3.0
coverage                      7.6.3
cycler                        0.12.1
docutils                      0.21.2
fonttools                     4.54.1
idna                          3.10
imagesize                     1.4.1
iniconfig                     2.0.0
Jinja2                        3.1.4
kiwisolver                    1.4.7
MarkupSafe                    3.0.1
matplotlib                    3.9.2
numpy                         2.1.2
numpydoc                      1.8.0
packaging                     24.1
pillow                        11.0.0
pip                           24.2
pluggy                        1.5.0
Pygments                      2.18.0
pyparsing                     3.2.0
pytest                        8.3.3
pytest-cov                    5.0.0
python-dateutil               2.9.0.post0
requests                      2.32.3
six                           1.16.0
snowballstemmer               2.2.0
Sphinx                        8.1.3
sphinxcontrib-applehelp       2.0.0
sphinxcontrib-devhelp         2.0.0
sphinxcontrib-htmlhelp        2.1.0
sphinxcontrib-jsmath          1.0.1
sphinxcontrib-qthelp          2.0.0
sphinxcontrib-serializinghtml 2.0.0
tabulate                      0.9.0
urllib3                       2.2.3

Sphinx extensions

numpydoc

Additional context

No response

[AA-Turner]

Indeed, mocking sphinx.application.Sphinx isn't a supported use-case, so I don't consider this a bug -- but adding the private _exception_on_warning attribute is the correct resolution.

A