CI: Fail build on "UserWarning: Code input with no code at..."

[rhshadrach]

Blank lines in ipython blocks cause the warning "UserWarning: Code input with no code at..." to be emitted when building the docs. #41400 removed the blank lines so there are currently no warnings, but they will likely reappear, specially since double blank lines around classes/functions looks correct. Should we fail the doc build on such a warning (not sure why that doesn't already happen) or enforce this through style checks?

[jorisvandenbossche]

I think that would indeed be good to fail the doc build on such a warning.

not sure why that doesn't already happen

The doc build is configured to fail for any Sphinx warning, but this is a plain python warning from the ipyhon directive (https://github.com/ipython/ipython/blob/0e4d6390b2174fb1b352a082b72ad387ae696e87/IPython/sphinxext/ipython_directive.py#L1016-L1023)

A relatively easy fix might actually be to fix this upstream and to make that warnings.warn(..) into a sphinx logger.warning(..) so we can use the standard sphinx machinery to catch this.
Somewhat related issue about having exceptions turned into sphinx logger's warnings/errors: ipython/ipython#11547

[rhshadrach]

Thanks @jorisvandenbossche! I hope to find some time to follow up on this and put in a PR to ipython in a few weeks.

[rhshadrach]

ipython/ipython#13030 has been merged, to be included in 7.26. Will test after 7.26 is released and can then close this issue.

[MrMino]

@rhshadrach Update: 7.26 was released today.

[rhshadrach]

Thanks @MrMino! Confirmed the CI fails when warnings are present in #42858.

[jorisvandenbossche]

Thanks @rhshadrach for fixing this on the IPython side!