VOUnit to issue VOWarning rather than generic UnitsWarning

astropy.io.votable.exceptions.W50 is defined to be the warning for units that do not adhere to the VO standards: https://www.ivoa.net/documents/VOUnits

yet, astropy.units.format.vounit.VOUnit issues a generic UnitsWarning for the exact same reason.

This is somewhat unexpected in practice, and while in the example below we could ignore both, I don’t think that’s the right approach:

https://github.com/astropy/astroquery/issues/2352

[eerovaher]

If we look at the code that parses the VOTable then we see that it should already properly replace any UnitsWarning instances with W50: https://github.com/astropy/astropy/blob/acdefa2cdde7b1dd5923e7acece15eca7812d127/astropy/io/votable/tree.py#L1390-L1404 The fact that we still get an UnitsWarning despite parse_strict='silent' means that there is a bug that would not be fixed by changing the warnings hierarchy, so that does not sound like a solution we should pursue. Indeed, the underlying bug can be easily confirmed if we try out different unit formats. With e.g. format='cds' everything works as expected:

>>> from astropy import units as u
>>> u.Unit('e-/s', format='cds', parse_strict='raise')
...
ValueError: 'e-/s' did not parse as cds unit: Syntax error If this is meant to be a custom unit...
>>> u.Unit('e-/s', format='cds', parse_strict='warn')
WARNING: UnitsWarning: 'e-/s' did not parse as cds unit: Syntax error If this is meant to be a custom unit...
UnrecognizedUnit(e-/s)
>>> u.Unit('e-/s', format='cds', parse_strict='silent')
UnrecognizedUnit(e-/s)

But now with format='vounit':

>>> u.Unit('e-/s', format='vounit', parse_strict='raise')
WARNING: UnitsWarning: Unit 'e' not supported by the VOUnit standard.  [astropy.units.format.vounit]
...
ValueError: 'e-/s' did not parse as vounit unit: Invalid character at col 1 If this is meant to be a custom unit...
>>> u.Unit('e-/s', format='vounit', parse_strict='warn')
WARNING: UnitsWarning: Unit 'e' not supported by the VOUnit standard.  [astropy.units.format.vounit]
WARNING: UnitsWarning: 'e-/s' did not parse as vounit unit: Invalid character at col 1 If this is meant to be a custom unit...
UnrecognizedUnit(e-/s)
>>> u.Unit('e-/s', format='vounit', parse_strict='silent')
WARNING: UnitsWarning: Unit 'e' not supported by the VOUnit standard.  [astropy.units.format.vounit]
UnrecognizedUnit(e-/s)

We can see that using format='vounit' always results in an additional warning, which is the real bug that needs to be fixed.

[eerovaher]

Turns out the bug we uncovered here was reported already years ago: #6302