multi-valued tags for artists + albumartists sort and credits

[jmbannon]

As title, for the PR: beetbox/beets#4743

On Ubuntu, I'm hitting a pre-existing test failure on WAVETest.test_read_audio_properties when reading the bitrate tag: 88200 != 705600

[sampsyo]

Hi there! Thanks for getting this started!

I notice that you have used "plural" names for the underlying on-disk metadata formats for each field here. That means that, for example, artist_sort will use completely different metadata from artists_sort: one will never affect the other. Is this intentional? Is there external software you have in mind that is expecting these fields to work this way?

It isn't how we do it for other fields that have both "singular" and "plural" equivalents. For example, here is the mapping for the genre singular field, which is based on genres (plural):

mediafile/mediafile.py

Lines 1811 to 1817 in 57a330e

	genres = ListMediaField(
	MP3ListStorageStyle('TCON'),
	MP4ListStorageStyle('\xa9gen'),
	ListStorageStyle('GENRE'),
	ASFStorageStyle('WM/Genre'),
	)
	genre = genres.single_field()

It uses the same underlying storage as genres, meaning that we stay interoperable with other software using those fields. Would that be appropriate here?

I realize we don't do this for artists, but perhaps we should…

[sampsyo]

Looks good overall! I wouldn't worry too much about the bitrate mismatch; it likely has to do with the underlying version of Mutagen (but in any case it's not a big deal).

Could you please add a quick changelog entry here?
https://github.com/beetbox/mediafile/blob/master/docs/index.rst#changelog

Then we should be good to go!

[jmbannon]

@sampsyo Done!

[sampsyo]

Excellent; thanks!

[jmbannon]

When you get some time @sampsyo , mind updating pypi with 0.11.1? Then I can update the beets PR's pip dep with this change

[sampsyo]

I've actually published it as v0.12.0 (following the SemVer recommendation to bump the minor version for new features). Thanks again!