Adjusting the default behavior for writing tags when adding music

[jtpavlock]

Currently, Moe reads and writes tags when music is added to the library by default. This can produce unexpected results as tags can be interpreted and written differently than initially read based on how the underlying tag infrastructure (mediafile and mutagen) operate, as seen in #332.

This made me wonder if writing tags when adding music should be the default behavior or not. Of note, Moe will write tags based on how it's underlying tag infrastructure reads the tags, and ultimately how the track is represented in the library database. Having a mismatch between a track's actual tags, and how Moe is able to actually read and write its tags, may produce some unexpected behavior in future write operations if the user decides not to just accept Moe's interpretation of the music when first adding to the library.

[jpfieber]

After looking at the mediafile documentation, I see:

For fields that have multiple possible storage keys, MediaFile optimizes for interoperability: it accepts any of the possible storage keys and writes all of them. This may result in duplicated information in the tags, but it ensures that other players with slightly divergent opinions on tag names will all be able to interact with beets_.

This is consistant with the tag changes I saw happen on my files. Though I feared Musicbrainz data was being pulled in, the tags that were added were apparently done so for compatability, based on existing tag data, and the tags that were modified had their 'formatting' changed, but still retained the same information.

With a better understanding of what's happening, I'm less nervous about the changes being made, but can still understand there may be people that don't want ANY changes made. I would be fine with 'compatability' changes being made to tags on import/add by default, but think there should be a way (ideally through config/env variable and one time command argument) where someone can change that behavior to prevent those changes should they want/need complete control over their tags.

[jtpavlock]

That's a good find from the documentation and makes sense with what we saw. I'm also leaning towards compatibility changes being okay as a default behavior.