A beets plugin with a boosted modify command to add/remove values in multi-value tags

[Morikko]

Hi all,

I needed to modify multi-value tags atomically, like adding or removing an element without modifying the others. Also, I wanted to be able to use a string with a chosen separator.

Both combined were required to leverage navidrome custom tags splitting behavior since v0.55.0. I started by using it to batch add some grouping like RIP CD, Original Soundtrack, Game Soundtrack, Christmas... But other fields may benefit from it like genre.

Preview:

# Initial: genre: Rock

# Add a value
beet multimodify grouping+="Hard Rock" <query>
# genre: Rock,Hard Rock

# Add and remove values
beet mmod genre+="Classic Rock" genre-="Hard Rock" <query>
# genre: Rock,Classic Rock

# Original modify command still applies
beet mmod genre+="Classic Rock" genre-="Hard Rock" year! title="Best song"

As it is very useful, I decided to finalize and share it. Enjoy :)

Link: https://github.com/Morikko/beets-multivalue