title: abstracting the conversion between id3 tags, m4a tags, flac tags
body: <p>I'm looking for a resource in python or bash that will make it easy to take, for example, mp3 file X and m4a file Y and say "copy X's tags to Y".</p>
  
  <p>Python's "mutagen" module is great for manupulating tags in general, but there's no abstract concept of "artist field" that spans different types of tag; I want a library that handles all the fiddly bits and knows fieldname equivalences. For things not all tag systems can express, I'm okay with information being lost or best-guessed.</p>
  
  <p>(Use case: I encode lossless files to mp3, then go use the mp3s for listening. Every month or so, I want to be able to update the 'master' lossless files with whatever tag changes I've made to the mp3s. I'm tired of stubbing my toes on implementation differences among formats.)</p>