ID3v2.3 multi-value field separator is always "/"

[Moonbase59]

For some libraries, users might decide to store ID3v2.3 tags for greater compatibility with older software (as opposed to the ID3v2.4 default).

Now ID3v2.4 allows storing multiple tags of the same name (for multi-value entries like artists, genres and so on) and ID3v2.3 traditionally used a slash "/" character (which brought us the "AC/DC problem").

People (and tagging software like MusicBrainz Picard) have started to support other separators to store multiple values on one ID3v2.3 field. Typical separators used today are the semicolon ;, the vertical bar |, a comma , the NULL character \0 and even character sequences like semicolon blank ; for better readability. The underlying Mutagen supports all of these by specifying v23_sep.

This is a missing feature in MediaFile (and beets) and I’d like to add it to the MediaFile class so it could also be supported via a beets config entry:

class MediaFile(object):
    """Represents a multimedia file on disk and provides access to its
    metadata.
    """
    def __init__(self, path, id3v23=False, id3v23_sep='/'):
        """Constructs a new `MediaFile` reflecting the file at path. May
        throw `UnreadableFileError`.

        By default, MP3 files are saved with ID3v2.4 tags. You can use
        the older ID3v2.3 standard by specifying the `id3v23` option
        and (optionally) the desired separator `id3v23_sep` for
        multi-value fields (defaults to a slash character).

        To use a NULL character as terminator, pass `id3v23_sep=None`.
        """

Objections?
Would it need a u'/' instead?

N.B.: Making this a core config entry in beets should also help some plugins who currently specify their own separators.

[sampsyo]

Sounds pretty reasonable to me, despite my general resistance to additional configuration options. Because Mutagen has first-class support, I can see this being effective. Here are a few miscellaneous comments:

• MediaFile and certainly beets currently don't have great support for multi-valued tags in the first place, so realizing the full potential (e.g., superseding those plugin configuration options) will not happen automatically.
• Any particular reason to have None stand in for the null byte? It seems more predictable to let users specify '\0' if they want to use the null byte.
• One general question: how much does ID3v2.3 really matter? There are certainly tools that are still stuck on ID3v2.3, but I don't currently have a clear picture of which ones are really important. Windows Explorer, for example, finally added v2.4 support in 2017, AFAICT.

[Moonbase59]

• Yeah, multi-value tags in MP3s are very badly supported—everywhere I look. Maybe due to all these separator problems in ID3v2.3. ID3v2.4 was supposed to better this (and UTF-8), but it came too late so nobody cared too much anymore.

• I’m also not too happy with None for the NULL byte, but that’s how Mutagen decided to do it.

• You’d think everybody uses ID3v2.4 now, but actually the overwhelming majority of the files out there are still ID3v2.3, for many different reasons. My own library of about 150k tracks also still has some 120k MP3s tagged using ID3v2.3, for compatibility reasons.

Would it break anything if we started out with the extra param in MediaFile and then later care about it in beets?

[JuniorJPDJ]

Would it break anything if we started out with the extra param in MediaFile and then later care about it in beets?

It's the idea I would like to see as it's the most reasonable IMO.