mbsync: Double output

Recently the mbsync plugin has started to produce double output.

$ beet mbsync -p black sabbath vol 4
Black Sabbath - Vol 4 - Wheels of Confusion
 script:  -> Latn
 language:  -> eng
 script:  -> Latn
 language:  -> eng

...snip...

Black Sabbath - Vol 4 - Under the Sun
 script:  -> Latn
 language:  -> eng
 script:  -> Latn
 language:  -> eng

A quick git bisect reveals this was introduced in 72515448ad505ef0b73c0d0aa4aa424419b55755.

It looks like this could just be a cosmetic issue but I haven’t had time to dive into the new code from #2988 to confirm yet.

[wisp3rwind]

I am a bit confused how this is an issue though, as all_keys already converts model_keys to a set…

The call trace is actually slightly different, you looked at beets.library.FormattedItemMapping.__iter__, what’s actually being iterated over in show_model_changes is not this formatted view, but the beets.library.Item directly. That in turn inherits beets.dbcore.db.Model.__iter__, which just returns iter(self.keys()).

So it appears to be correct that Item.keys() should always return a list, but if with_album=True, it should remove duplicates first.

In addition, I have the impression that show_model_changes can be improved performance-wise. I guess it’s correct that it iterates over the item rather than the formatted view, since it should compare the actual values, not any formatted output. However, it creates a new view for each key by calling Item.formatted(). It should keep this view around instead.

[jackwilsdon]

@FichteFoll it looks like this:

https://github.com/beetbox/beets/blob/c4347960eab78be2ad54f4e35b32928c47e287d1/beetsplug/mbsync.py#L161

Calls show_model_changes, which iterates over fields:

https://github.com/beetbox/beets/blob/c4347960eab78be2ad54f4e35b32928c47e287d1/beets/ui/__init__.py#L713

Which are returned from __iter__:

https://github.com/beetbox/beets/blob/c4347960eab78be2ad54f4e35b32928c47e287d1/beets/library.py#L432-L433

And it seems like all_keys is an amalgamation of model_keys and album_keys:

https://github.com/beetbox/beets/blob/c4347960eab78be2ad54f4e35b32928c47e287d1/beets/library.py#L385-L387

And model_keys is populated from the keys method:

https://github.com/beetbox/beets/blob/c4347960eab78be2ad54f4e35b32928c47e287d1/beets/library.py#L382

I am a bit confused how this is an issue though, as all_keys already converts model_keys to a set…