MPDStats Plugin: handle update_rating when item is None / start tests for mpdstats

[CodyReichert]

I had a small issue where I would play a song with MPD that wasn't imported by beets. So, when mpdstats tried to update the rating it would throw up an AttributeError that looks like:

mpdstats: skipped /media/music/tagged/Z-RO/Relvis Presley/09 One More Time.mp3

Traceback (most recent call last):
  File "/usr/bin/beet", line 9, in <module>
    load_entry_point('beets==1.3.14', 'console_scripts', 'beet')()
  File "/home/cody/src/beets/beets/ui/__init__.py", line 1104, in main
    _raw_main(args)
  File "/home/cody/src/beets/beets/ui/__init__.py", line 1094, in _raw_main
    subcommand.func(lib, suboptions, subargs)
  File "/home/cody/src/beets/beetsplug/mpdstats.py", line 360, in func
    MPDStats(lib, self._log).run()
  File "/home/cody/src/beets/beetsplug/mpdstats.py", line 306, in run
    handler(status)
  File "/home/cody/src/beets/beetsplug/mpdstats.py", line 253, in on_stop
    self.handle_song_change(self.now_playing)
  File "/home/cody/src/beets/beetsplug/mpdstats.py", line 233, in handle_song_change
    self.update_rating(song['beets_item'], skipped)
  File "/home/cody/src/beets/beetsplug/mpdstats.py", line 206, in update_rating
    item.load()
AttributeError: 'NoneType' object has no attribute 'load'

Obviously the better fix would be to import the song with beets ;) - but since MPD can play songs that beets may or may not have, it would be good to sanity check it and not kill the plugin.

Also started some tests for mpdstats which checks this!

[sampsyo]

Can you add a note to the docstring here? Something like

`item` can either be a beets `Item` or None, in which case nothing changes

[CodyReichert]

Good idea - done!

[sampsyo]

Awesome! Thanks for getting to the bottom of this, and even adding tests. 😃 ✨ 🐟

It looks like this case is already handled for the other events: handle_played and hanlde_skipped call update_item immediately, which already has the null check. Maybe this is worth documenting to make this clearer—the item in the song dict can always be None, so these utilities had better have checks.

[CodyReichert]

Not a problem!

I did see where update_item handled the None item, so I tried to keep it consistent since update_rating needs to load the item as well.

[CodyReichert]

Alright, squashed commits - and hopefully that fixes the tests!

[sampsyo]

Great! Thank you!