cannot modify albumartist on single files

[azrdev]

beet modify $query albumartist!
fails with the following backtrace:

Modifying 9 items.
Traceback (most recent call last):
  File "/usr/local/bin/beet", line 9, in <module>
    load_entry_point('beets==1.3.9', 'console_scripts', 'beet')()
  File "/usr/local/lib/python2.7/dist-packages/beets/ui/__init__.py", line 935, in main
    _raw_main(args)
  File "/usr/local/lib/python2.7/dist-packages/beets/ui/__init__.py", line 925, in _raw_main
    subcommand.func(lib, suboptions, subargs)
  File "/usr/local/lib/python2.7/dist-packages/beets/ui/commands.py", line 1327, in modify_func
    not opts.yes)
  File "/usr/local/lib/python2.7/dist-packages/beets/ui/commands.py", line 1265, in modify_items
    del obj[field]
  File "/usr/local/lib/python2.7/dist-packages/beets/dbcore/db.py", line 256, in __delitem__
    raise KeyError('fixed field {0} cannot be deleted'.format(key))
KeyError: 'fixed field albumartist cannot be deleted'

#561 already states that this wouldn't work, but not that it crashes.

[sampsyo]

Thanks for pointing this out. The error is actually related to deletion, not track-level scoping. It's currently not possible to delete built-in fields (see #919 for some background). Something like beet modify albumartist=foo should work fine, though, since item-level fields are a superset of album-level fields.

I've just pushed a fix that avoids the crash in the modify command when this comes up. This, of course, is only a stopgap until we finish full-fledged deletion.