As detailed here:
https://github.com/beetbox/beets/issues/2406#issuecomment-274423601
In a *function-style* definition, we didn't properly *un-define* the
values for a given item after each function invocation. So when a field
wasn't defined, it would get the value for the previously-formatted
object instead. It now properly throws a NameError.
Include import of __future__ features division, absolute_imports and
print_function everywhere. Don't add unicode_literals yet for it is
harder to convert.
Goal is smoothing the transition to python 3.
A second base class, LibModel, maintains a reference to the Library and should
take care of database-related tasks like load and store. This is the beginning
of the end of the terrible incongruity between Item and Album objects (only
the latter had a library reference). More refactoring to come.
One large side effect: Album objects no longer automatically store
modifications. You have to call album.store(). Several places in the code
assume otherwise; they need cleaning up.
ResultIterator is now polymorphic (it takes a type parameter, which must be a
subclass of LibModel).
An earlier change (due to @pedros) added the ability for plugins to define
template fields that work with Albums as well as Items. This enables some
cool new use cases but required that every template field definition check the
type of its arguments. Instead, this iteration on the idea distinguishes
between fields meant for Items and those meant for Albums.
In addition to simplifying the implementation of these functions, this also
enables the creation of album fields with identical names to item fields.
(For example, a user contacted me recently about adding a $bitrate field for
albums, which would be the average bitrate of the items. They can do this now
using a plugin.)
I also changed the docs to stop using the decorator approach to registering
template fields. We're moving toward removing those.
I'm transitioning to using exclusively instance-level fields instead of
class-level fields in plugin objects, but I neglected to bring inline and
rewrite into the future. This manifested as silent inaction on the part of
these plugins.
This change restores the old behavior (for compatibility) but also updates the
plugins to use the new behavior.
This should be backwards compatible. In case the the path field
isn't a statement, beets will assume it's a block of code that
stores the value in a special '_' variable.
