Plugins can't extend MediaFile anymore

Backported from #607.
See https://github.com/sampsyo/beets/pull/607#issuecomment-38387100

This will be return in #644.

beets/plugins.py

@@ -19,7 +19,6 @@ import traceback
 from collections import defaultdict
 
 import beets
-from beets import mediafile
 
 PLUGIN_NAMESPACE = 'beetsplug'
 
@@ -40,7 +39,6 @@ class BeetsPlugin(object):
     def __init__(self, name=None):
         """Perform one-time plugin setup.
         """
-        _add_media_fields(self.item_fields())
         self.import_stages = []
         self.name = name or self.__module__.split('.')[-1]
         self.config = beets.config[self.name]
@@ -86,14 +84,6 @@ class BeetsPlugin(object):
         """
         return ()
 
-    def item_fields(self):
-        """Returns field descriptors to be added to the MediaFile class,
-        in the form of a dictionary whose keys are field names and whose
-        values are descriptor (e.g., MediaField) instances. The Library
-        database schema is not (currently) extended.
-        """
-        return {}
-
     def album_for_id(self, album_id):
         """Return an AlbumInfo object or None if no matching release was
         found.
@@ -297,13 +287,6 @@ def template_funcs():
             funcs.update(plugin.template_funcs)
     return funcs
 
-def _add_media_fields(fields):
-    """Adds a {name: descriptor} dictionary of fields to the MediaFile
-    class. Called during the plugin initialization.
-    """
-    for key, value in fields.iteritems():
-        setattr(mediafile.MediaFile, key, value)
-
 def import_stages():
     """Get a list of import stage functions defined by plugins."""
     stages = []

docs/dev/plugins.rst

@@ -297,41 +297,6 @@ This field works for *item* templates. Similarly, you can register *album*
 template fields by adding a function accepting an ``Album`` argument to the
 ``album_template_fields`` dict.
 
-Extend MediaFile
-^^^^^^^^^^^^^^^^
-
-`MediaFile`_ is the file tag abstraction layer that beets uses to make
-cross-format metadata manipulation simple. Plugins can add fields to MediaFile
-to extend the kinds of metadata that they can easily manage.
-
-The ``item_fields`` method on plugins should be overridden to return a
-dictionary whose keys are field names and whose values are descriptor objects
-that provide the field in question. The descriptors should probably be
-``MediaField`` instances (defined in ``beets.mediafile``). Here's an example
-plugin that provides a meaningless new field "foo"::
-
-    from beets import mediafile, plugins, ui
-    class FooPlugin(plugins.BeetsPlugin):
-        def item_fields(self):
-            return {
-                'foo': mediafile.MediaField(
-                    mp3 = mediafile.StorageStyle(
-                        'TXXX', id3_desc=u'Foo Field'),
-                    mp4 = mediafile.StorageStyle(
-                        '----:com.apple.iTunes:Foo Field'),
-                    etc = mediafile.StorageStyle('FOO FIELD')
-                ),
-            }
-
-Later, the plugin can manipulate this new field by saying something like
-``mf.foo = 'bar'`` where ``mf`` is a ``MediaFile`` instance.
-
-Note that, currently, these additional fields are *only* applied to
-``MediaFile`` itself. The beets library database schema and the ``Item`` class
-are not extended, so the fields are second-class citizens. This may change
-eventually.
-
-.. _MediaFile: https://github.com/sampsyo/beets/wiki/MediaFile
 
 Add Import Pipeline Stages
 ^^^^^^^^^^^^^^^^^^^^^^^^^^