plugin API to extend MediaFile (#324)

beets/plugins.py

@@ -19,6 +19,8 @@ import itertools
 import traceback
 from collections import defaultdict
 
+from beets import mediafile
+
 PLUGIN_NAMESPACE = 'beetsplug'
 DEFAULT_PLUGINS = []
 
@@ -36,6 +38,12 @@ class BeetsPlugin(object):
     functionality by defining a subclass of BeetsPlugin and overriding
     the abstract methods defined here.
     """
+    def __init__(self):
+        """Perform one-time plugin setup. There is probably no reason to
+        override this method.
+        """
+        _add_media_fields(self.item_fields())
+
     def commands(self):
         """Should return a list of beets.ui.Subcommand objects for
         commands that should be added to beets' CLI.
@@ -72,6 +80,14 @@ class BeetsPlugin(object):
         """
         pass
 
+    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 {}
+
     listeners = None
 
     @classmethod
@@ -246,6 +262,13 @@ def template_values(item):
                 values[name] = unicode(func(item))
     return values
 
+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)
+
 
 # Event dispatch.
 

docs/changelog.rst

@@ -22,6 +22,8 @@ Changelog
 * Filename extensions are now always lower-cased when copying and moving files.
 * The ``inline`` plugin now prints a more comprehensible error when exceptions
   occur in Python snippets.
+* New plugin API: plugins can now add fields to the MediaFile tag abstraction
+  layer. See :ref:`writing-plugins`.
 * A reasonable error message is now shown when the import log file cannot be
   opened.
 * Fix a bug in the ``rewrite`` plugin that broke the use of multiple rules for

docs/plugins/index.rst

@@ -301,3 +301,39 @@ that adds a ``$disc_and_track`` field::
 
 With this plugin enabled, templates can reference ``$disc_and_track`` as they
 can any standard metadata field.
+
+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