The Music Library Data Structures

This page contains a thorough introduction to the data structures used for the music library items[1]. The data structures are implemented in the soco.data_structures module and they are used to represent the metadata for music items, such as music tracks, albums, genres and playlists.

Many music related items have a lot of metadata in common. For example, a music track and an album may both have artist and title metadata. It is therefore possible and useful to derive a hierarchy of items, and to implement them as a class hierarchy. The hierarchy which Sonos has adopted is represented by the DIDL Lite xml schema (DIDL stands for ‘Digital Item Description Language’. For more details, see the UPnP specifications (PDF).

In the data_structures module, each class represents a particular DIDL-Lite object and is illustrated in the figure below. The black lines are the lines of inheritance, going from left to right.

Inheritance diagram of soco.data_structures

All data structures are subclasses of the abstract Didl Object item class. You should never need to instantiate this directly. The subclasses are divided into Containers and Items. In general, Containers are things, like playlists, which are intended to contain other items.

At the bottom of the class hierarchy are 10 types of DIDL items. On each of these classes, relevant metadata items are available as attributes (though they may be implemented as properties). Each has a title, a URI, an item id and a UPnP class. Some have other attributes. For example, DidlMusicTrack and DidlMusicAlbum have some extra fields such as album, album_art_uri and creator.

One of the more important attributes which each class has is didl_metadata. It is used to produce the metadata that is sent to the Sonos® units in the form of XML. This metadata is created in an almost identical way for each class, which is why it is implemented in DidlObject. It uses the URI, the UPnP class and the title that the items are instantiated with, along with the two class variables parent_id and _translation.


[1]Text of the first footnote.