Add Discogs and general source weight penalty (configurable, undocumented).

Discogs (and other individual plugins) should return a penalty between
0.0 and 0.1, which will be multiplied by the general source weight. This
allows individual plugins to be weighted against relative to each other,
and amplified together against other penalty categories (track, title,
etc.)

beets/config_default.yaml

@@ -73,6 +73,7 @@ match:
         tracklength: strong
         tracknumber: strong
     weight:
+        source: 3.0
         artist: 3.0
         album: 3.0
         track: 1.0
@@ -85,3 +86,6 @@ match:
         track_length_max: 30
         track_length: 2.0
         track_id: 5.0
+
+discogs:
+    source_weight: 0.5

beetsplug/discogs.py

@@ -15,6 +15,7 @@
 """Adds Discogs album search support to the autotagger. Requires the
 discogs-client library.
 """
+from beets import config
 from beets.autotag.hooks import AlbumInfo, TrackInfo
 from beets.autotag.match import current_metadata, VA_ARTISTS
 from beets.plugins import BeetsPlugin
@@ -27,6 +28,10 @@ import time
 
 log = logging.getLogger('beets')
 
+# Distance parameters.
+DISCOGS_SOURCE_WEIGHT = config['discogs']['source_weight'].as_number()
+SOURCE_WEIGHT = config['match']['weight']['source'].as_number()
+
 # Silence spurious INFO log lines generated by urllib3.
 urllib3_logger = logging.getLogger('requests.packages.urllib3')
 urllib3_logger.setLevel(logging.CRITICAL)
@@ -36,6 +41,11 @@ discogs_client.user_agent = 'beets/%s +http://beets.radbox.org/' % \
     beets.__version__
 
 class DiscogsPlugin(BeetsPlugin):
+    def album_distance(self, items, album_info, mapping):
+        """Returns the discogs source weight and the maximum source weight.
+        """
+        return DISCOGS_SOURCE_WEIGHT * SOURCE_WEIGHT, SOURCE_WEIGHT
+
     def candidates(self, items, artist, album, va_likely):
         """Returns a list of AlbumInfo objects for discogs search results
         matching an album and artist (if not various).