discogs: revise position regexp, add tests, tox

Revise the regular expression used for extracting information from a
discogs position, making the subtrack_index only be matched if one of
the other groups is matched as well. Split the definition into several
lines in order to try to document it a bit more clearly.
Add unit tests for position parsing and non standard positions.
Fix the underscore on tox dependency.

beetsplug/discogs.py

@@ -416,17 +416,30 @@ class DiscogsPlugin(BeetsPlugin):
         """Returns the medium, medium index and subtrack index for a discogs
         track position.
         """
-        # TODO: revise comment
-        # medium_index is a number at the end of position. medium is everything
-        # else. E.g. (A)(1), (Side A, Track )(1), (A)(), ()(1), etc.
-        match = re.match(r'^(.*?)(\d*?)(\.[\w]+|[A-Z]+)?$', position.upper())
+        # Match the standard Discogs positions (12.2.9), which can have several
+        # forms (1, 1-1, A1, A1.1, A1a, ...).
+        match = re.match(
+            r'^(.*?)'           # medium: everything before medium_index.
+            r'(\d*?)'           # medium_index: a number at the end of
+                                # `position`, except if followed by a subtrack
+                                # index.
+                                # subtrack_index: can only be matched if medium
+                                # or medium_index have been matched, and can be
+            r'((?<=\w)\.[\w]+'  # - a dot followed by a string (A.1, 2.A)
+            r'|(?<=\d)[A-Z]+'   # - a string that follows a number (1A, B2a)
+            r')?'
+            r'$',
+            position.upper()
+        )
+
         if match:
             medium, index, subindex = match.groups()
+
             if subindex and subindex.startswith('.'):
                 subindex = subindex[1:]
         else:
             self._log.debug(u'Invalid position: {0}', position)
-            medium = index, subindex = None
+            medium = index = subindex = None
         return medium or None, index or None, subindex or None
 
     def get_track_length(self, duration):

test/test_discogs.py

@@ -92,6 +92,26 @@ class DGAlbumInfoTest(_common.TestCase):
         self.assertEqual(t[0].media, d.media)
         self.assertEqual(t[1].media, d.media)
 
+    def test_parse_position(self):
+        """Test the conversion of discogs `position` to medium, medium_index
+        and subtrack_index."""
+        # List of tuples (discogs_position, (medium, medium_index, subindex)
+        positions = [('1',       (None,   '1',  None)),
+                     ('A12',     ('A',    '12', None)),
+                     ('12-34',   ('12-',  '34', None)),
+                     ('CD1-1',   ('CD1-', '1',  None)),
+                     ('1.12',    (None,   '1',  '12')),
+                     ('12.a',    (None,   '12', 'A')),
+                     ('12.34',   (None,   '12', '34')),
+                     ('1ab',     (None,   '1',  'AB')),
+                     # Non-standard
+                     ('IV',      ('IV',   None, None)),
+                     ]
+
+        d = DiscogsPlugin()
+        for position, expected in positions:
+            self.assertEqual(d.get_track_index(position), expected)
+
     def test_parse_tracklist_without_sides(self):
         """Test standard Discogs position 12.2.9#1: "without sides"."""
         release = self._make_release_from_positions(['1', '2', '3'])
@@ -125,6 +145,14 @@ class DGAlbumInfoTest(_common.TestCase):
         self.assertEqual(d.mediums, 3)
         self.assertEqual(len(d.tracks), 4)
 
+    def test_parse_tracklist_non_standard(self):
+        """Test non standard Discogs position."""
+        release = self._make_release_from_positions(['I', 'II', 'III', 'IV'])
+        d = DiscogsPlugin().get_album_info(release)
+
+        self.assertEqual(d.mediums, 1)
+        self.assertEqual(len(d.tracks), 4)
+
     def test_parse_tracklist_subtracks_dot(self):
         """Test standard Discogs position 12.2.9#5: "sub tracks, dots"."""
         release = self._make_release_from_positions(['1', '2.1', '2.2', '3'])

tox.ini

@@ -23,7 +23,7 @@ deps =
     jellyfish
     python-mpd2
     coverage
-    discogs_client
+    discogs-client
 
 [_flake8]
 deps =