fix field inference w/ null first item (closes #14 on GitHub)

When a partial match is found, its first item (task.items[0]) may be None, and
_infer_album_fields would crash in this case. This solution walks through the
items list and finds the first non-None item.

beets/importer.py

@@ -170,18 +170,25 @@ def _infer_album_fields(task):
     elif task.choice_flag == action.APPLY:
         # Applying autotagged metadata. Just get AA from the first
         # item.
-        if not task.items[0].albumartist:
-            changes['albumartist'] = task.items[0].artist
-        if not task.items[0].mb_albumartistid:
-            changes['mb_albumartistid'] = task.items[0].mb_artistid
+        for item in task.items:
+            if item is not None:
+                first_item = item
+                break
+        else:
+            assert False, "all items are None"
+        if not first_item.albumartist:
+            changes['albumartist'] = first_item.artist
+        if not first_item.mb_albumartistid:
+            changes['mb_albumartistid'] = first_item.mb_artistid
 
     else:
         assert False
 
     # Apply new metadata.
     for item in task.items:
-        for k, v in changes.iteritems():
-            setattr(item, k, v)
+        if item is not None:
+            for k, v in changes.iteritems():
+                setattr(item, k, v)
 
 def _open_state():
     """Reads the state file, returning a dictionary."""

docs/changelog.rst

@@ -13,6 +13,7 @@ This release focuses on making beets' path formatting vastly more powerful.
   functions via a plugin, see :ref:`writing-plugins`.
 * Fix an incompatibility in BPD with libmpc (the library that powers mpc and
   ncmpc).
+* Fix a crash when importing a partial match whose first track was missing.
 
 1.0b11 (December 12, 2011)
 --------------------------

test/test_importer.py

@@ -541,6 +541,13 @@ class InferAlbumDataTest(unittest.TestCase):
         self._infer()
         self.assertFalse(self.items[0].comp)
 
+    def test_first_item_null_apply(self):
+        self.items[0] = None
+        self.task.set_choice(({}, self.items)) # APPLY
+        self._infer()
+        self.assertFalse(self.items[1].comp)
+        self.assertEqual(self.items[1].albumartist, self.items[2].artist)
+
 class DuplicateCheckTest(unittest.TestCase):
     def setUp(self):
         self.lib = library.Library(':memory:')