Implement __format__ on Album and Item

Cut the need to format manually (and often incorrectly) when logging by
implementing the __format__ magic method (see PEP 3101) on LibModel
(the parent class of Album & Item).

Based on a discussion in PR #1262

beets/library.py

@@ -247,6 +247,21 @@ class LibModel(dbcore.Model):
         super(LibModel, self).add(lib)
         plugins.send('database_change', lib=self._db)
 
+    def __format__(self, spec):
+        if not spec:
+            spec = beets.config[self._format_config_key].get(unicode)
+        result = self.evaluate_template(spec)
+        if isinstance(spec, bytes):
+            return result.encode('utf8')
+        else:
+            return result
+
+    def __str__(self):
+        return format(self).encode('utf8')
+
+    def __unicode__(self):
+        return format(self)
+
 
 class FormattedItemMapping(dbcore.db.FormattedMapping):
     """Add lookup for album-level fields.
@@ -383,6 +398,8 @@ class Item(LibModel):
 
     _sorts = {'artist': SmartArtistSort}
 
+    _format_config_key = 'list_format_item'
+
     @classmethod
     def _getters(cls):
         getters = plugins.item_field_getters()
@@ -791,6 +808,8 @@ class Album(LibModel):
     """List of keys that are set on an album's items.
     """
 
+    _format_config_key = 'list_format_album'
+
     @classmethod
     def _getters(cls):
         # In addition to plugin-provided computed fields, also expose

test/test_library.py

@@ -1,3 +1,4 @@
+# -*- coding: utf-8 -*-
 # This file is part of beets.
 # Copyright 2015, Adrian Sampson.
 #
@@ -1087,6 +1088,23 @@ class TemplateTest(_common.LibTestCase):
         self.album.store()
         self.assertEqual(self.i.evaluate_template('$foo'), 'baz')
 
+    def test_album_and_item_format(self):
+        config['list_format_album'] = u'foö $foo'
+        album = beets.library.Album()
+        album.foo = 'bar'
+        album.tagada = 'togodo'
+        self.assertEqual(u"{0}".format(album), u"foö bar")
+        self.assertEqual(u"{0:$tagada}".format(album), u"togodo")
+        self.assertEqual(unicode(album), u"foö bar")
+        self.assertEqual(str(album), b"fo\xc3\xb6 bar")
+
+        config['list_format_item'] = 'bar $foo'
+        item = beets.library.Item()
+        item.foo = 'bar'
+        item.tagada = 'togodo'
+        self.assertEqual("{0}".format(item), "bar bar")
+        self.assertEqual("{0:$tagada}".format(item), "togodo")
+
 
 class UnicodePathTest(_common.LibTestCase):
     def test_unicode_path(self):