Add MediaFile.update() method to supersede save()

beets/library.py

@@ -420,16 +420,14 @@ class Item(LibModel):
         Can raise either a `ReadError` or a `WriteError`.
         """
         try:
-            f = MediaFile(syspath(self.path))
+            mediafile = MediaFile(syspath(self.path))
         except (OSError, IOError) as exc:
             raise ReadError(self.path, exc)
 
         plugins.send('write', item=self)
 
-        for key in ITEM_KEYS_WRITABLE:
-            setattr(f, key, self[key])
         try:
-            f.save(id3v23=beets.config['id3v23'].get(bool))
+            mediafile.update(self, id3v23=beets.config['id3v23'].get(bool))
         except (OSError, IOError, MutagenError) as exc:
             raise WriteError(self.path, exc)
 

beets/mediafile.py

@@ -1219,6 +1219,7 @@ class MediaFile(object):
         if self.mgfile.tags is None:
             self.mgfile.add_tags()
 
+    # FIXME Deprecated. Use `update` instead
     def save(self, id3v23=False):
         """Write the object's tags back to the file.
 
@@ -1264,6 +1265,25 @@ class MediaFile(object):
             if isinstance(descriptor, MediaField):
                 yield property
 
+    def update(self, dict, id3v23=False):
+        """Update tags from the dictionary and write them to the file.
+
+        For any key in ``dict`` that is also a field to store tags the
+        method retrieves the corresponding value from ``dict`` and
+        updates the ``MediaFile``. If any of the tags are changed, these
+        changes are written to the disk.
+
+        By default, MP3 files are saved with ID3v2.4 tags. You can use
+        the older ID3v2.3 standard by specifying the `id3v23` option.
+        """
+        updated = False
+        for field in self.fields():
+            if field in dict and getattr(self, field) != dict[field]:
+                updated = True
+                setattr(self, field, dict[field])
+        if updated:
+            self.save(id3v23)
+
 
     # Field definitions.
 

test/test_mediafile.py

@@ -353,6 +353,15 @@ class ReadWriteTestBase(ArtTestMixin, GenreListTestMixin):
         mediafile = MediaFile(mediafile.path)
         self.assertTags(mediafile, tags)
 
+    def test_update_empty(self):
+        mediafile = self._mediafile_fixture('empty')
+        tags = self._generate_tags()
+
+        mediafile.update(tags)
+
+        mediafile = MediaFile(mediafile.path)
+        self.assertTags(mediafile, tags)
+
     def test_overwrite_full(self):
         mediafile = self._mediafile_fixture('full')
         tags = self._generate_tags()
@@ -369,6 +378,17 @@ class ReadWriteTestBase(ArtTestMixin, GenreListTestMixin):
         mediafile = MediaFile(mediafile.path)
         self.assertTags(mediafile, tags)
 
+    def test_update_full(self):
+        mediafile = self._mediafile_fixture('full')
+        tags = self._generate_tags()
+
+        mediafile.update(tags)
+        # Make sure the tags are already set when writing a second time
+        mediafile.update(tags)
+
+        mediafile = MediaFile(mediafile.path)
+        self.assertTags(mediafile, tags)
+
     def test_write_date_components(self):
         mediafile = self._mediafile_fixture('full')
         mediafile.year = 2001