write: Do not try to write non-writable fields

Fix #1268.

beets/library.py

@@ -402,6 +402,14 @@ class Item(LibModel):
     `write`.
     """
 
+    _media_tag_fields = set(MediaFile.fields()).intersection(_fields.keys())
+    """Set of item fields that are backed by *writable* `MediaFile` tag
+    fields.
+
+    This excludes fields that represent audio data, such as `bitrate` or
+    `length`.
+    """
+
     _formatter = FormattedItemMapping
 
     _sorts = {'artist': SmartArtistSort}

beets/ui/commands.py

@@ -1428,7 +1428,7 @@ def write_items(lib, query, pretend, force):
 
         # Check for and display changes.
         changed = ui.show_model_changes(item, clean_item,
-                                        library.Item._media_fields, force)
+                                        library.Item._media_tag_fields, force)
         if (changed or force) and not pretend:
             item.try_sync()
 

docs/changelog.rst

@@ -75,6 +75,8 @@ Fixes:
 * :doc:`/plugins/importfeeds` and :doc:`/plugins/smartplaylist`: Automatically
   create parent directories for playlist files (instead of crashing when the
   parent directory does not exist). :bug:`1266`
+* The :ref:`write-cmd` command no longer tries to "write" non-writable fields
+  like the bitrate. :bug:`1268`
 
 For developers:
 

test/test_ui.py

@@ -318,6 +318,37 @@ class WriteTest(unittest.TestCase, TestHelper):
         item = self.lib.items().get()
         self.assertEqual(item.mtime, item.current_mtime())
 
+    def test_non_metadata_field_unchanged(self):
+        """Changing a non-"tag" field like `bitrate` and writing should
+        have no effect.
+        """
+        # An item that starts out "clean".
+        item = self.add_item_fixture()
+        item.read()
+
+        # ... but with a mismatched bitrate.
+        item.bitrate = 123
+        item.store()
+
+        with capture_stdout() as stdout:
+            self.write_cmd()
+
+        self.assertEqual(stdout.getvalue(), '')
+
+    def test_write_metadata_field(self):
+        item = self.add_item_fixture()
+        item.read()
+        old_title = item.title
+
+        item.title = 'new title'
+        item.store()
+
+        with capture_stdout() as stdout:
+            self.write_cmd()
+
+        self.assertTrue('{0} -> new title'.format(old_title)
+                        in stdout.getvalue())
+
 
 class MoveTest(_common.TestCase):
     def setUp(self):