Add whitelist feature to zero plugin

beetsplug/zero.py

@@ -41,26 +41,54 @@ def __init__(self):
 
         self.config.add({
             'fields': [],
+            'keep_fields': [],
             'update_database': False,
         })
 
         self.patterns = {}
         self.warned = False
 
-        for field in self.config['fields'].as_str_seq():
-            if field in ('id', 'path', 'album_id'):
-                self._log.warn(u'field \'{0}\' ignored, zeroing '
-                               u'it would be dangerous', field)
-                continue
+        if self.config['fields']:
+            self.validate_config('fields')
+            for field in self.config['fields'].as_str_seq():
+                self.set_pattern(field)
+
+        elif self.config['keep_fields']:
+            self.validate_config('keep_fields')
+
+            for field in MediaFile.fields():
+                if field in self.config['keep_fields'].as_str_seq():
+                    continue
+                self.set_pattern(field)
+
+            # These fields should be preserved
+            for key in ('id', 'path', 'album_id'):
+                if key in self.patterns:
+                    del self.patterns[key]
+
+    def validate_config(self, mode):
+        """Check if fields written in config are correct."""
+        if self.config['fields'] and self.config['keep_fields']:
+            self._log.warn(u'cannot blacklist and whitelist at the same time')
+        for field in self.config[mode].as_str_seq():
             if field not in MediaFile.fields():
                 self._log.error(u'invalid field: {0}', field)
                 continue
+            if mode == 'fields' and field in ('id', 'path', 'album_id'):
+                self._log.warn(u'field \'{0}\' ignored, zeroing '
+                               u'it would be dangerous', field)
+                continue
 
-            try:
-                self.patterns[field] = self.config[field].as_str_seq()
-            except confit.NotFoundError:
-                # Matches everything
-                self.patterns[field] = True
+    def set_pattern(self, field):
+        """Set a field in `self.patterns` to a string list corresponding to
+        the configuration, or `True` if the field has no specific
+        configuration.
+        """
+        try:
+            self.patterns[field] = self.config[field].as_str_seq()
+        except confit.NotFoundError:
+            # Matches everything
+            self.patterns[field] = True
 
     def import_task_choice_event(self, session, task):
         """Listen for import_task_choice event."""

docs/plugins/zero.rst

@@ -18,6 +18,9 @@ fields to nullify and the conditions for nullifying them:
   get the list of all available fields by running ``beet fields``. In
   addition, the ``images`` field allows you to remove any images
   embedded in the media file.
+* Set ``keep_fields`` to *invert* the logic of the plugin. Only these fields
+  will be kept; other fields will be removed. Remember to set only
+  ``fields`` or ``keep_fields``, not both!
 * To conditionally filter a field, use ``field: [regexp, regexp]`` to specify
   regular expressions.
 * By default this plugin only affects files' tags ; the beets database is left
@@ -31,6 +34,10 @@ For example::
         genre: [rnb, 'power metal']
         update_database: true
 
+The plugin can work in one of two modes. The first mode, the default,
+is a blacklist, where you choose the tags you want to remove. The second mode
+is a whitelist where you instead specify the tags you want to keep.
+
 If a custom pattern is not defined for a given field, the field will be nulled
 unconditionally.