Move configuration check to plugin initalization

beetsplug/cleantags.py

@@ -11,6 +11,8 @@ class CleanTagsPlugin(BeetsPlugin):
             'keep': [], # Tags to write to files.
             'discard': [] # Tags to discard
         })
+        if self.keep_tags and self.discard_tags:
+            raise ConfigValueError('Cannot specify both "keep" and "discard" for cleantags plugin.')
         self.register_listener('write', self.clear_metadata)
             
     @property
@@ -26,8 +28,6 @@ class CleanTagsPlugin(BeetsPlugin):
         return self.config['discard'].as_str_seq()
     
     def clear_metadata(self, item, path, tags):
-        if self.keep_tags and self.discard_tags:
-            raise ConfigValueError('Cannot specify both "keep" and "discard" for cleantags plugin.')
         if self.clean:
             file = File(path)
             file.delete()