Use simpler approach of asserting that at most one handler of import_task_before_choice returns an action.

Author: Lanny

beets/plugins.py

@@ -501,22 +501,6 @@ def send(event, **arguments):
     return results
 
 
-def send_seq(event, **arguments):
-    """Like `send` but passes the result of the previous event handler to the
-    next, and returns only the result of the last non-None event handler.
-
-    `event` is the name of  the event to send, all other named arguments
-    are passed along to the handlers.
-    """
-    log.debug(u'Sequentially sending event: {0}', event)
-    previous = None
-    for handler in event_handlers()[event]:
-        result = handler(previous=previous, **arguments)
-        previous = result or previous
-
-    return previous
-
-
 def feat_tokens(for_artist=True):
     """Return a regular expression that matches phrases like "featuring"
     that separate a main artist or a song title from secondary artists.

beets/ui/commands.py

@@ -700,10 +700,16 @@ def choose_match(self, task):
 
         # Let plugins display info or prompt the user before we go through the
         # process of selecting candidate.
-        action = plugins.send_seq('import_task_before_choice',
-                                  session=self, task=task)
-        if action is not None:
-            return action
+        results = plugins.send('import_task_before_choice',
+                               session=self, task=task)
+        actions = [action for action in results if action]
+
+        if len(actions) == 1:
+            return actions[0]
+        elif len(actions) > 1:
+            raise Exception(
+                u'Only one handler for `import_task_before_choice` may return '
+                u'an action.')
 
         # Take immediate action if appropriate.
         action = _summary_judgment(task.rec)

beetsplug/badfiles.py

@@ -173,11 +173,7 @@ def on_import_task_start(self, task, session):
         if checks_failed:
             task._badfiles_checks_failed = checks_failed
 
-    def on_import_task_before_choice(self, task, session, previous):
-        # Already skipping, so no need to notify the user of anything
-        if previous == importer.action.SKIP:
-            return None
-
+    def on_import_task_before_choice(self, task, session):
         if hasattr(task, '_badfiles_checks_failed'):
             ui.print_('{} one or more files failed checks:'
                       .format(ui.colorize('text_warning', 'BAD')))

docs/dev/plugins.rst

@@ -204,9 +204,8 @@ The events currently available are:
   before any decision is made about how/if to import or tag. Can be used to
   present information about the task or initiate interaction with the user
   before importing occurs. Return an importer action to take a specific action.
-  Parameters: ``task``, ``session``, and ``previous`` (which is the action
-  specified by the previous event handler for the same task, allowing an early
-  exist if e.g. a previous plugin has already selected to skip the import)
+  Only one handler may return a non-None result.
+  Parameters: ``task`` and ``session``
 
 * `import_task_choice`: called after a decision has been made about an import
   task. This event can be used to initiate further interaction with the user.