use (1, 'iteration') if  arg is None
@@ -186,7 +186,9 @@ class Trainer(object):
trigger (tuple or Trigger): Trigger object that determines when to
invoke the extension. If it is ``None``, ``extension.trigger``
is used instead. If the trigger is not callable, it is passed
- to :class:`IntervalTrigger` to build an interval trigger.
+ to :class:`IntervalTrigger` to build an interval trigger. If
+ ``extension.trigger`` is ``None``, the extension is triggered
+ at every iteration as default.
priority (int): Invocation priority of the extension. Extensions
are invoked in the descending order of priorities in each
iteration. If this is ``None``, ``extension.priority`` is used
@@ -213,7 +215,7 @@ class Trainer(object):
'the name "training" is prohibited as an extension name')
if trigger is None:
- trigger = getattr(extension, 'trigger', None)
+ trigger = getattr(extension, 'trigger', (1, 'iteration'))
trigger = trigger_module.get_trigger(trigger)
if priority is None:
