<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_action_handler(self, controller_name, action_name):
 """ Return action of controller as callable. If requested controller isn't found - return 'not_found' action of requested controller or Index controller. """

        try_actions = [
            controller_name + '/' + action_name,
            controller_name + '/not_found',
            # call Index controller to catch all unhandled pages
            'index/not_found'
        ]
        # search first appropriate action handler
        for path in try_actions:
            if path in self._controllers:
                return self._controllers[path]
        return None