code: def _partition_tasks(worker):
      task_history = worker._add_task_history
      pending_tasks = {task for(task, status, ext) in task_history if status == 'PENDING'}
      set_tasks = {}
      set_tasks["completed"] = {task for (task, status, ext) in task_history if status == 'DONE' and task in pending_tasks}
      set_tasks["already_done"] = {task for (task, status, ext) in task_history
                                   if status == 'DONE' and task not in pending_tasks and task not in set_tasks["completed"]}
      set_tasks["ever_failed"] = {task for (task, status, ext) in task_history if status == 'FAILED'}
      set_tasks["failed"] = set_tasks["ever_failed"] - set_tasks["completed"]
      set_tasks["scheduling_error"] = {task for(task, status, ext) in task_history if status == 'UNKNOWN'}
      set_tasks["still_pending_ext"] = {task for (task, status, ext) in task_history
                                        if status == 'PENDING' and task not in set_tasks["ever_failed"] and task not in set_tasks["completed"] and not ext}
      set_tasks["still_pending_not_ext"] = {task for (task, status, ext) in task_history
                                            if status == 'PENDING' and task not in set_tasks["ever_failed"] and task not in set_tasks["completed"] and ext}
      set_tasks["run_by_other_worker"] = set()
      set_tasks["upstream_failure"] = set()
      set_tasks["upstream_missing_dependency"] = set()
      set_tasks["upstream_run_by_other_worker"] = set()
      set_tasks["upstream_scheduling_error"] = set()
      set_tasks["not_run"] = set()
      return set_tasks