本文档不保证在其中运行哪些线程回调。唯一记录的保证是，回调将在添加了回调的进程所属的线程中运行，但是可以是任何线程，因为您使用的是ThreadPoolExecutor而不是ProcessPoolExecutor：
添加的可调用对象按添加顺序被调用，并且始终在属于添加它们的进程的线程中调用。
在当前的ThreadPoolExecutor实现中，执行回调的线程取决于
Future添加回调时的状态，以及是否
Future取消。这些是实施细节；您不应该依赖它们，因为它们在不同的Python实现或不同版本中可能有所不同，并且如有更改，恕不另行通知。
如果在
Future完成后添加回调，则该回调将在您调用的任何线程中执行
add_done_callback。您可以通过查看
add_done_callback源代码来看到此信息：
def add_done_callback(self, fn): """Attaches a callable that will be called when the future finishes. Args: fn: A callable that will be called with this future as its only argument when the future completes or is cancelled. The callable will always be called by a thread in the same process in which it was added. If the future has already completed or been cancelled then the callable will be called immediately. These callables are called in the order that they were added. """ with self._condition: if self._state not in [CANCELLED, CANCELLED_AND_NOTIFIED, FINISHED]: self._done_callbacks.append(fn) return fn(self)
如果状态
Future指示已取消或已完成，
fn则立即在当前执行线程中调用它。否则，它将添加到内部回调列表中，以在
Future完成时运行。
例如：
>>> def func(*args):... time.sleep(5)... print("func {}".format(threading.current_thread()))>>> def cb(a): print("cb {}".format(threading.current_thread()))... >>> fut = ex.submit(func)>>> func <Thread(Thread-1, started daemon 140084551563008)>>>> fut = e.add_done_callback(cb)cb <_MainThread(MainThread, started 140084622018368)>如果成功
cancel调用取消了future ，则执行取消的线程会立即调用所有回调：
def cancel(self): """Cancel the future if possible. Returns True if the future was cancelled, False otherwise. A future cannot be cancelled if it is running or has already completed. """ with self._condition: if self._state in [RUNNING, FINISHED]: return False if self._state in [CANCELLED, CANCELLED_AND_NOTIFIED]: return True self._state = CANCELLED self._condition.notify_all() self._invoke_callbacks() return True
否则，执行将来任务的线程将调用回调。
