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。
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