ThreadPoolExecutor - callback guaranteed to run in same thread as the submitted function?

[Praveen Gollakota]

Hello,

In the ThreadPoolExecutor (TPE), is the callback always guaranteed to run in the same thread as the submitted function?

 For example, I tested this with the following code. I ran it many times and it seemed like `func` and `callback` always ran in the same thread.

    import concurrent.futures
    import random
    import threading
    import time

    executor = concurrent.futures.ThreadPoolExecutor(max_workers=3)

    def func(x):
        time.sleep(random.random())
        return threading.current_thread().name

    def callback(future):
        time.sleep(random.random())
        x = future.result()
        cur_thread = threading.current_thread().name
        if (cur_thread != x):
            print(cur_thread, x)

    print('main thread: %s' % threading.current_thread())
    for i in range(10000):
        future = executor.submit(func, i)
        future.add_done_callback(callback)

However, it seemed to fail when I removed the `time.sleep(random.random())` statements, i.e. at least a few `func` functions and `callbacks` **did not** run in the same thread. 

For a project that I am working on, the callback must always run on the same thread as the submitted function, so I wanted to be sure that this is guaranteed by TPE. (And also the results of the test without the random sleep seemed puzzling).

I looked at the source code for executors (see https://hg.python.org/cpython/file/6d44906344f4/Lib/concurrent/futures/_base.py#l297) and it does not seem like we switch the thread to the main thread before we run the callback. But just wanted to be sure. 

Any insight would be appreciated!

Thanks,
Praveen