Python3 concurrent future doesn't seem to work async

Question

I'm new to python and just trying a simple threading example. But I can't explain myself why this code is working synchronous:

from concurrent.futures import ThreadPoolExecutor
import time

def return_after_3_secs():
    time.sleep(3)
    return

def callback_method(future):
    print ("Hello World.")

with ThreadPoolExecutor(2) as pool:
    future = pool.submit(return_after_3_secs)
    future.add_done_callback(callback_method)

print ("1")
time.sleep(1)
print ("2")
time.sleep(1)
print ("3")

print (future.result())

I'm basically from C# and thinking of a future as a Task in C#. So this is a handle or token for the new Thread.

I would expect following output:

1. 1
2. 2
3. 3
4. Hello World.
5. None

But I'm getting:

1. Hello World.
2. 1
3. 2
4. 3
5. None

Before something is printed the Console wait for 3 seconds. So this code is running synchron. Can someone help me up with the understanding of futures and tell me why the time.sleep(3) isn't running on second thread?

Answer 1

The statements after the with ThreadPoolExecutor(..) is executed after the statements inside the with .. statements done.

(because with ThreadPoolExecutor(..) internally calls executor.shutdown(wait=True) to wait the pending futures done, and related resources freed. See concurrent.futures.Executor.shutdown )

By indenting those statement ( print , time.sleep ) inside the with statement, you will get what you want.

with ThreadPoolExecutor(2) as pool:
    future = pool.submit(return_after_3_secs)
    future.add_done_callback(callback_method)

    print ("1")
    time.sleep(1)
    print ("2")
    time.sleep(1)
    print ("3")