function of `with` in `concurrent.futures`

Question

The code below executes as expected by returning the total finish time as almost zero because it doesn't wait for the threads to finish every job.

import concurrent.futures
import time

start = time.perf_counter()


def do_something(seconds):
    print(f'Sleeping {seconds} second(s)...')
    time.sleep(seconds)
    return f'Done Sleeping...{seconds}'



executor= concurrent.futures.ThreadPoolExecutor()
secs = [10, 4, 3, 2, 1]

fs = [executor.submit(do_something, sec) for sec in secs]


finish = time.perf_counter()

print(f'Finished in {round(finish-start, 2)} second(s)')

But with the with command it does wait:

with concurrent.futures.ThreadPoolExecutor() as executor:
    secs = [10, 4, 3, 2, 1]

    fs = [executor.submit(do_something, sec) for sec in secs]

Why? What is the reason that with has this behavior with multithreading?

Answer 1

Using a concurrent.futures.Executor in a with statement is equivalent to calling Executor.shutdown after using it – causing the executor to wait for all tasks to complete. An Executor used in a with guarantees proper shutdown of concurrent tasks even if an error occurs inside the with block.

Executor.shutdown(wait=True)

Signal the executor that it should free any resources that it is using when the currently pending futures are done executing. Calls to Executor.submit() and Executor.map() made after shutdown will raise RuntimeError.

[...]

You can avoid having to call this method explicitly if you use the with statement, which will shutdown the Executor (waiting as if Executor.shutdown() were called with wait set to True): [...]