Python ThreadPoolExecutor 未并行运行

Question

Python ThreadPoolExecutor not running parallelly, it is calling cube method in sequence and waiting for complete,

where I need to run 20 parallell threads

from concurrent.futures import ThreadPoolExecutor
from time import sleep

def cube(x):
    sleep(2)
    print(f'Cube of {x}:{x*x*x}')

count = 0
while True:
    with ThreadPoolExecutor(max_workers=20) as exe:
        exe.submit(cube,2)
    count += 1
    if count > 50:
        break   

Answer 1

"with" statement uses __enter__ method for ThreadPoolExecutor, it initialize thread pool at this moment. So, for function execution in thread pool we should use it inside "with" statement.

with ThreadPoolExecutor(max_workers=20) as exe:  # for default it will be number of os.cpu_count()
    futures = []
    for i in range(50):
        futures.append(executor.submit(cube, i))
    for future in concurrent.futures.as_completed(futures):
        print(future.result())

Answer 2

I guess want something like this:

from concurrent import futures
from time import sleep

THREADS = 20

def cube(x):
    sleep(2)
    res = x*x*x
    print(f'Cube of {x}:{res}')
    return res

with futures.ThreadPoolExecutor(max_workers=THREADS) as exe:
    jobs = {exe.submit(cube, i): i for i in range(THREADS)}

    for job in futures.as_completed(jobs):
        try:
            data = job.result()
        except Exception as exc:
            print('%r generated an exception: %s' % (jobs[job], exc))
        else:
            print(f"Result of thread {jobs[job]}: {data})")

Out:

Cube of 2:8
Cube of 1:1
Cube of 3:27
Cube of 0:0
Cube of 4:64
Result of thread 2: 8)
Result of thread 1: 1)
Result of thread 0: 0)
Result of thread 4: 64)
Cube of 5:125
Cube of 6:216
Cube of 8:512
Cube of 14:2744
Cube of 12:1728
Cube of 17:4913
Result of thread 3: 27)
Cube of 13:2197
Cube of 7:343
Cube of 16:4096
Cube of 15:3375
Cube of 18:5832
Cube of 11:1331
Cube of 9:729
Cube of 19:6859
Cube of 10:1000
Result of thread 5: 125)
Result of thread 6: 216)
Result of thread 8: 512)
Result of thread 14: 2744)
Result of thread 12: 1728)
Result of thread 17: 4913)
Result of thread 13: 2197)
Result of thread 7: 343)
Result of thread 16: 4096)
Result of thread 15: 3375)
Result of thread 18: 5832)
Result of thread 11: 1331)
Result of thread 10: 1000)
Result of thread 19: 6859)
Result of thread 9: 729)