lesson06_wait_error.py

# SuperFastPython.com
# example of waiting for the first task to fail
from time import sleep
from random import random
from concurrent.futures import ProcessPoolExecutor
from concurrent.futures import wait
from concurrent.futures import FIRST_EXCEPTION

# custom task function executed by a worker process
def task(number):
 # generate a random number between 0 and 1
 value = random()
 # block for a fraction of a second
 sleep(value)
 # check for a failure
 if value < 0.5:
 raise Exception(f'Task {number} Failed')
 # report that the task completed
 print(f'>task {number} is done', flush=True)
 # return the task number
 return number

# protect the entry point
if __name__ == '__main__':
 # start the process pool
 with ProcessPoolExecutor(10) as exe:
 # submit tasks and collect futures
 futs = [exe.submit(task, i) for i in range(10)]
 # wait until any task completes
 done,_ = wait(futs, return_when=FIRST_EXCEPTION)
 # check that no tasks failed
 if len(done) == len(futs):
 print('No tasks failed')
 else:
 # get the first task to complete
 future = done.pop()
 # report the exception
 print(f'First fail: {future.exception()}')