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1. alirzaev

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Oct 30, 2023
2. terryjreedy

   added

   topic-multiprocessing

   on Oct 30, 2023
3. terryjreedy

   added this to Multiprocessing issues and removed this from Multiprocessing issueson Oct 30, 2023
4. terryjreedy

   removed

   topic-multiprocessing

   on Oct 30, 2023
5. iritkatriel

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   on Nov 26, 2023
6. cwindolf commented on Feb 1, 2024

   

   cwindolf

   on Feb 1, 2024

   Last edited by cwindolf

   More actions

   I'm observing the same issue (using the same test case, with some added prints to make sure the first map completes) on Python 3.11.4 and Python 3.12.1 on Linux as well. Using range(10) is enough to see the issue. By adding print(f"work {i=}") inside the work function, I can observe that the second map only makes it to i=4, indicating that only one child process was created.

   In other words, the test case:

   import time
   from concurrent.futures import ProcessPoolExecutor
   
   
   def work(i):
       print(f"work {i=}", flush=True)
       time.sleep(1)
       return i * i
   
   
   def main():
       print("main")
       with ProcessPoolExecutor(max_workers=1) as executor:
           print("start A", flush=True)
           _ = list(executor.map(work, range(10)))
           # completes
       print("done A", flush=True)
   
       with ProcessPoolExecutor(max_workers=1, max_tasks_per_child=5) as executor:
           print("start B", flush=True)
           _ = list(executor.map(work, range(10)))
           # hangs forever
       print("done B", flush=True)
   
   
   if __name__ == '__main__':
       main()
   

   produces the output

   $ python test.py
   main
   start A
   work i=0
   work i=1
   work i=2
   work i=3
   work i=4
   work i=5
   work i=6
   work i=7
   work i=8
   work i=9
   done A
   start B
   work i=0
   work i=1
   work i=2
   work i=3
   work i=4
   

   and hangs.

   👀React with 👀1Reacted by Brandon Smith
7. gpshead commented on Jul 3, 2026

   

   gpshead

   on Jul 3, 2026

   Member

   More actions

   Confirmed fixed by GH-140900. Same root cause, map() manifestation. Reproducer hangs on unpatched main (killed at 60s), completes in ~1s with the fix.
8. gpshead

   closed this as a duplicateof ProcessPoolExecutor hangs when 1<max_tasks_per_child<num_submitted//max_workers #115634 on Jul 3, 2026