[New-bugs-announce] [issue39021] multiprocessing is_alive() between children processes

Matt report at bugs.python.org
Tue Dec 10 18:22:54 EST 2019

New submission from Matt <matthewtimms at live.com.au>:

I'm trying to evaluate process' state between two "sibling" processes (processes created by the same parent process); using the .is_alive() and exitcode to evaluate whether a process has been init'd, started, finished successfully or unsuccessfully. 
The reference to one process is passed to the other and I'd like to call .is_alive(). This raises the following assertion error:

Process C-2:
Traceback (most recent call last):
  File "/usr/lib/python3.7/multiprocessing/process.py", line 297, in _bootstrap
  File "/home/timms/.PyCharm2019.2/config/scratches/is_alive_method.py", line 59, in run
    print("brother - ",self.brother.state)
  File "/home/timms/.PyCharm2019.2/config/scratches/is_alive_method.py", line 16, in state
    if self.is_alive():
  File "/usr/lib/python3.7/multiprocessing/process.py", line 151, in is_alive
    assert self._parent_pid == os.getpid(), 'can only test a child process'
AssertionError: can only test a child process

It's obvious that the assertion fails given the family structure of the processes. Overwriting the is_alive() method in my own process class appears to produce my desired output behaviour - with assertion and discarding self removed (see attachment).

Is there something fundamental to how process' operate that I should be weary of? I understand that is_alive also joins itself if possible; is that the sole reason for the assertion? Could a method that mirrors is_alive() without the assertion and discard method work with the desired intention I've described above?

files: is_alive_method.py
messages: 358232
nosy: matttimms
priority: normal
severity: normal
status: open
title: multiprocessing is_alive() between children processes
type: behavior
versions: Python 3.7
Added file: https://bugs.python.org/file48770/is_alive_method.py

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