multiprocessing is_alive() between children processes

[MattTimms]

BPO	39021
Nosy	@MattTimms
Files	is_alive_method.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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assignee = None
closed_at = None
created_at = <Date 2019-12-10.23:22:54.787>
labels = ['type-bug', '3.7']
title = 'multiprocessing is_alive() between children processes'
updated_at = <Date 2019-12-10.23:22:54.787>
user = 'https://github.com/MattTimms'

bugs.python.org fields:

activity = <Date 2019-12-10.23:22:54.787>
actor = 'matttimms'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2019-12-10.23:22:54.787>
creator = 'matttimms'
dependencies = []
files = ['48770']
hgrepos = []
issue_num = 39021
keywords = []
message_count = 1.0
messages = ['358232']
nosy_count = 1.0
nosy_names = ['matttimms']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39021'
versions = ['Python 3.7']

[MattTimms]

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
    self.run()
  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?

[catchthemonster]

Thank you MattTimmis for raising this issue!
The same thing is happening to me... In my case, I am using a main process to bootstrap mp "fork" processes as daemons. These are long-running processes. Now another also forked daemon receives a dict of process objects but these proc objects references loose for example information: from :
the same becomes in new forked daemon
I see no difference in object structure, all process attributes are there and are valid.
however, I can't use is_alive (like you).I can terminate the process from a secondary process but this creates a defunct process. A possible solution to this problem I think is to join() in the main process after the secondary process actually performs termination(). I am all Unix, so this might be a double fork issue when creating a daemon in Unix world but frankly I am not sure how a daemon is implemented exactly in Python multiprocessing ...
I am also not clear why forking an object would not pass a proc reference in full and allow actions on it that main process could catch and acknowledge it ...
Thank you for the workaround on is_alive()