[issue30051] Document that the random module doesn't support fork

[STINNER Victor]

New submission from STINNER Victor:

When reviewing the issue #30030, it reminded me that the random module "doesn't support fork": after fork, the parent and the child produce the same "random" number sequence.

I suggest to add a quick note about that: not a warning, just a note, as a reminder.

There is an exception: SystemRandom produces a different sequence after fork, since it uses os.urandom() (which runs in the kernel).

I am tempted to propose a solution in the note like using SystemRandom, but I'm not sure that it's a good idea. Some users may misunderstood the note and always use SystemRandom where random.Random is just fine for their needs.

Proposed note:

The random module doesn't support fork: the parent and the child process will produce the same number sequence.

Proposed solution:

If your code uses os.fork(), a workaround is to check if os.getpid() changes and in that case, create a new Random instance or reseed the RNG in the child process.

--

The tempfile module reminds the pid and instanciates a new RNG on fork.

Another option is to not add a note, but implement the workaround directly into the random module. But I don't think that it's worth it. Forking is a rare usecase, and calling os.getpid() may slowdown the random module. (I don't recall if os.getpid() requires a syscall or not, I'm quite sure that it's optimized at least on Linux to avoid a real syscall.)

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messages: 291534
nosy: haypo
priority: normal
severity: normal
status: open
title: Document that the random module doesn't support fork
versions: Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30051>
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