[New-bugs-announce] [issue9816] random.jumpahead and PRNG sequence independence

[Joseph Schaeffer]

New submission from Joseph Schaeffer <thiryal at gmail.com>:

Reading the Python 2.6 docs, it appeared that using random.jumpahead would allow the initialization of several generators with the same seed but having much different internal states. While the resulting PRNG appear to have different internal states, the produced random numbers [via .random()] are exactly the same after a small initial segment.

Attached is some example code which shows the first point at which they all agree - in my testing (Mac OS X, Python versions 2.5, 2.6, 2.7) the generated numbers all agreed on the 12th number generated. For smaller differences in jumpahead it was noticeable a lot earlier - n=1,2 differ only in the first sample from each.

The internal state of the PRNGs is indeed different even after the successive sampling, so it may be that this is intended - however if so the docs may cause confusion: my particular case was where I need random numbers for a stochastic markov process and in addition needed many such generators [one for each trajectory] and was hoping to use random.jumpahead to have indepedent PRNG's without having to generate [and prove] my own independent set of seeds. Thus having a long sequence of non-independent random numbers near the initial start condition causes random.jumpahead to be unusable for my situation.

It appears that Python 3.1 removed random.jumpahead - if so, it may be useful to note in the 2.6 docs why this was / the issues with random.jumpahead: reading how it changed after 2.3 made it sound like it was exactly what I wanted. 

Possible cause: I suspect the issue may be related to how a Mersenne Twister algorithm can take a while to recover from poor seeding (excessive 0's), but do not know enough to explore that idea.

----------
components: Library (Lib)
files: random_test.py
messages: 115985
nosy: Joseph.Schaeffer
priority: normal
severity: normal
status: open
title: random.jumpahead and PRNG sequence independence
type: behavior
versions: Python 2.5, Python 2.6, Python 2.7
Added file: http://bugs.python.org/file18816/random_test.py

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<http://bugs.python.org/issue9816>
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