Return-Path: <python-python-list@m.gmane.org>
X-Original-To: python-list@python.org
Delivered-To: python-list@mail.python.org
X-Spam-Status: OK 0.000
X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'fixes': 0.05; 'seemed': 0.07; 'python': 0.09; 'fixed,': 0.09; 'happens.': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'fork().': 0.16; 'generator.': 0.16; 'hint?': 0.16; 'illustrates': 0.16; 'received:80.91.229.3': 0.16; 'received:dip.t-dialin.net': 0.16; 'received:plane.gmane.org': 0.16; 'received:t-dialin.net': 0.16; 'wrote:': 0.17; 'copied': 0.17; 'everyone,': 0.17; 'import': 0.21; 'random': 0.24; 'header:User-Agent:1': 0.26; 'thanks!': 0.26; 'header:X-Complaints-To:1': 0.28; 'behaviour': 0.29; 'this.': 0.29; "i'm": 0.29; 'related': 0.30; 'expect': 0.31; 'anybody': 0.32; 'print': 0.32; 'problem': 0.33; 'to:addr:python-list': 0.33; 'program,': 0.34; 'sequence': 0.35; 'received:org': 0.36; 'but': 0.36; 'anything': 0.36; 'why': 0.37; 'subject:: ': 0.38; 'fact': 0.38; 'to:addr:python.org': 0.39; 'easily': 0.39; 'skip:" 10': 0.40; 'header:Received:5': 0.40; 'today': 0.67
X-Injected-Via-Gmane: http://gmane.org/
To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: Random and fork
Date: Wed, 06 Feb 2013 17:47:38 +0100
Organization: None
References: <9efb072f-b671-45da-bc4e-ef224b2ffe76@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 7Bit
X-Gmane-NNTP-Posting-Host: p5084936f.dip.t-dialin.net
User-Agent: KNode/4.7.3
X-BeenThere: python-list@python.org
X-Mailman-Version: 2.1.15
Precedence: list
List-Id: General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe: <http://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive: <http://mail.python.org/pipermail/python-list/>
List-Post: <mailto:python-list@python.org>
List-Help: <mailto:python-list-request@python.org?subject=help>
List-Subscribe: <http://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups: comp.lang.python
Message-ID: <mailman.1420.1360169270.2939.python-list@python.org>
Lines: 33
NNTP-Posting-Host: 2001:888:2000:d::a6
X-Trace: 1360169270 news.xs4all.nl 6853 [2001:888:2000:d::a6]:52184
X-Complaints-To: abuse@xs4all.nl
Path: csiph.com!usenet.pasdenom.info!news.franciliens.net!feed.ac-versailles.fr!nerim.net!novso.com!news2.euro.net!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Xref: csiph.com comp.lang.python:38294

Julien Le Goff wrote:

> Hi everyone,
> 
> Today I came accross a behaviour I did not expect in python (I am using
> 2.7). In my program, random.random() always seemed to return the same
> number; it turned out to be related to the fact that I was using os.fork.
> 
> See below a small program that illustrates this. It is easily fixed, but
> I'm interested in knowing why this happens. Can anybody give me a hint?
> Thanks!

Those numbers are "pseudo"-random numbers -- the sequence of numbers 
generated is fully determined by the state of the random number generator. 
That state like anything else is copied by fork(). I you need "better" 
randomness you can use random.SystemRandom:

> import random
> import os

random = random.SystemRandom()
 
> for i in xrange(10):
>     pid = os.fork()
>     if pid == 0:
>         # uncommenting this fixes the problem
>         # random.seed(os.getpid())
>         print random.random()
>         os._exit(0)
> 
> os.wait()