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Re: difference between random module in python 2.6 and 3.2?

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On 2/5/2012 11:01 PM, Steven D'Aprano wrote:

> Reading the docs, I would expect that when using an int as seed, you
> should get identical results.

That is similar to expecting hash to be consistent from version to version.

> There is no mention that the PRNG has changed between 2.6 and 3.2;

There is at best an informal policy. This was discussed in
http://bugs.python.org/issue9025
Antoine argued that if there were a written policy, it should be limited 
to bug-fix releases within a version. I agree.

> It appears to be a bug in 3.2, because 3.1 gives the same results as 2.6:

This change is a side effect of fixing the bug of non-uniformity 
discussed in that issue. In any case, in 2.7 and probably 3.1:

     def choice(self, seq):
         """Choose a random element from a non-empty sequence."""
         return seq[int(self.random() * len(seq))]  # raises IndexError

whereas in 3.2:

     def choice(self, seq):
         """Choose a random element from a non-empty sequence."""
         try:
             i = self._randbelow(len(seq))
         except ValueError:
             raise IndexError('Cannot choose from an empty sequence')
         return seq[i]

The change was announced in What's New in 3.2

random
The integer methods in the random module now do a better job of 
producing uniform distributions. Previously, they computed selections 
with int(n*random()) which had a slight bias whenever n was not a power 
of two. Now, multiple selections are made from a range up to the next 
power of two and a selection is kept only when it falls within the range 
0 <= x < n. The functions and methods affected are randrange(), 
randint(), choice(), shuffle() and sample().

-- 
Terry Jan Reedy

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difference between random module in python 2.6 and 3.2? Matej Cepl <mcepl@redhat.com> - 2012-02-06 02:27 +0100
  Re: difference between random module in python 2.6 and 3.2? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-02-06 04:01 +0000
    Re: difference between random module in python 2.6 and 3.2? Terry Reedy <tjreedy@udel.edu> - 2012-02-06 00:07 -0500
      Re: difference between random module in python 2.6 and 3.2? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-02-06 05:56 +0000
        Re: difference between random module in python 2.6 and 3.2? Terry Reedy <tjreedy@udel.edu> - 2012-02-06 02:27 -0500
          Re: difference between random module in python 2.6 and 3.2? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-02-06 08:05 +0000
            Re: difference between random module in python 2.6 and 3.2? Matej Cepl <mcepl@redhat.com> - 2012-02-06 09:45 +0100
              Re: difference between random module in python 2.6 and 3.2? Matej Cepl <mcepl@redhat.com> - 2012-02-06 09:57 +0100
                Re: difference between random module in python 2.6 and 3.2? Tim Chase <python.list@tim.thechases.com> - 2012-02-06 13:26 -0600
                Re: difference between random module in python 2.6 and 3.2? Matej Cepl <mcepl@redhat.com> - 2012-02-06 23:06 +0100
                Re: difference between random module in python 2.6 and 3.2? Serhiy Storchaka <storchaka@gmail.com> - 2012-02-07 12:05 +0200
              Re: difference between random module in python 2.6 and 3.2? Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2012-02-07 16:11 +0100
            Re: difference between random module in python 2.6 and 3.2? Mel Wilson <mwilson@the-wire.com> - 2012-02-06 10:20 -0500

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