[Python-Dev] a note in random.shuffle.__doc__ ...
Raymond Hettinger
rhettinger at ewtllc.com
Mon Jun 12 15:52:18 CEST 2006
Alex Martelli wrote:
>...claims:
>
>Note that for even rather small len(x), the total number of
>permutations of x is larger than the period of most random number
>generators; this implies that "most" permutations of a long
>sequence can never be generated.
>
>Now -- why would the behavior of "most" random number generators be
>relevant here? The module's docs claim, for its specific Mersenne
>Twister generator, a period of 2**19997-1, which is (e.g.) a
>comfortable
>130128673800676351960752618754658780303412233749552410245124492452914636
>028095467780746435724876612802011164778042889281426609505759158196749438
>742986040468247017174321241233929215223326801091468184945617565998894057
>859403269022650639413550466514556014961826309062543 times larger than
>the number of permutations of 2000 items, which doesn't really feel
>to me like a "rather small len(x)" in this context (what I'm most
>often shuffling is just a pack of cards -- len(x)==52 -- for example).
>
>I suspect that the note is just a fossil from a time when the default
>random number generator was Whichman-Hill, with a much shorter
>period. Should this note just be removed, or instead somehow
>reworded to point out that this is not in fact a problem for the
>module's current default random number generator? Opinions welcome!
>
>
I think the note is still useful, but the "rather small" wording
should be replaced by something most precise (such as the
value of n=len(x) where n! > 2**19997).
Raymond
More information about the Python-Dev
mailing list