[Python-Dev] a note in random.shuffle.doc ... (original) (raw)

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

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