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

Terry Jones terry at jon.es
Sun Jun 11 03:04:38 CEST 2006

"Greg" == Greg Ewing <greg.ewing at canterbury.ac.nz> writes: Greg> A generator with only N possible internal states can't Greg> possibly result in more than N different outcomes from Greg> any algorithm that uses its results.

I don't mean to pick nits, but I do find this a bit too general.

Suppose you have a RNG with a cycle length of 5. There's nothing to stop an algorithm from taking multiple already returned values and combining them in some (deterministic) way to generate > 5 outcomes. (Yes, those outcomes might be more, or less, predictable but that's not the point). If you expanded what you meant by "internal states" to include the state of the algorithm (as well as the state of the RNG), then I'd be more inclined to agree.

Worse, if you have multiple threads / processes using the same RNG, the individual threads could exhibit much more random behavior if individual thread outcomes depend on multiple RNG return values (as is the case with random.shuffle) and the scheduler is mixing things up. Here you'd have to include the state of the operating system to claim you can't get more outcomes than the number of internal states. But that's getting pretty far away from what we'd ordinarily think of as the internal state of the RNG.

Terry

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