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Re: [Python-Dev] PEP 506 secrets module

From: Guido van Rossum <gui...@python.org>
Sat, 17 Oct 2015 16:05:42 -0700
OK, so just randbelow() then.

--Guido (mobile)
On Oct 17, 2015 2:13 PM, "Tim Peters" <tim....@gmail.com> wrote:

> [Steven D'Aprano]> >> ...> >> I think it is fair to say that out of the three functions, there is> >> consensus that randbelow has the most useful functionality in a crypto> >> context. Otherwise, people seem roughly equally split between the three> >> functions. There doesn't seem to be any use-case for the three argument> >> version of randrange.>> [Guido]> > I'm fine with dropping the 3rd arg. But I find the argument to introduce> a> > new spelling for 1-arg randrange() weak.>> Note the inherent absurdity here ;-)  That is, we're proposing to add> a module _all_ of whose functions are merely trivial (to the educated)> respellings of functions that already exist elsewhere.  Is there,> e.g., really "a strong" argument for giving a different way to spell> os.urandom()?>> That depends.  I'm playing along with the basic assumption:  that this> module intends to be as idiot-proof as possible.  In which case, sure,> rename away.>> And in which case, no, randbelow is not a new spelling for 1-arg> randrange.  To the contrary, all the integer-like functions (including> randrange, randint, and choice) in the random module are currently> built on top of the private Random._randbelow() method.  The latter is> "the right" fundamental building block for implementing uniform choice> free of statistical bias.  It's also overwhelmingly most useful _on> its own_ in the `secrets` context, and has the crushing (to me, in> this context) advantage that its very name strongly suggests its> argument is not a possible return value.  Giving a strongly mnemonic> name to a function with a dead simple "one required integer argument"> signature is likely "as idiot-proof as possible" as it gets.>> BTW, it also gives a simple answer to "I'm used to using> arc4random_uniform() in OpenBSD - how do I spell that in `secrets`?".> Answer:  `randbelow()` is identical, except in Python the argument> isn't limited to uint32_t".>>> > I definitely thing that randint() is an attractive nuisance so we should> > drop that.>> `randrange()` isn't a nuisance at all in Python, but its signature is> more convoluted than putative users of the proposed module seem to> want, and long experience has shown its name is not idiot-proof.> `secrets` users aren't looking to pick something uniformly from "a> range" - they're looking to pick a non-negative integer less than some> upper bound.  Unneeded generalization beyond just that much is also an> attractive nuisance, in context.>> "Simplest thing that can possibly suffice" is always a good way to start> :-)>

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