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Random string of digits?

Steven D'Aprano steve+comp.lang.python at pearwood.info
Sun Dec 25 12:18:53 EST 2011 
On Mon, 26 Dec 2011 03:11:56 +1100, Chris Angelico wrote:

> On Mon, Dec 26, 2011 at 2:46 AM, Steven D'Aprano
> <steve+comp.lang.python at pearwood.info> wrote:
>> Use the Source, Luke, er, Chris :)
>>
>> If I've read the source correctly, randint() will generate sufficient
>> bits of randomness to ensure that the entire int is random.
>>
>> http://hg.python.org/cpython/file/default/Lib/random.py
> 
> I prefer not to rely on the source. That tells me what happens, not
> what's guaranteed to happen. 

In this case, the source explicitly tells you that the API includes 
support for arbitrary large ranges if you include a getrandbits() method:

     Optionally, implement a getrandbits() method so that randrange()
     can cover arbitrarily large ranges.

I call that a pretty strong guarantee.


> However... bit of poking around can't hurt.
> That file doesn't actually justify anything, because random.Random()
> does not define getrandbits() - that, it seems, comes from _random();
> turns out that getrandbits is actually doing pretty much the same thing
> I suggested:
> 
> http://hg.python.org/cpython/file/745f9fd9856d/Modules/
_randommodule.c#l371
> 
> Need a 64-bit random number? Take two 32-bit numbers and concatenate.
> So, it's going to be easier and clearer to just take the simple option,
> since it's actually doing the same thing underneath anyway.

Um, I'm not sure what you consider "the simple option" in this context. I 
would hope you mean to use the high level API of randint:

# need a random number with exactly 20 decimal digits
random.randint(10**20, 10**21-1)

rather than manually assembling a 20 digit number from smaller pieces.


-- 
Steven
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