[issue18844] allow weights in random.choice

[Madison May]

Madison May added the comment:

[Raymond Hettinger]
> The sticking point is going to be that we don't want to recompute the 
> cumulative weights for every call to weighted_choice.

> So there should probably be two functions:

>  cw = make_cumulate_weights(weight_list) 
>  x = choice(choice_list, cw)

That's pretty much how I broke things up when I decided to test out optimization with lru_cache.  That version of the patch is now attached.

[Serhiy Storchaka]
> I like the idea about adding a family of distribution generators. 
> They should check input parameters and make a precomputation and then > generate infinite sequence of specially distributed random numbers.

Would these distribution generators be implemented internally (see attached patch) or publicly exposed?

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Added file: http://bugs.python.org/file31546/weighted_choice_v2.diff

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