Coolest Python recipe of all time

[geremy condra]

On Fri, May 6, 2011 at 9:59 AM, Steven D'Aprano
<steve+comp.lang.python at pearwood.info> wrote:
> On Mon, 02 May 2011 10:33:31 -0700, Raymond Hettinger wrote:
>
>> I think it is time to give some visibility to some of the instructive
>> and very cool recipes in ActiveState's python cookbook.
> [...]
>> What are your favorites?
>
>
> I'm not sure if favourite is the right word, but I'm amazed by this one:
> McCarthy's "amb" (ambiguous) operator.
>
> http://code.activestate.com/recipes/577368
>
> Here's how I might use it to solve the problem if making change. In
> Australian currency, I have 5, 10, 20, 50 cent and $1 and $2 coins.
> Ignoring the dollar coins, how can I make up change for any multiple of
> five cents up to a dollar?
>
> Suppose I have more 5 cent coins that I can deal with, and I want to make
> sure I hand out at least three of them. Here's how to make 45 cents worth
> of change:
>
>>>> amb = Amb()
>>>> a = amb(range(3, 21))  # number of 5 cent coins
>>>> b = amb(range(11))  # number of 10 cent coins
>>>> c = amb(range(6))  # number of 20 cent coins
>>>> d = amb(range(3))  # number of 50 cent coins
>>>> for _ in amb(lambda a,b,c,d: 5*a+10*b+20*c+50*d == 45):
> ...     print a, b, c, d
> ...
> 3 1 1 0
> 3 3 0 0
> 5 0 1 0
> 5 2 0 0
> 7 1 0 0
> 9 0 0 0
>
>
> Suppose I have some words, and want to put them together so that there
> are a certain number of vowels:
>
>>>> amb = Amb()
>>>> a = amb(['quick', 'slow', 'hungry', 'wise-old'])
>>>> b = amb(['fox', 'hare', 'turtle', 'kangaroo'])
>>>> c = amb(['lazy', 'stupid', 'sleepy', 'confused'])
>>>> d = amb(['dog', 'aardvark', 'sloth', 'wombat'])
>>>>
>>>> def test(a, b, c, d):
> ...     s = "The %s brown %s jumped over the %s %s." % (a, b, c, d)
> ...     num_vowels = sum(s.count(c) for c in 'aeiou')
> ...     return num_vowels in (12, 18, 19)
> ...
>>>> for _ in amb(test):
> ...     print a, b, c, d
> ...
> quick fox lazy sloth
> quick fox lazy dog
> quick kangaroo stupid aardvark
> [...more solutions cut for brevity]
> hungry kangaroo confused aardvark
>
>
>
> As written, amb is just a brute-force solver using more magic than is
> good for any code, but it's fun to play with.

I use a similar technique *a lot* for various kinds of constraint
solvers, but I have yet to come up with a really satisfying spelling
for it. Have you looked at the way this is done in Sage?

Geremy Condra