Coolest Python recipe of all time

[Steven D'Aprano]

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.



-- 
Steven