[Tutor] Double loop and performances

[Benjamin Klein-Fignier]

Hi,

I am wondering if there is a way to efficiently run a double loop in  
Python. To be more precise, I would like to compute the sum of a  
function f(x,y) for every possible (x,y) pair.

I have tried several alternatives so far:

1) the classic double loop:
     for x in X:
         for y in Y:
             sum += f(x,y)

2) using list comprehension:
     sum([f(x,y) for x in X for y in Y])

3) I figured out that the loop was slow, so I used map() to generate  
a list of every combination and then map to compute the result. But  
it is not very memory efficient and still pretty slow.

I know I might try to use SciPy & co to do it but I would prefer to  
be able to run my script on any default python installation. Any  
suggestion ?

Regards,

     -- Ben