enumerate is very useful, here's mine:

    def equi ( A ):
        right = sum(A)
        left = 0
        for i,x in enumerate(A):
            right -= x
            if right == left:
                return i
            left += x
        return -1

What's interesting is that all the versions I've seen so far are pretty much identical apart from very minor things. This would seem to me to be obviously the correct way to do it but I'm interested if it's just a way. Did anyone solve this in a different way of equal or greater elegance?