Python how to reduce multiple lists?

The first argument of the lambda function is the sum so far and the second argument is the next pair of elements:

value = reduce(lambda sum, (x, y): sum + x*y, zip(a, b), 0)

A solution using reduce and map,

from operator import add,mul

a = [1,2,3]
b = [4,5,6]

print reduce(add,map(mul,a,b))

I would do it this way (I don't think you need lambda)...

sum(x*y for x, y in zip(a, b))

This also seems slightly more explicit. Zip AB, multiply them, and sum up the terms.