Project Euler #1 Sum of multiples of 3 and 5 python implementation
It'd be more pythonic to use the built-in sum function instead of writing one yourself:

sum(xrange(num, limit, num))


However, this is still doing way too much work -- you don't need to do a for-loop for a series sum, there's a closed-form solution:

def sum_multiples(n, lim):
    last = (lim - 1) // n
    return n * (last) * (last+1) // 2


EDIT: Also, don't call your own function sum, since you hide the built-in one that way. 

def sum35(limit):
    return (sum_multiples(3, limit) +
            sum_multiples(5, limit) -
            sum_multiples(15, limit))

print sum35(10)   # 23
print sum35(1000) # 233168