gistfile1.py

def str_to_int(input):
    base = ord('0')

    def char_to_int(c, d):
        n = ord(d) - base   
        if not (0 <= n <= 9):
            raise ValueError('Bad input')
        return 10 * c + n

    if input[0] == '-':
        return -reduce(char_to_int, input[1:], 0)

    return reduce(char_to_int, input, 0)