Convert hex, oct stings to integer, because there is no base for int() for Python1.5.2

atoi.py

HEX_CHARS = {
    '0': 0,
    '1': 1,
    '2': 2,
    '3': 3,
    '4': 4,
    '5': 5,
    '6': 6,
    '7': 7,
    '8': 8,
    '9': 9,
    'a': 10,
    'b': 11,
    'c': 12,
    'd': 13,
    'e': 14,
    'f': 15,
    'A': 10,
    'B': 11,
    'C': 12,
    'D': 13,
    'E': 14,
    'F': 15,
}

def atoi(inp):
    if inp.startswith('0x'):
        inp = inp[2:]
        base = 16
    elif inp.startswith('0'):
        inp = inp[1:]
        base = 8
    else:
        raise ValueError('Input string should start from "0x" for hex '
                         'or from "0" for octal value.')
    res = 0
    for index in xrange(len(inp)):
        res = res + HEX_CHARS[inp[len(inp) - index - 1]] * base ** index
    return res