simple binary conversion for numbers and letters

binary_conversion.py

# coding=utf-8 

CONSTANT_BEGIN = 201416920417
CONSTANT_CHAR = 'JTXD9hz6SBqpINCrx5Memk1LO2ylcPYjFnwbd4tv3VW0aiRK8oGE7fuQgsAUHZ'
CONSTANT_LENGTH = len(CONSTANT_CHAR)

def encode_b64(num):
    if not isinstance(num, (int, long)):
        raise TypeError('Must int num')
    if num <= 0:
        raise ValueError('Must great than 0')
    code_str = ''
    source = num + CONSTANT_BEGIN
    while source > 0:
        code_str += CONSTANT_CHAR[source % CONSTANT_LENGTH]
        source /= CONSTANT_LENGTH
    return code_str[::-1].rjust(6, CONSTANT_CHAR[0])


def decode_b64(code_str):
    res = 0
    for idx in xrange(len(code_str)):
        res *= CONSTANT_LENGTH
        res += CONSTANT_CHAR.index(code_str[idx])
    code_id = res - CONSTANT_BEGIN
    if code_id >= 0:
        return code_id
    else:
        raise ValueError('Code Error')

if __name__ == '__main__':
    n = 1080
    s = encode_b64(n)
    n2 = decode_b64(s)
    print(s, n2)