urldecode.py

import re

urldecoder = re.compile('%([0-9a-fA-F]{2})')

def urldecode(s):
    bs = bytearray(s)
    offset = 0
    for encoded in urldecoder.finditer(s):
        start, end = encoded.span()
        bs[start + offset:end + offset] = chr(int(encoded.group(1), 16))
        offset += (start - end + 1)
    return str(bs)