python产生随机字符串

def GenerateRandomString(len, basechars = []):
     if (basechars == []):
         x = range(ord('a'), ord('z') + )
         x.extend(range(ord('A'), ord('Z') + ))
         x.extend(range(ord(''), ord('') + ))
         basechars = [chr(i) for i in x]
         ret = ''
         for i in range(len):
            ret += random.choice(basechars)
         return ret