剑指offer python版 字符串变成整数

def str_to_int(string):
    if not string:  # 空字符返回异常
        raise Exception('string cannot be None', string)
    flag = 0  # 用来表示第一个字符是否为+、-
    ret = 0  # 结果
    for k, s in enumerate(string):
        if s.isdigit():  # 数字直接运算
            val = ord(s) - ord('0')
            ret = ret * 10 + val
        else:
            if not flag:
                if s == '+' and k == 0:  # 避免中间出现+、-
                    flag = 1
                elif s == '-' and k == 0:
                    flag = -1
                else:
                    raise Exception('digit is need', string)
            else:
                raise Exception('digit is need', string)
    if flag and len(string) == 1:  # 判断是不是只有+、-
        raise Exception('digit is need', string)
    return ret if flag >= 0 else -ret


if __name__ == '__main__':
    test = '12399+'
print str_to_int(test)