完善以下Python代码，实现从用户输入的原字符串中查找指定子串并替换为新字符串的功能：

s = input("请输入待查找的字符串：")
key = input("请输入需查找的子字符串：")
new = input("请输入要替换的字符串：")
def findstr(key, s, begin):
    for i in range(begin, len(s) - len(key) + 1):
        if s[i:    ①    ] == key:
            return i
    return -1
def replace(key, new, s):
    begin = 0
    while begin <    ②    :
        pos = findstr(key, s, begin)
        if pos == -1:
            break
        else:
            s = s[0:pos] + new + s[pos + len(key):    ③    ]
            begin = pos + len(key)
    return s
rst =    ④    # 调用函数替换字符
print(rst)