使用yield生成给定长度的所有子字符串

def generate_substrings(s, length):
    if length == 0:  # 递归基，长度为0时返回空字符串
        yield ""
    elif len(s) < length:  # 如果字符串长度小于指定长度，无法生成子字符串
        return
    else:
        for i in range(len(s) - length + 1):
            substring = s[i:i+length]  # 使用切片生成当前位置的子字符串
            yield substring  # 返回当前子字符串
            yield from generate_substrings(s[i+1:], length)  # 递归生成剩余子字符串

# 使用示例
s = "abcd"
length = 2
substrings = generate_substrings(s, length)
for substring in substrings:
    print(substring)