匹配嵌套字典中的项并返回外部键

def find_key(nested_dict, target_value):
    for key, value in nested_dict.items():
        if value == target_value:
            return key
        elif isinstance(value, dict):
            result = find_key(value, target_value)
            if result is not None:
                return key + '.' + result
    return None

# 示例字典
nested_dict = {
    'a': {
        'b': {
            'c': 1,
            'd': 2
        },
        'e': {
            'f': 3,
            'g': 4
        }
    },
    'h': {
        'i': {
            'j': 5,
            'k': 6
        },
        'l': {
            'm': 7,
            'n': 8
        }
    }
}

# 查找值为3的项
result = find_key(nested_dict, 3)
print(result)  # 输出：a.e.f

# 查找值为8的项
result = find_key(nested_dict, 8)
print(result)  # 输出：h.l.n

# 查找值为9的项（不存在）
result = find_key(nested_dict, 9)
print(result)  # 输出：None