import time

class Cache:
    def __init__(self):
        Cache.cache = {}
    # ttl 单位秒
    def set(self, key, value, ttl=None):
        Cache.cache[key] = {
            'value': value,
            'expires': time.time() + ttl if ttl else None
        }

    def get(self, key):
        item = Cache.cache.get(key)
        if item:
            if item['expires'] is None or item['expires'] > time.time():
                return item['value']
            else:
                del Cache.cache[key]
        return None

    def delete(self, key):
        if key in Cache.cache:
            del Cache.cache[key]

    def clear(self):
        Cache.cache.clear()

if __name__ == "__main__":
    # 测试永久存储
    cache = Cache()
    cache1 = Cache()
    cache.set("key1", "value1")
    print("永久存储测试:", cache1.get("key1"))

    # 测试带过期时间的存储
    cache.set("key2", "value2", ttl=2)
    print("带过期时间存储测试(立即查询):", cache.get("key2"))
    time.sleep(3)
    print("带过期时间存储测试(过期后查询):", cache.get("key2"))

    # 测试删除功能
    cache.set("key3", "value3")
    cache.delete("key3")
    print("删除功能测试:", cache.get("key3"))

    # 测试清空功能
    cache.set("key4", "value4")
    cache.clear()
    print("清空功能测试:", cache.get("key4"))