自动过期变量

Question

如何在Python中实现一个会自动过期的变量？比如说，让程序运行一个小时。我想实现一个包含6个变量的数组，每个变量在数组中会在10分钟后自动删除自己。而在一个小时后，数组中将不会有任何变量。

Answer 1

嗯，这听起来有点奇怪，但也是有可能的。

你可能需要一个类，用来记录当 __init__ 被调用时的时间。然后，重写 __getitem__ 方法，检查它被调用的时间，只有在时间还不算晚的时候才返回那个项目。（这样做可能比让一个“后台进程”一直在运行，主动删除你没请求的项目要简单。）

Answer 2

我实际上是为了字典做这个的。也许你会觉得这段代码有用：

"""Cache which has data that expires after a given period of time."""
from datetime import datetime, timedelta

class KeyExpiredError(KeyError): pass 

def __hax():
    class NoArg: pass
    return NoArg()
NoArg = __hax()

class DataCache(object):
    def __init__(self, defaultExpireTime=timedelta(1, 0, 0), dbg=True):
        self.defaultExpireTime = defaultExpireTime

        self.cache = {}
        self.dbg = dbg

        self.processExpires = True

    def setProcessExpires(self, b):
        self.processExpires = b

    def __getitem__(self, key):
        c = self.cache[key]

        n = datetime.now()
        if (n - c['timestamp']) < c['expireTime'] or not self.processExpires:
            return c['data']

        del self.cache[key]

        if self.dbg:
            print "DataCache: Key %s expired" % repr(key)

        raise KeyExpiredError(key)

    def __contains__(self, key):
        try:
            self[key]
            return True
        except KeyError:
            return False

    def __setitem__(self, key, val):
        self.cache[key] = {
            'data': val,
            'timestamp': datetime.now(),
            'expireTime': self.defaultExpireTime,
            }

    def items(self):
        keys = list(self.cache)
        for k in keys:
            try:
                val = self[k]
                yield (k, val)                                             
            except:
                pass

    def get(self, key, default=NoArg, expired=NoArg):
        try:
            return self[key]
        except KeyExpiredError:
            if expired is NoArg and default is not NoArg:
                return default
            if expired is NoArg: return None
            return expired
        except KeyError:
            if default is NoArg: return None
            return default

    def set(self, key, val, expireTime=None):
        if expireTime is None:
            expireTime = self.defaultExpireTime

        self.cache[key] = {
            'data': val,
            'timestamp': datetime.now(),
            'expireTime': expireTime,
            }

    def tryremove(self, key):
        if key in self.cache:
            del self.cache[key]
            return True
        return False

    #the following you can call without triggering any expirations
    def getTotalExpireTime(self, key):
        """Get the total amount of time the key will be in the cache for"""
        c = self.cache[key]
        return c['expireTime']

    def getExpirationTime(self, key):
        """Return the datetime when the given key will expire"""
        c = self.cache[key]
        return c['timestamp'] + c['expireTime']

    def getTimeRemaining(self, key):
        """Get the time left until the item will expire"""
        return self.getExpirationTime(key) - datetime.now()

    def getTimestamp(self, key):
        return self.cache[key]['timestamp']

    def __len__(self):
        return len(self.cache)

使用方法：

>>> dc = DataCache(timedelta(0, 5, 0)) #expire in 5 seconds
>>> dc[4] = 3
>>> dc[4]
3
>>> import time
>>> time.sleep(5)
>>> dc[4]
DataCache: Key 4 expired
Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    dc[4]
  File "datacache.py", line 35, in __getitem__
    raise KeyExpiredError(key)
KeyExpiredError: 4
>>> 

Answer 3

你可以创建一个后台进程，来检查已经过去了多少时间，然后删除合适的项目……或者如果你想创建一个列表的子类，让它在一定时间后自动删除内容，你也可以这样做，只需要在init里调用它就行。

def __init__(self, time):
    #run subprocess to chek_espired elements

补充：

我写了一个例子，但其实可以做得更好！

class MyList(list):
    def __init__(self,elems,  expires_time):
        list.__init__(self, elems)
        self.created = time.time()
        self.expires_time = expires_time
    def __getitem__(self, index):
        t = time.time()
        print t -  self.created
        if t - self.created > self.expires_time:
            self.created += self.expires_time
            self.pop(index)
            self.__getitem__(index)
        return list.__getitem__(self, index)

顺便说一下，如果程序试图从一个空列表中获取索引，你当然可以很容易地抛出一个自定义错误。