Automatically expiring variable

Question

How to implement an automatically expiring variable in python? For example, Let the program running For one hour. I want implement an array of 6 variables, each variable in array will be automatically deleted themselves after 10 mins. And after 1 hour, there will be no variable in the array.

Answer 1

Hmmm, seems weird, but possible.

Sounds like you need a class which records the time when __init__ is called. Then, implement __getitem__ to check the time when it is called, and only return the item if it's not too late. (It's probably easier to do this than to have a process "running in the background" which actively deletes items even when you don't ask for them.)

Answer 2

I actually had to do this for dictionaries. Maybe you'll find the code useful:

"""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)

Usage:

>>> 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
>>>