timedcache

timedcache.py

from collections import namedtuple
from datetime import datetime, timedelta


timedvalue = namedtuple('TimedValue', 'expires value')


class ExpiredException(Exception):
    pass


class TimedCache(dict):

    def __getitem__(self, key):
        """ Normal lookup, but checks to see if the item is expired, and if so,
        raises an ExpiredException
        """
        val = dict.__getitem__(self, key)
        if val.expires and val.expires <= datetime.now():
            raise ExpiredException()
        return val.value

    def __setitem__(self, packed, val):
        """ Accepts (tuple(key, expiration), value), the tuple is unpacked and
        used to set the key an expiration.
        """
        key = packed # assume they're only doing x['k'] and not x['k','y']
        expires = None
        if isinstance(packed, tuple):
            key, expires = packed
        if expires: # assume they're (re)setting the timer.
            expires = datetime.now() + timedelta(seconds=expires)
            val = timedvalue(expires, val)
        elif key in self:
            val = timedvalue(dict.__getitem__(self, key).expires, val)
        dict.__setitem__(self, key, val)