Simple cache: dictionary with expire

cache.py

#vim:set fileencoding=utf-8

import time

# Cache with Expire
class Cache:
  def __init__(self, timeout):
    # Actual expire is more than timeout and less than timeout*3.
    self.timeout_ = timeout
    self.clear()

  def clear(self):
    self.old_ = {}
    self.current_ = {}
    self.limit_ = 0

  def __len__(self):
    return len(self.old_) + len(self.current_)

  def __setitem__(self, key, value):
    self.housekeep()
    self.current_[key] = value
    # Don't care even if old_ has the key.

  def __getitem__(self, key):
    # KeyError raised if key is not found.
    self.housekeep()
    try:
      return self.current_[key]
    except KeyError:
      return self.old_[key]

  def housekeep(self):
    now = time.time()
    if now > self.limit_:
      if now > self.limit_ + self.timeout_:
        self.old_ = {}
      else:
        self.old_ = self.current_
      self.current_ = {}
      self.limit_ = now + self.timeout_

class AccurateCache(Cache):
  def __init__(self, timeout):
    Cache.__init__(self, timeout)

  def __setitem__(self, key, value):
    Cache.__setitem__(self, key, (value, time.time()))

  def __getitem__(self, key):
    value, added = Cache.__getitem__(self, key)
    if time.time() > added + self.timeout_:
      raise KeyError
    return value