title: Dictionary in Python which keeps the last x accessed keys
body: <p>Is there a dictionary in python which will only keep the most recently accessed keys. Specifically, I am caching relatively large blobs of data in a dictionary, and I am looking for a way of preventing the dictionary from ballooning in size, and to drop to the variables which were only accessed a long time ago [i.e. to only keep the say the 1000 most recently accessed keys - and when a new key gets added, to drop the key that was accessed the longest ago].</p>
  
  <p>I suspect this is not part of the standard dictionary class, but am hoping there is something analogous.</p>