title: Why does Python's Queue return an approximate size in qsize()?
body: <p>In the <a href="http://docs.python.org/library/queue.html#queue-objects" rel="noreferrer">doc</a> of qsize() it says: Return the approximate size of the queue. </p>
  
  <p>Why can't it just return an exact size of this Queue? I understand the Queue may be accessed by multiple threads, but at the moment I call the function I think it's still possible to return the exact size of that moment.</p>