title: What exactly is __weakref__ in Python?
body: <p>Surprisingly, there's no explicit documentation for <code>__weakref__</code>. Weak references are explained <a href="https://docs.python.org/2/library/weakref.html">here</a>. <code>__weakref__</code> is also shortly mentioned in the documentation of <code>__slots__</code>. But I could not find anything about <code>__weakref__</code> itself.</p>
  
  <p>What exactly is <code>__weakref__</code>?
  - Is it just a member acting as a flag: If present, the object may be weakly-referenced?
  - Or is it a function/variable that can be overridden/assigned to get a desired behavior? How?</p>