5.3.4 Making it possible to create weak references to extension classes

By default, extension types do not support having weak references made to them. You can enable weak referencing by declaring a C attribute of type object called __weakref__. For example,

cdef class MyRing:
   """
   There is supposed to be at most one instance of this ring, so 
   we keep a weak reference pointer to it, so it will be deleted
   from memory when there are no more user references to it. 
   """
    cdef object __weakref__

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