Objetos de referencia débil
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Python soporta *referencias débiles* como objetos de primera clase.
Hay dos tipos de objetos específicos que implementan directamente
referencias débiles. El primero es un objeto con referencia simple, y
el segundo actúa como un proxy del objeto original tanto como pueda.

int PyWeakref_Check(ob)

   Retorna verdadero (true) si *ob* es una referencia o un objeto
   proxy. Esta función siempre finaliza con éxito.

int PyWeakref_CheckRef(ob)

   Retorna verdadero (true) si *ob* es un objeto de referencia. Esta
   función siempre finaliza con éxito.

int PyWeakref_CheckProxy(ob)

   Retorna verdadero (true) si *ob* es un objeto proxy. Esta función
   siempre finaliza con éxito.

PyObject *PyWeakref_NewRef(PyObject *ob, PyObject *callback)
    *Return value: New reference.** Part of the Stable ABI.*

   Return a weak reference object for the object *ob*.  This will
   always return a new reference, but is not guaranteed to create a
   new object; an existing reference object may be returned.  The
   second parameter, *callback*, can be a callable object that
   receives notification when *ob* is garbage collected; it should
   accept a single parameter, which will be the weak reference object
   itself. *callback* may also be "None" or "NULL".  If *ob* is not a
   weakly referencable object, or if *callback* is not callable,
   "None", or "NULL", this will return "NULL" and raise "TypeError".

PyObject *PyWeakref_NewProxy(PyObject *ob, PyObject *callback)
    *Return value: New reference.** Part of the Stable ABI.*

   Return a weak reference proxy object for the object *ob*.  This
   will always return a new reference, but is not guaranteed to create
   a new object; an existing proxy object may be returned.  The second
   parameter, *callback*, can be a callable object that receives
   notification when *ob* is garbage collected; it should accept a
   single parameter, which will be the weak reference object itself.
   *callback* may also be "None" or "NULL".  If *ob* is not a weakly
   referencable object, or if *callback* is not callable, "None", or
   "NULL", this will return "NULL" and raise "TypeError".

PyObject *PyWeakref_GetObject(PyObject *ref)
    *Return value: Borrowed reference.** Part of the Stable ABI.*

   Retorna el objeto referenciado desde una referencia débil, *ref*.
   Si el referente no está vivo, retornará "Py_None".

   Nota:

     Esta función retorna una referencia *borrowed reference* al
     objeto referenciado. Esto significa que siempre debe llamar a
     "Py_INCREF()" sobre el objeto, excepto cuando no pueda ser
     destruido antes del último uso de la referencia prestada.

PyObject *PyWeakref_GET_OBJECT(PyObject *ref)
    *Return value: Borrowed reference.*

   Similar a "PyWeakref_GetObject()", pero implementado como un macro
   que no verifica errores.

void PyObject_ClearWeakRefs(PyObject *object)
    * Part of the Stable ABI.*

   This function is called by the "tp_dealloc" handler to clear weak
   references.

   This iterates through the weak references for *object* and calls
   callbacks for those references which have one. It returns when all
   callbacks have been attempted.
