弱參照物件
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Python supports *weak references* as first-class objects.  There are
two specific object types which directly implement weak references.
The first is a simple reference object, and the second acts as a proxy
for the original object as much as it can.

int PyWeakref_Check(ob)

   Return true if *ob* is either a reference or proxy object.

int PyWeakref_CheckRef(ob)

   Return true if *ob* is a reference object.

int PyWeakref_CheckProxy(ob)

   Return true if *ob* is a proxy object.

PyObject* PyWeakref_NewRef(PyObject *ob, PyObject *callback)
    *Return value: New reference.*

   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.*

   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.*

   Return the referenced object from a weak reference, *ref*.  If the
   referent is no longer live, returns "Py_None".

   備註: This function returns a **borrowed reference** to the
     referenced object. This means that you should always call
     "Py_INCREF()" on the object except if you know that it cannot be
     destroyed while you are still using it.

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

   Similar to "PyWeakref_GetObject()", but implemented as a macro that
   does no error checking.
