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(PyObject *ob)

   Return non-zero if *ob* is either a reference or proxy object.
   This function always succeeds.

int PyWeakref_CheckRef(PyObject *ob)

   Return non-zero if *ob* is a reference object or a subclass of the
   reference type.  This function always succeeds.

int PyWeakref_CheckRefExact(PyObject *ob)

   Return non-zero if *ob* is a reference object, but not a subclass
   of the reference type.  This function always succeeds.

int PyWeakref_CheckProxy(PyObject *ob)

   Return non-zero if *ob* is a proxy object.  This function always
   succeeds.

PyObject *PyWeakref_NewRef(PyObject *ob, PyObject *callback)
    *回傳值：新的參照。** 為 穩定 ABI 的一部分.*

   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 referenceable object, or if *callback* is not callable,
   "None", or "NULL", this will return "NULL" and raise "TypeError".

   也參考:

     "PyType_SUPPORTS_WEAKREFS()" for checking if *ob* is weakly
     referenceable.

PyObject *PyWeakref_NewProxy(PyObject *ob, PyObject *callback)
    *回傳值：新的參照。** 為 穩定 ABI 的一部分.*

   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
   referenceable object, or if *callback* is not callable, "None", or
   "NULL", this will return "NULL" and raise "TypeError".

   也參考:

     "PyType_SUPPORTS_WEAKREFS()" for checking if *ob* is weakly
     referenceable.

int PyWeakref_GetRef(PyObject *ref, PyObject **pobj)
    * 為 穩定 ABI 的一部分 自 3.13 版本開始.*

   Get a *strong reference* to the referenced object from a weak
   reference, *ref*, into **pobj*.

   * On success, set **pobj* to a new *strong reference* to the
     referenced object and return 1.

   * If the reference is dead, set **pobj* to "NULL" and return 0.

   * On error, raise an exception and return -1.

   在 3.13 版被加入.

int PyWeakref_IsDead(PyObject *ref)

   Test if the weak reference *ref* is dead. Returns 1 if the
   reference is dead, 0 if it is alive, and -1 with an error set if
   *ref* is not a weak reference object.

   在 3.14 版被加入.

void PyObject_ClearWeakRefs(PyObject *object)
    * 為 穩定 ABI 的一部分.*

   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.

void PyUnstable_Object_ClearWeakRefsNoCallbacks(PyObject *object)

   *這是 不穩定 API，它可能在小版本發布中沒有任何警告地被變更。*

   Clears the weakrefs for *object* without calling the callbacks.

   This function is called by the "tp_dealloc" handler for types with
   finalizers (i.e., "__del__()").  The handler for those objects
   first calls "PyObject_ClearWeakRefs()" to clear weakrefs and call
   their callbacks, then the finalizer, and finally this function to
   clear any weakrefs that may have been created by the finalizer.

   In most circumstances, it's more appropriate to use
   "PyObject_ClearWeakRefs()" to clear weakrefs instead of this
   function.

   在 3.13 版被加入.
