弱參照物件
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Python 支持 “弱引用” 作为一类对象。具体来说，有两种直接实现弱引用的对
象。第一种就是简单的引用对象，第二种尽可能地作用为一个原对象的代理。

int PyWeakref_Check(ob)

   如果 *ob* 是一个引用或代理对象则返回真值。 此函数总是会成功执行。

int PyWeakref_CheckRef(ob)

   如果 *ob* 是一个引用对象则返回真值。 此函数总是会成功执行。

int PyWeakref_CheckProxy(ob)

   如果 *ob* 是一个代理对象则返回真值。 此函数总是会成功执行。

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.*

   返回弱引用对象 ref 的被引用对象。如果被引用对象不再存在，则返回
   "Py_None"。

   備註:

     该函数返回被引用对象的一个 *borrowed reference*。 这意味着应该总
     是在该对象上调用 "Py_INCREF()"，除非是当它在借入引用的最后一次被
     使用之前无法被销毁的时候。

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

   类似 "PyWeakref_GetObject()"，但实现为一个不做类型检查的宏。

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.
