弱引用对象
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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” 是一个引用或者一个代理对象，则返回 true。

int PyWeakref_CheckRef(ob)

   如果 “ob” 是一个引用，则返回 true。

int PyWeakref_CheckProxy(ob)

   如果 “ob” 是一个代理对象，则返回 true。

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.*

   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"。

   注解:

     该函数返回被引用对象的一个**借来的引用**。这意味着除非你很清楚在
     你使用期间这个对象不可能被销毁，否则你应该始终对该对象调用
     "Py_INCREF()"。

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。
