약한 참조 객체
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파이썬은 *약한 참조*를 1급 객체로 지원합니다. 약한 참조를 직접 구현하
는 두 가지 구체적인 객체 형이 있습니다. 첫 번째는 간단한 참조 객체이며
, 두 번째는 가능한 한 원래 객체의 프락시 역할을 합니다.

int PyWeakref_Check(PyObject *ob)

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

int PyWeakref_CheckRef(PyObject *ob)

   Return true if *ob* is a reference object.  This function always
   succeeds.

int PyWeakref_CheckProxy(PyObject *ob)

   Return true if *ob* is a proxy object.  This function always
   succeeds.

PyObject *PyWeakref_NewRef(PyObject *ob, PyObject *callback)
    *반환값: 새 참조.** Part of the 안정 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)
    *반환값: 새 참조.** Part of the 안정 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)
    *반환값: 빌린 참조.** Part of the 안정 ABI.*

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

   참고:

     이 함수는 참조된 객체에 대한 *빌린 참조*를 반환합니다. 이는 빌린
     참조의 마지막 사용 전에 파괴될 수 없을 때를 제외하고, 객체에 대해
     항상 "Py_INCREF()"를 호출해야 함을 뜻합니다.

PyObject *PyWeakref_GET_OBJECT(PyObject *ref)
    *반환값: 빌린 참조.*

   "PyWeakref_GetObject()"와 유사하지만, 에러 검사를 수행하지 않습니다
   .

void PyObject_ClearWeakRefs(PyObject *object)
    * Part of the 안정 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.
