# 弱参照オブジェクト

Python は 弱参照 を第一級オブジェクト (first-class object) としてサポートします。弱参照を直接実装する二種類の固有のオブジェクト型があります。第一は単純な参照オブジェクトで、第二はオリジナルのオブジェクトに対して可能な限りプロキシとして振舞うオブジェクトです。

int `PyWeakref_Check`(ob)

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

int `PyWeakref_CheckRef`(ob)

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

int `PyWeakref_CheckProxy`(ob)

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

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.

  弱参照 ref が参照しているオブジェクトを返します。被参照オブジェクトがすでに存続していない場合、 `Py_None` を返します。

  注釈

  This function returns a borrowed reference to the referenced object. This means that you should always call `Py_INCREF()` on the object except when it cannot be destroyed before the last usage of the borrowed reference.

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

  `PyWeakref_GetObject()` に似ていますが、マクロで実装されていて、エラーチェックを行いません。

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.