Weak Reference Objects

.. highlightlang:: c

.. _weakrefobjects:

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.

.. c:function:: int PyWeakref_Check(ob)

Return true if ob is either a reference or proxy object.

.. c:function:: int PyWeakref_CheckRef(ob)

Return true if ob is a reference object.

.. c:function:: int PyWeakref_CheckProxy(ob)

Return true if ob is a proxy object.

.. c:function:: PyObject* PyWeakref_NewRef(PyObject *ob, PyObject *callback)

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 :exc:TypeError.

.. c:function:: PyObject* PyWeakref_NewProxy(PyObject *ob, PyObject *callback)

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 :exc:TypeError.

.. c:function:: PyObject* PyWeakref_GetObject(PyObject *ref)

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

.. note::

  This function returns a **borrowed reference** to the referenced object.
  This means that you should always call :c:func:`Py_INCREF` on the object
  except if you know that it cannot be destroyed while you are still
  using it.

.. c:function:: PyObject* PyWeakref_GET_OBJECT(PyObject *ref)

Similar to :c:func:PyWeakref_GetObject, but implemented as a macro that does no error checking.