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.. highlight:: c
.. _weakrefobjects:
Weak Reference Objects
----------------------
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(PyObject *ob)
Return true if *ob* is either a reference or proxy object. This function
always succeeds.
.. c:function:: int PyWeakref_CheckRef(PyObject *ob)
Return true if *ob* is a reference object. This function always succeeds.
.. c:function:: int PyWeakref_CheckProxy(PyObject *ob)
Return true if *ob* is a proxy object. This function always succeeds.
.. 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 ``Py_None``.
.. note::
This function returns a :term:`borrowed reference` to the referenced object.
This means that you should always call :c:func:`Py_INCREF` on the object
except when it cannot be destroyed before the last usage of the borrowed
reference.
.. c:function:: PyObject* PyWeakref_GET_OBJECT(PyObject *ref)
Similar to :c:func:`PyWeakref_GetObject`, but does no error checking.
.. c:function:: void PyObject_ClearWeakRefs(PyObject *object)
This function is called by the :c:member:`~PyTypeObject.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.
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