Doc/c-api/weakref.rst

.. 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 non-zero if *ob* is either a reference or proxy object.  This function
   always succeeds.


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

   Return non-zero if *ob* is a reference object or a subclass of the reference
   type.  This function always succeeds.


.. c:function:: int PyWeakref_CheckRefExact(PyObject *ob)

   Return non-zero if *ob* is a reference object, but not a subclass of the
   reference type.  This function always succeeds.


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

   Return non-zero 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 referenceable object, or if *callback* is not callable, ``None``, or
   ``NULL``, this will raise :exc:`TypeError` and return ``NULL``.

   .. versionchanged:: 3.16
      Raise :exc:`!TypeError` if *callback* is not callable, ``None``, or
      ``NULL``.

   .. seealso::
      :c:func:`PyType_SUPPORTS_WEAKREFS` for checking if *ob* is weakly
      referenceable.


.. 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 referenceable object, or if *callback* is not callable,
   ``NULL``, this will raise :exc:`TypeError` and return ``NULL``.

   .. versionchanged:: 3.16
      Raise :exc:`!TypeError` if *callback* is not callable, ``None``, or
      ``NULL``.

   .. seealso::
      :c:func:`PyType_SUPPORTS_WEAKREFS` for checking if *ob* is weakly
      referenceable.


.. c:function:: int PyWeakref_GetRef(PyObject *ref, PyObject **pobj)

   Get a :term:`strong reference` to the referenced object from a weak
   reference, *ref*, into *\*pobj*.

   * On success, set *\*pobj* to a new :term:`strong reference` to the
     referenced object and return 1.
   * If the reference is dead, set *\*pobj* to ``NULL`` and return 0.
   * On error, raise an exception and return -1.

   .. versionadded:: 3.13


.. c:function:: int PyWeakref_IsDead(PyObject *ref)

   Test if the weak reference *ref* is dead. Returns 1 if the reference is
   dead, 0 if it is alive, and -1 with an error set if *ref* is not a weak
   reference object.

   .. versionadded:: 3.14


.. 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.


.. c:function:: void PyUnstable_Object_ClearWeakRefsNoCallbacks(PyObject *object)

   Clears the weakrefs for *object* without calling the callbacks.

   This function is called by the :c:member:`~PyTypeObject.tp_dealloc` handler
   for types with finalizers (i.e., :meth:`~object.__del__`).  The handler for
   those objects first calls :c:func:`PyObject_ClearWeakRefs` to clear weakrefs
   and call their callbacks, then the finalizer, and finally this function to
   clear any weakrefs that may have been created by the finalizer.

   In most circumstances, it's more appropriate to use
   :c:func:`PyObject_ClearWeakRefs` to clear weakrefs instead of this function.

   .. versionadded:: 3.13