cpython/Doc/c-api/refcounting.rst

.. highlight:: c


.. _countingrefs:

******************
Reference Counting
******************

The functions and macros in this section are used for managing reference counts
of Python objects.


.. c:function:: Py_ssize_t Py_REFCNT(PyObject *o)

   Get the reference count of the Python object *o*.

   Use the :c:func:`Py_SET_REFCNT()` function to set an object reference count.

   .. versionchanged:: 3.11
      The parameter type is no longer :c:expr:`const PyObject*`.

   .. versionchanged:: 3.10
      :c:func:`Py_REFCNT()` is changed to the inline static function.


.. c:function:: void Py_SET_REFCNT(PyObject *o, Py_ssize_t refcnt)

   Set the object *o* reference counter to *refcnt*.

   .. versionadded:: 3.9


.. c:function:: void Py_INCREF(PyObject *o)

   Increment the reference count for object *o*.

   This function is usually used to convert a :term:`borrowed reference` to a
   :term:`strong reference` in-place. The :c:func:`Py_NewRef` function can be
   used to create a new :term:`strong reference`.

   The object must not be ``NULL``; if you aren't sure that it isn't
   ``NULL``, use :c:func:`Py_XINCREF`.


.. c:function:: void Py_XINCREF(PyObject *o)

   Increment the reference count for object *o*.  The object may be ``NULL``, in
   which case the macro has no effect.

   See also :c:func:`Py_XNewRef`.


.. c:function:: PyObject* Py_NewRef(PyObject *o)

   Create a new :term:`strong reference` to an object: increment the reference
   count of the object *o* and return the object *o*.

   When the :term:`strong reference` is no longer needed, :c:func:`Py_DECREF`
   should be called on it to decrement the object reference count.

   The object *o* must not be ``NULL``; use :c:func:`Py_XNewRef` if *o* can be
   ``NULL``.

   For example::

       Py_INCREF(obj);
       self->attr = obj;

   can be written as::

       self->attr = Py_NewRef(obj);

   See also :c:func:`Py_INCREF`.

   .. versionadded:: 3.10


.. c:function:: PyObject* Py_XNewRef(PyObject *o)

   Similar to :c:func:`Py_NewRef`, but the object *o* can be NULL.

   If the object *o* is ``NULL``, the function just returns ``NULL``.

   .. versionadded:: 3.10


.. c:function:: void Py_DECREF(PyObject *o)

   Decrement the reference count for object *o*.

   If the reference count reaches zero, the object's type's deallocation
   function (which must not be ``NULL``) is invoked.

   This function is usually used to delete a :term:`strong reference` before
   exiting its scope.

   The object must not be ``NULL``; if you aren't sure that it isn't ``NULL``,
   use :c:func:`Py_XDECREF`.

   .. warning::

      The deallocation function can cause arbitrary Python code to be invoked (e.g.
      when a class instance with a :meth:`__del__` method is deallocated).  While
      exceptions in such code are not propagated, the executed code has free access to
      all Python global variables.  This means that any object that is reachable from
      a global variable should be in a consistent state before :c:func:`Py_DECREF` is
      invoked.  For example, code to delete an object from a list should copy a
      reference to the deleted object in a temporary variable, update the list data
      structure, and then call :c:func:`Py_DECREF` for the temporary variable.


.. c:function:: void Py_XDECREF(PyObject *o)

   Decrement the reference count for object *o*.  The object may be ``NULL``, in
   which case the macro has no effect; otherwise the effect is the same as for
   :c:func:`Py_DECREF`, and the same warning applies.


.. c:function:: void Py_CLEAR(PyObject *o)

   Decrement the reference count for object *o*.  The object may be ``NULL``, in
   which case the macro has no effect; otherwise the effect is the same as for
   :c:func:`Py_DECREF`, except that the argument is also set to ``NULL``.  The warning
   for :c:func:`Py_DECREF` does not apply with respect to the object passed because
   the macro carefully uses a temporary variable and sets the argument to ``NULL``
   before decrementing its reference count.

   It is a good idea to use this macro whenever decrementing the reference
   count of an object that might be traversed during garbage collection.

   .. versionchanged:: 3.12
      The macro argument is now only evaluated once. If the argument has side
      effects, these are no longer duplicated.


.. c:function:: void Py_IncRef(PyObject *o)

   Increment the reference count for object *o*. A function version of :c:func:`Py_XINCREF`.
   It can be used for runtime dynamic embedding of Python.


.. c:function:: void Py_DecRef(PyObject *o)

   Decrement the reference count for object *o*. A function version of :c:func:`Py_XDECREF`.
   It can be used for runtime dynamic embedding of Python.


.. c:macro:: Py_SETREF(dst, src)

   Macro safely decrementing the `dst` reference count and setting `dst` to
   `src`.

   As in case of :c:func:`Py_CLEAR`, "the obvious" code can be deadly::

       Py_DECREF(dst);
       dst = src;

   The safe way is::

        Py_SETREF(dst, src);

   That arranges to set `dst` to `src` _before_ decrementing reference count of
   *dst* old value, so that any code triggered as a side-effect of `dst`
   getting torn down no longer believes `dst` points to a valid object.

   .. versionadded:: 3.6

   .. versionchanged:: 3.12
      The macro arguments are now only evaluated once. If an argument has side
      effects, these are no longer duplicated.


.. c:macro:: Py_XSETREF(dst, src)

   Variant of :c:macro:`Py_SETREF` macro that uses :c:func:`Py_XDECREF` instead
   of :c:func:`Py_DECREF`.

   .. versionadded:: 3.6

   .. versionchanged:: 3.12
      The macro arguments are now only evaluated once. If an argument has side
      effects, these are no longer duplicated.