.. highlightlang:: c


.. _countingrefs:

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

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


.. cfunction:: void Py_INCREF(PyObject *o)

   Increment the reference count for object *o*.  The object must not be *NULL*; if
   you aren't sure that it isn't *NULL*, use :cfunc:`Py_XINCREF`.


.. cfunction:: 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.


.. cfunction:: void Py_DECREF(PyObject *o)

   Decrement the reference count for object *o*.  The object must not be *NULL*; if
   you aren't sure that it isn't *NULL*, use :cfunc:`Py_XDECREF`.  If the reference
   count reaches zero, the object's type's deallocation function (which must not be
   *NULL*) is invoked.

   .. 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 :cfunc:`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 :cfunc:`Py_DECREF` for the temporary variable.


.. cfunction:: 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
   :cfunc:`Py_DECREF`, and the same warning applies.


.. cfunction:: 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
   :cfunc:`Py_DECREF`, except that the argument is also set to *NULL*.  The warning
   for :cfunc:`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 value of a
   variable that might be traversed during garbage collection.


The following functions are for runtime dynamic embedding of Python:
``Py_IncRef(PyObject *o)``, ``Py_DecRef(PyObject *o)``. They are
simply exported function versions of :cfunc:`Py_XINCREF` and
:cfunc:`Py_XDECREF`, respectively.

The following functions or macros are only for use within the interpreter core:
:cfunc:`_Py_Dealloc`, :cfunc:`_Py_ForgetReference`, :cfunc:`_Py_NewReference`,
as well as the global variable :cdata:`_Py_RefTotal`.