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Reference Counting

Doc/c-api/refcounting.rst

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.. 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.

Note that the returned value may not actually reflect how many references to the object are actually held. For example, some objects are :term:immortal and have a very high refcount that does not reflect the actual number of references. Consequently, do not rely on the returned value to be accurate, other than a value of 0 or 1.

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

.. note::

  On :term:`free-threaded builds <free-threaded build>` of Python, returning 1
  isn't sufficient to determine if it's safe to treat *o* as having no
  access by other threads. Use :c:func:`PyUnstable_Object_IsUniquelyReferenced`
  for that instead.

  See also the function :c:func:`PyUnstable_Object_IsUniqueReferencedTemporary()`.

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

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

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

Set the object o reference counter to refcnt.

On :ref:Python build with Free Threading <free-threading-build>, if refcnt is larger than UINT32_MAX, the object is made :term:immortal.

This function has no effect on :term:immortal objects.

.. versionadded:: 3.9

.. versionchanged:: 3.12 Immortal objects are not modified.

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

Indicate taking a new :term:strong reference to object o, indicating it is in use and should not be destroyed.

This function has no effect on :term:immortal objects.

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.

When done using the object, release is by calling :c:func:Py_DECREF.

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

Do not expect this function to actually modify o in any way. For at least :pep:some objects <0683>, this function has no effect.

.. versionchanged:: 3.12 Immortal objects are not modified.

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

Similar to :c:func:Py_INCREF, but the object o can be NULL, in which case this 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: call :c:func:Py_INCREF on 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 release the reference.

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)

Release a :term:strong reference to object o, indicating the reference is no longer used.

This function has no effect on :term:immortal objects.

Once the last :term:strong reference is released (i.e. the object's reference count reaches 0), 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.

Do not expect this function to actually modify o in any way. For at least :pep:some objects <683>, this function has no effect.

.. warning::

  The deallocation function can cause arbitrary Python code to be invoked (e.g.
  when a class instance with a :meth:`~object.__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.

.. versionchanged:: 3.12 Immortal objects are not modified.

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

Similar to :c:func:Py_DECREF, but the object o can be NULL, in which case this has no effect. The same warning from :c:func:Py_DECREF applies here as well.

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

Release a :term:strong reference 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 releasing the reference.

It is a good idea to use this macro whenever releasing a reference to 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)

Indicate taking a new :term:strong reference to 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)

Release a :term:strong reference to 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 releasing a :term:strong reference to object dst 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 releasing the reference to the old value of dst, 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.