.. 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.
.. 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.
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