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PEP 683 (immortal objects) revealed some ways in which the Python documentation has been unnecessarily coupled to the implementation details of reference counts. In the end users should focus on reference ownership, including taking references and releasing them, rather than on how many reference counts an object has. This change updates the documentation to reflect that perspective. It also updates the docs relative to immortal objects in a handful of places.
220 lines
6.8 KiB
ReStructuredText
220 lines
6.8 KiB
ReStructuredText
.. highlight:: c
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.. _countingrefs:
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******************
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Reference Counting
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******************
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The functions and macros in this section are used for managing reference counts
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of Python objects.
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.. c:function:: Py_ssize_t Py_REFCNT(PyObject *o)
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Get the reference count of the Python object *o*.
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Note that the returned value may not actually reflect how many
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references to the object are actually held. For example, some
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objects are "immortal" and have a very high refcount that does not
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reflect the actual number of references. Consequently, do not rely
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on the returned value to be accurate, other than a value of 0 or 1.
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Use the :c:func:`Py_SET_REFCNT()` function to set an object reference count.
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.. versionchanged:: 3.11
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The parameter type is no longer :c:expr:`const PyObject*`.
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.. versionchanged:: 3.10
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:c:func:`Py_REFCNT()` is changed to the inline static function.
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.. c:function:: void Py_SET_REFCNT(PyObject *o, Py_ssize_t refcnt)
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Set the object *o* reference counter to *refcnt*.
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Note that this function has no effect on
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`immortal <https://peps.python.org/pep-0683/>`_
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objects.
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.. versionadded:: 3.9
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.. versionchanged:: 3.12
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Immortal objects are not modified.
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.. c:function:: void Py_INCREF(PyObject *o)
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Indicate taking a new :term:`strong reference` to object *o*,
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indicating it is in use and should not be destroyed.
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This function is usually used to convert a :term:`borrowed reference` to a
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:term:`strong reference` in-place. The :c:func:`Py_NewRef` function can be
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used to create a new :term:`strong reference`.
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When done using the object, release is by calling :c:func:`Py_DECREF`.
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The object must not be ``NULL``; if you aren't sure that it isn't
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``NULL``, use :c:func:`Py_XINCREF`.
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Do not expect this function to actually modify *o* in any way.
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For at least `some objects <https://peps.python.org/pep-0683/>`_,
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this function has no effect.
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.. versionchanged:: 3.12
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Immortal objects are not modified.
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.. c:function:: void Py_XINCREF(PyObject *o)
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Similar to :c:func:`Py_INCREF`, but the object *o* can be ``NULL``,
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in which case this has no effect.
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See also :c:func:`Py_XNewRef`.
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.. c:function:: PyObject* Py_NewRef(PyObject *o)
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Create a new :term:`strong reference` to an object:
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call :c:func:`Py_INCREF` on *o* and return the object *o*.
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When the :term:`strong reference` is no longer needed, :c:func:`Py_DECREF`
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should be called on it to release the reference.
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The object *o* must not be ``NULL``; use :c:func:`Py_XNewRef` if *o* can be
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``NULL``.
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For example::
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Py_INCREF(obj);
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self->attr = obj;
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can be written as::
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self->attr = Py_NewRef(obj);
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See also :c:func:`Py_INCREF`.
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.. versionadded:: 3.10
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.. c:function:: PyObject* Py_XNewRef(PyObject *o)
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Similar to :c:func:`Py_NewRef`, but the object *o* can be NULL.
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If the object *o* is ``NULL``, the function just returns ``NULL``.
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.. versionadded:: 3.10
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.. c:function:: void Py_DECREF(PyObject *o)
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Release a :term:`strong reference` to object *o*, indicating the
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reference is no longer used.
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Once the last :term:`strong reference` is released
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(i.e. the object's reference count reaches 0),
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the object's type's deallocation
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function (which must not be ``NULL``) is invoked.
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This function is usually used to delete a :term:`strong reference` before
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exiting its scope.
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The object must not be ``NULL``; if you aren't sure that it isn't ``NULL``,
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use :c:func:`Py_XDECREF`.
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Do not expect this function to actually modify *o* in any way.
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For at least `some objects <https://peps.python.org/pep-0683/>`_,
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this function has no effect.
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.. warning::
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The deallocation function can cause arbitrary Python code to be invoked (e.g.
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when a class instance with a :meth:`~object.__del__` method is deallocated). While
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exceptions in such code are not propagated, the executed code has free access to
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all Python global variables. This means that any object that is reachable from
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a global variable should be in a consistent state before :c:func:`Py_DECREF` is
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invoked. For example, code to delete an object from a list should copy a
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reference to the deleted object in a temporary variable, update the list data
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structure, and then call :c:func:`Py_DECREF` for the temporary variable.
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.. versionchanged:: 3.12
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Immortal objects are not modified.
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.. c:function:: void Py_XDECREF(PyObject *o)
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Similar to :c:func:`Py_DECREF`, but the object *o* can be ``NULL``,
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in which case this has no effect.
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The same warning from :c:func:`Py_DECREF` applies here as well.
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.. c:function:: void Py_CLEAR(PyObject *o)
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Release a :term:`strong reference` for object *o*.
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The object may be ``NULL``, in
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which case the macro has no effect; otherwise the effect is the same as for
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:c:func:`Py_DECREF`, except that the argument is also set to ``NULL``. The warning
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for :c:func:`Py_DECREF` does not apply with respect to the object passed because
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the macro carefully uses a temporary variable and sets the argument to ``NULL``
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before releasing the reference.
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It is a good idea to use this macro whenever releasing a reference
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to an object that might be traversed during garbage collection.
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.. versionchanged:: 3.12
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The macro argument is now only evaluated once. If the argument has side
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effects, these are no longer duplicated.
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.. c:function:: void Py_IncRef(PyObject *o)
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Indicate taking a new :term:`strong reference` to object *o*.
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A function version of :c:func:`Py_XINCREF`.
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It can be used for runtime dynamic embedding of Python.
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.. c:function:: void Py_DecRef(PyObject *o)
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Release a :term:`strong reference` to object *o*.
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A function version of :c:func:`Py_XDECREF`.
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It can be used for runtime dynamic embedding of Python.
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.. c:macro:: Py_SETREF(dst, src)
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Macro safely releasing a :term:`strong reference` to object *dst*
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and setting *dst* to *src*.
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As in case of :c:func:`Py_CLEAR`, "the obvious" code can be deadly::
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Py_DECREF(dst);
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dst = src;
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The safe way is::
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Py_SETREF(dst, src);
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That arranges to set *dst* to *src* _before_ releasing the reference
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to the old value of *dst*, so that any code triggered as a side-effect
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of *dst* getting torn down no longer believes *dst* points
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to a valid object.
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.. versionadded:: 3.6
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.. versionchanged:: 3.12
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The macro arguments are now only evaluated once. If an argument has side
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effects, these are no longer duplicated.
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.. c:macro:: Py_XSETREF(dst, src)
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Variant of :c:macro:`Py_SETREF` macro that uses :c:func:`Py_XDECREF` instead
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of :c:func:`Py_DECREF`.
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.. versionadded:: 3.6
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.. versionchanged:: 3.12
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The macro arguments are now only evaluated once. If an argument has side
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effects, these are no longer duplicated.
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