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diff --git a/Doc/c-api/lifecycle.rst b/Doc/c-api/lifecycle.rst new file mode 100644 index 00000000000..0e2ffc096ca --- /dev/null +++ b/Doc/c-api/lifecycle.rst @@ -0,0 +1,273 @@ +.. highlight:: c + +.. _life-cycle: + +Object Life Cycle +================= + +This section explains how a type's slots relate to each other throughout the +life of an object. It is not intended to be a complete canonical reference for +the slots; instead, refer to the slot-specific documentation in +:ref:`type-structs` for details about a particular slot. + + +Life Events +----------- + +The figure below illustrates the order of events that can occur throughout an +object's life. An arrow from *A* to *B* indicates that event *B* can occur +after event *A* has occurred, with the arrow's label indicating the condition +that must be true for *B* to occur after *A*. + +.. only:: html and not epub + + .. raw:: html + + <style type="text/css"> + + .. raw:: html + :file: lifecycle.dot.css + + .. raw:: html + + </style> + + .. raw:: html + :file: lifecycle.dot.svg + + .. raw:: html + + <script> + (() => { + const g = document.getElementById('life_events_graph'); + const title = g.querySelector(':scope > title'); + title.id = 'life-events-graph-title'; + const svg = g.closest('svg'); + svg.role = 'img'; + svg.setAttribute('aria-describedby', + 'life-events-graph-description'); + svg.setAttribute('aria-labelledby', 'life-events-graph-title'); + })(); + </script> + +.. only:: epub or not (html or latex) + + .. image:: lifecycle.dot.svg + :align: center + :class: invert-in-dark-mode + :alt: Diagram showing events in an object's life. Explained in detail + below. + +.. only:: latex + + .. image:: lifecycle.dot.pdf + :align: center + :class: invert-in-dark-mode + :alt: Diagram showing events in an object's life. Explained in detail + below. + +.. container:: + :name: life-events-graph-description + + Explanation: + + * When a new object is constructed by calling its type: + + #. :c:member:`~PyTypeObject.tp_new` is called to create a new object. + #. :c:member:`~PyTypeObject.tp_alloc` is directly called by + :c:member:`~PyTypeObject.tp_new` to allocate the memory for the new + object. + #. :c:member:`~PyTypeObject.tp_init` initializes the newly created object. + :c:member:`!tp_init` can be called again to re-initialize an object, if + desired. The :c:member:`!tp_init` call can also be skipped entirely, + for example by Python code calling :py:meth:`~object.__new__`. + + * After :c:member:`!tp_init` completes, the object is ready to use. + * Some time after the last reference to an object is removed: + + #. If an object is not marked as *finalized*, it might be finalized by + marking it as *finalized* and calling its + :c:member:`~PyTypeObject.tp_finalize` function. Python does + *not* finalize an object when the last reference to it is deleted; use + :c:func:`PyObject_CallFinalizerFromDealloc` to ensure that + :c:member:`~PyTypeObject.tp_finalize` is always called. + #. If the object is marked as finalized, + :c:member:`~PyTypeObject.tp_clear` might be called by the garbage collector + to clear references held by the object. It is *not* called when the + object's reference count reaches zero. + #. :c:member:`~PyTypeObject.tp_dealloc` is called to destroy the object. + To avoid code duplication, :c:member:`~PyTypeObject.tp_dealloc` typically + calls into :c:member:`~PyTypeObject.tp_clear` to free up the object's + references. + #. When :c:member:`~PyTypeObject.tp_dealloc` finishes object destruction, + it directly calls :c:member:`~PyTypeObject.tp_free` (usually set to + :c:func:`PyObject_Free` or :c:func:`PyObject_GC_Del` automatically as + appropriate for the type) to deallocate the memory. + + * The :c:member:`~PyTypeObject.tp_finalize` function is permitted to add a + reference to the object if desired. If it does, the object is + *resurrected*, preventing its pending destruction. (Only + :c:member:`!tp_finalize` is allowed to resurrect an object; + :c:member:`~PyTypeObject.tp_clear` and + :c:member:`~PyTypeObject.tp_dealloc` cannot without calling into + :c:member:`!tp_finalize`.) Resurrecting an object may + or may not cause the object's *finalized* mark to be removed. Currently, + Python does not remove the *finalized* mark from a resurrected object if + it supports garbage collection (i.e., the :c:macro:`Py_TPFLAGS_HAVE_GC` + flag is set) but does remove the mark if the object does not support + garbage collection; either or both of these behaviors may change in the + future. + * :c:member:`~PyTypeObject.tp_dealloc` can optionally call + :c:member:`~PyTypeObject.tp_finalize` via + :c:func:`PyObject_CallFinalizerFromDealloc` if it wishes to reuse that + code to help with object destruction. This is recommended because it + guarantees that :c:member:`!tp_finalize` is always called before + destruction. See the :c:member:`~PyTypeObject.tp_dealloc` documentation + for example code. + * If the object is a member of a :term:`cyclic isolate` and either + :c:member:`~PyTypeObject.tp_clear` fails to break the reference cycle or + the cyclic isolate is not detected (perhaps :func:`gc.disable` was called, + or the :c:macro:`Py_TPFLAGS_HAVE_GC` flag was erroneously omitted in one + of the involved types), the objects remain indefinitely uncollectable + (they "leak"). See :data:`gc.garbage`. + + If the object is marked as supporting garbage collection (the + :c:macro:`Py_TPFLAGS_HAVE_GC` flag is set in + :c:member:`~PyTypeObject.tp_flags`), the following events are also possible: + + * The garbage collector occasionally calls + :c:member:`~PyTypeObject.tp_traverse` to identify :term:`cyclic isolates + <cyclic isolate>`. + * When the garbage collector discovers a :term:`cyclic isolate`, it + finalizes one of the objects in the group by marking it as *finalized* and + calling its :c:member:`~PyTypeObject.tp_finalize` function, if it has one. + This repeats until the cyclic isolate doesn't exist or all of the objects + have been finalized. + * :c:member:`~PyTypeObject.tp_finalize` is permitted to resurrect the object + by adding a reference from outside the :term:`cyclic isolate`. The new + reference causes the group of objects to no longer form a cyclic isolate + (the reference cycle may still exist, but if it does the objects are no + longer isolated). + * When the garbage collector discovers a :term:`cyclic isolate` and all of + the objects in the group have already been marked as *finalized*, the + garbage collector clears one or more of the uncleared objects in the group + (possibly concurrently) by calling each's + :c:member:`~PyTypeObject.tp_clear` function. This repeats as long as the + cyclic isolate still exists and not all of the objects have been cleared. + + +Cyclic Isolate Destruction +-------------------------- + +Listed below are the stages of life of a hypothetical :term:`cyclic isolate` +that continues to exist after each member object is finalized or cleared. It +is a memory leak if a cyclic isolate progresses through all of these stages; it should +vanish once all objects are cleared, if not sooner. A cyclic isolate can +vanish either because the reference cycle is broken or because the objects are +no longer isolated due to finalizer resurrection (see +:c:member:`~PyTypeObject.tp_finalize`). + +0. **Reachable** (not yet a cyclic isolate): All objects are in their normal, + reachable state. A reference cycle could exist, but an external reference + means the objects are not yet isolated. +#. **Unreachable but consistent:** The final reference from outside the cyclic + group of objects has been removed, causing the objects to become isolated + (thus a cyclic isolate is born). None of the group's objects have been + finalized or cleared yet. The cyclic isolate remains at this stage until + some future run of the garbage collector (not necessarily the next run + because the next run might not scan every object). +#. **Mix of finalized and not finalized:** Objects in a cyclic isolate are + finalized one at a time, which means that there is a period of time when the + cyclic isolate is composed of a mix of finalized and non-finalized objects. + Finalization order is unspecified, so it can appear random. A finalized + object must behave in a sane manner when non-finalized objects interact with + it, and a non-finalized object must be able to tolerate the finalization of + an arbitrary subset of its referents. +#. **All finalized:** All objects in a cyclic isolate are finalized before any + of them are cleared. +#. **Mix of finalized and cleared:** The objects can be cleared serially or + concurrently (but with the :term:`GIL` held); either way, some will finish + before others. A finalized object must be able to tolerate the clearing of + a subset of its referents. :pep:`442` calls this stage "cyclic trash". +#. **Leaked:** If a cyclic isolate still exists after all objects in the group + have been finalized and cleared, then the objects remain indefinitely + uncollectable (see :data:`gc.garbage`). It is a bug if a cyclic isolate + reaches this stage---it means the :c:member:`~PyTypeObject.tp_clear` methods + of the participating objects have failed to break the reference cycle as + required. + +If :c:member:`~PyTypeObject.tp_clear` did not exist, then Python would have no +way to safely break a reference cycle. Simply destroying an object in a cyclic +isolate would result in a dangling pointer, triggering undefined behavior when +an object referencing the destroyed object is itself destroyed. The clearing +step makes object destruction a two-phase process: first +:c:member:`~PyTypeObject.tp_clear` is called to partially destroy the objects +enough to detangle them from each other, then +:c:member:`~PyTypeObject.tp_dealloc` is called to complete the destruction. + +Unlike clearing, finalization is not a phase of destruction. A finalized +object must still behave properly by continuing to fulfill its design +contracts. An object's finalizer is allowed to execute arbitrary Python code, +and is even allowed to prevent the impending destruction by adding a reference. +The finalizer is only related to destruction by call order---if it runs, it runs +before destruction, which starts with :c:member:`~PyTypeObject.tp_clear` (if +called) and concludes with :c:member:`~PyTypeObject.tp_dealloc`. + +The finalization step is not necessary to safely reclaim the objects in a +cyclic isolate, but its existence makes it easier to design types that behave +in a sane manner when objects are cleared. Clearing an object might +necessarily leave it in a broken, partially destroyed state---it might be +unsafe to call any of the cleared object's methods or access any of its +attributes. With finalization, only finalized objects can possibly interact +with cleared objects; non-finalized objects are guaranteed to interact with +only non-cleared (but potentially finalized) objects. + +To summarize the possible interactions: + +* A non-finalized object might have references to or from non-finalized and + finalized objects, but not to or from cleared objects. +* A finalized object might have references to or from non-finalized, finalized, + and cleared objects. +* A cleared object might have references to or from finalized and cleared + objects, but not to or from non-finalized objects. + +Without any reference cycles, an object can be simply destroyed once its last +reference is deleted; the finalization and clearing steps are not necessary to +safely reclaim unused objects. However, it can be useful to automatically call +:c:member:`~PyTypeObject.tp_finalize` and :c:member:`~PyTypeObject.tp_clear` +before destruction anyway because type design is simplified when all objects +always experience the same series of events regardless of whether they +participated in a cyclic isolate. Python currently only calls +:c:member:`~PyTypeObject.tp_finalize` and :c:member:`~PyTypeObject.tp_clear` as +needed to destroy a cyclic isolate; this may change in a future version. + + +Functions +--------- + +To allocate and free memory, see :ref:`allocating-objects`. + + +.. c:function:: void PyObject_CallFinalizer(PyObject *op) + + Finalizes the object as described in :c:member:`~PyTypeObject.tp_finalize`. + Call this function (or :c:func:`PyObject_CallFinalizerFromDealloc`) instead + of calling :c:member:`~PyTypeObject.tp_finalize` directly because this + function may deduplicate multiple calls to :c:member:`!tp_finalize`. + Currently, calls are only deduplicated if the type supports garbage + collection (i.e., the :c:macro:`Py_TPFLAGS_HAVE_GC` flag is set); this may + change in the future. + + +.. c:function:: int PyObject_CallFinalizerFromDealloc(PyObject *op) + + Same as :c:func:`PyObject_CallFinalizer` but meant to be called at the + beginning of the object's destructor (:c:member:`~PyTypeObject.tp_dealloc`). + There must not be any references to the object. If the object's finalizer + resurrects the object, this function returns -1; no further destruction + should happen. Otherwise, this function returns 0 and destruction can + continue normally. + + .. seealso:: + + :c:member:`~PyTypeObject.tp_dealloc` for example code. |