.. highlight:: c

.. _supporting-cycle-detection:

Supporting Cyclic Garbage Collection
====================================

Python's support for detecting and collecting garbage which involves circular
references requires support from object types which are "containers" for other
objects which may also be containers.  Types which do not store references to
other objects, or which only store references to atomic types (such as numbers
or strings), do not need to provide any explicit support for garbage
collection.

To create a container type, the :c:member:`~PyTypeObject.tp_flags` field of the type object must
include the :const:`Py_TPFLAGS_HAVE_GC` and provide an implementation of the
:c:member:`~PyTypeObject.tp_traverse` handler.  If instances of the type are mutable, a
:c:member:`~PyTypeObject.tp_clear` implementation must also be provided.


.. data:: Py_TPFLAGS_HAVE_GC
   :noindex:

   Objects with a type with this flag set must conform with the rules
   documented here.  For convenience these objects will be referred to as
   container objects.

Constructors for container types must conform to two rules:

#. The memory for the object must be allocated using :c:func:`PyObject_GC_New`
   or :c:func:`PyObject_GC_NewVar`.

#. Once all the fields which may contain references to other containers are
   initialized, it must call :c:func:`PyObject_GC_Track`.


.. c:function:: TYPE* PyObject_GC_New(TYPE, PyTypeObject *type)

   Analogous to :c:func:`PyObject_New` but for container objects with the
   :const:`Py_TPFLAGS_HAVE_GC` flag set.


.. c:function:: TYPE* PyObject_GC_NewVar(TYPE, PyTypeObject *type, Py_ssize_t size)

   Analogous to :c:func:`PyObject_NewVar` but for container objects with the
   :const:`Py_TPFLAGS_HAVE_GC` flag set.


.. c:function:: TYPE* PyObject_GC_Resize(TYPE, PyVarObject *op, Py_ssize_t newsize)

   Resize an object allocated by :c:func:`PyObject_NewVar`.  Returns the
   resized object or *NULL* on failure.  *op* must not be tracked by the collector yet.


.. c:function:: void PyObject_GC_Track(PyObject *op)

   Adds the object *op* to the set of container objects tracked by the
   collector.  The collector can run at unexpected times so objects must be
   valid while being tracked.  This should be called once all the fields
   followed by the :c:member:`~PyTypeObject.tp_traverse` handler become valid, usually near the
   end of the constructor.


Similarly, the deallocator for the object must conform to a similar pair of
rules:

#. Before fields which refer to other containers are invalidated,
   :c:func:`PyObject_GC_UnTrack` must be called.

#. The object's memory must be deallocated using :c:func:`PyObject_GC_Del`.


.. c:function:: void PyObject_GC_Del(void *op)

   Releases memory allocated to an object using :c:func:`PyObject_GC_New` or
   :c:func:`PyObject_GC_NewVar`.


.. c:function:: void PyObject_GC_UnTrack(void *op)

   Remove the object *op* from the set of container objects tracked by the
   collector.  Note that :c:func:`PyObject_GC_Track` can be called again on
   this object to add it back to the set of tracked objects.  The deallocator
   (:c:member:`~PyTypeObject.tp_dealloc` handler) should call this for the object before any of
   the fields used by the :c:member:`~PyTypeObject.tp_traverse` handler become invalid.


.. versionchanged:: 3.8

   The :c:func:`_PyObject_GC_TRACK` and :c:func:`_PyObject_GC_UNTRACK` macros
   have been removed from the public C API.

The :c:member:`~PyTypeObject.tp_traverse` handler accepts a function parameter of this type:


.. c:type:: int (*visitproc)(PyObject *object, void *arg)

   Type of the visitor function passed to the :c:member:`~PyTypeObject.tp_traverse` handler.
   The function should be called with an object to traverse as *object* and
   the third parameter to the :c:member:`~PyTypeObject.tp_traverse` handler as *arg*.  The
   Python core uses several visitor functions to implement cyclic garbage
   detection; it's not expected that users will need to write their own
   visitor functions.

The :c:member:`~PyTypeObject.tp_traverse` handler must have the following type:


.. c:type:: int (*traverseproc)(PyObject *self, visitproc visit, void *arg)

   Traversal function for a container object.  Implementations must call the
   *visit* function for each object directly contained by *self*, with the
   parameters to *visit* being the contained object and the *arg* value passed
   to the handler.  The *visit* function must not be called with a *NULL*
   object argument.  If *visit* returns a non-zero value that value should be
   returned immediately.

To simplify writing :c:member:`~PyTypeObject.tp_traverse` handlers, a :c:func:`Py_VISIT` macro is
provided.  In order to use this macro, the :c:member:`~PyTypeObject.tp_traverse` implementation
must name its arguments exactly *visit* and *arg*:


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

   If *o* is not *NULL*, call the *visit* callback, with arguments *o*
   and *arg*.  If *visit* returns a non-zero value, then return it.
   Using this macro, :c:member:`~PyTypeObject.tp_traverse` handlers
   look like::

      static int
      my_traverse(Noddy *self, visitproc visit, void *arg)
      {
          Py_VISIT(self->foo);
          Py_VISIT(self->bar);
          return 0;
      }

The :c:member:`~PyTypeObject.tp_clear` handler must be of the :c:type:`inquiry` type, or *NULL*
if the object is immutable.


.. c:type:: int (*inquiry)(PyObject *self)

   Drop references that may have created reference cycles.  Immutable objects
   do not have to define this method since they can never directly create
   reference cycles.  Note that the object must still be valid after calling
   this method (don't just call :c:func:`Py_DECREF` on a reference).  The
   collector will call this method if it detects that this object is involved
   in a reference cycle.