.. highlightlang:: 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.

.. An example showing the use of these interfaces can be found in "Supporting the

.. Cycle Collector (XXX not found: ../ext/example-cycle-support.html)".

To create a container type, the :attr:`tp_flags` field of the type object must

include the :const:`Py_TPFLAGS_HAVE_GC` and provide an implementation of the

:attr:`tp_traverse` handler.  If instances of the type are mutable, a

:attr:`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 :cfunc:`PyObject_GC_New` or

   :cfunc:`PyObject_GC_VarNew`.

#. Once all the fields which may contain references to other containers are

   initialized, it must call :cfunc:`PyObject_GC_Track`.

.. cfunction:: TYPE* PyObject_GC_New(TYPE, PyTypeObject *type)

   Analogous to :cfunc:`PyObject_New` but for container objects with the

   :const:`Py_TPFLAGS_HAVE_GC` flag set.

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

   Analogous to :cfunc:`PyObject_NewVar` but for container objects with the

   :const:`Py_TPFLAGS_HAVE_GC` flag set.

.. cfunction:: PyVarObject * PyObject_GC_Resize(PyVarObject *op, Py_ssize_t)

   Resize an object allocated by :cfunc:`PyObject_NewVar`.  Returns the resized

   object or *NULL* on failure.

.. cfunction:: 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

   :attr:`tp_traverse` handler become valid, usually near the end of the

   constructor.

.. cfunction:: void _PyObject_GC_TRACK(PyObject *op)

   A macro version of :cfunc:`PyObject_GC_Track`.  It should not be used for

   extension modules.

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

rules:

#. Before fields which refer to other containers are invalidated,

   :cfunc:`PyObject_GC_UnTrack` must be called.

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

.. cfunction:: void PyObject_GC_Del(void *op)

   Releases memory allocated to an object using :cfunc:`PyObject_GC_New` or

   :cfunc:`PyObject_GC_NewVar`.

.. cfunction:: void PyObject_GC_UnTrack(void *op)

   Remove the object *op* from the set of container objects tracked by the

   collector.  Note that :cfunc:`PyObject_GC_Track` can be called again on this

   object to add it back to the set of tracked objects.  The deallocator

   (:attr:`tp_dealloc` handler) should call this for the object before any of the

   fields used by the :attr:`tp_traverse` handler become invalid.

.. cfunction:: void _PyObject_GC_UNTRACK(PyObject *op)

   A macro version of :cfunc:`PyObject_GC_UnTrack`.  It should not be used for

   extension modules.

The :attr:`tp_traverse` handler accepts a function parameter of this type:

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

   Type of the visitor function passed to the :attr:`tp_traverse` handler.  The

   function should be called with an object to traverse as *object* and the third

   parameter to the :attr:`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 :attr:`tp_traverse` handler must have the following type:

.. ctype:: 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 :attr:`tp_traverse` handlers, a :cfunc:`Py_VISIT` macro is

provided.  In order to use this macro, the :attr:`tp_traverse` implementation

must name its arguments exactly *visit* and *arg*:

.. cfunction:: void Py_VISIT(PyObject *o)

   Call the *visit* callback, with arguments *o* and *arg*. If *visit* returns a

   non-zero value, then return it.  Using this macro, :attr:`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;

      }

   .. versionadded:: 2.4

The :attr:`tp_clear` handler must be of the :ctype:`inquiry` type, or *NULL* if

the object is immutable.

.. ctype:: 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 :cfunc:`Py_DECREF` on a reference).  The collector will call

   this method if it detects that this object is involved in a reference cycle.