:mod:`gc` --- Garbage Collector interface

=========================================

.. module:: gc

   :synopsis: Interface to the cycle-detecting garbage collector.

.. moduleauthor:: Neil Schemenauer <nas@arctrix.com>

.. sectionauthor:: Neil Schemenauer <nas@arctrix.com>

This module provides an interface to the optional garbage collector.  It

provides the ability to disable the collector, tune the collection frequency,

and set debugging options.  It also provides access to unreachable objects that

the collector found but cannot free.  Since the collector supplements the

reference counting already used in Python, you can disable the collector if you

are sure your program does not create reference cycles.  Automatic collection

can be disabled by calling ``gc.disable()``.  To debug a leaking program call

``gc.set_debug(gc.DEBUG_LEAK)``. Notice that this includes

``gc.DEBUG_SAVEALL``, causing garbage-collected objects to be saved in

gc.garbage for inspection.

The :mod:`gc` module provides the following functions:

.. function:: enable()

   Enable automatic garbage collection.

.. function:: disable()

   Disable automatic garbage collection.

.. function:: isenabled()

   Returns true if automatic collection is enabled.

.. function:: collect([generation])

   With no arguments, run a full collection.  The optional argument *generation*

   may be an integer specifying which generation to collect (from 0 to 2).  A

   :exc:`ValueError` is raised if the generation number  is invalid. The number of

   unreachable objects found is returned.

   .. versionchanged:: 2.5

      The optional *generation* argument was added.

   .. versionchanged:: 2.6

      The free lists maintained for a number of builtin types are cleared

      whenever a full collection or collection of the highest generation (2)

      is run.  Not all items in some free lists may be freed due to the

      particular implementation, in particular :class:`int` and :class:`float`.

.. function:: set_debug(flags)

   Set the garbage collection debugging flags. Debugging information will be

   written to ``sys.stderr``.  See below for a list of debugging flags which can be

   combined using bit operations to control debugging.

.. function:: get_debug()

   Return the debugging flags currently set.

.. function:: get_objects()

   Returns a list of all objects tracked by the collector, excluding the list

   returned.

   .. versionadded:: 2.2

.. function:: set_threshold(threshold0[, threshold1[, threshold2]])

   Set the garbage collection thresholds (the collection frequency). Setting

   *threshold0* to zero disables collection.

   The GC classifies objects into three generations depending on how many

   collection sweeps they have survived.  New objects are placed in the youngest

   generation (generation ``0``).  If an object survives a collection it is moved

   into the next older generation.  Since generation ``2`` is the oldest

   generation, objects in that generation remain there after a collection.  In

   order to decide when to run, the collector keeps track of the number object

   allocations and deallocations since the last collection.  When the number of

   allocations minus the number of deallocations exceeds *threshold0*, collection

   starts.  Initially only generation ``0`` is examined.  If generation ``0`` has

   been examined more than *threshold1* times since generation ``1`` has been

   examined, then generation ``1`` is examined as well.  Similarly, *threshold2*

   controls the number of collections of generation ``1`` before collecting

   generation ``2``.

.. function:: get_count()

   Return the current collection  counts as a tuple of ``(count0, count1,

   count2)``.

   .. versionadded:: 2.5

.. function:: get_threshold()

   Return the current collection thresholds as a tuple of ``(threshold0,

   threshold1, threshold2)``.

.. function:: get_referrers(*objs)

   Return the list of objects that directly refer to any of objs. This function

   will only locate those containers which support garbage collection; extension

   types which do refer to other objects but do not support garbage collection will

   not be found.

   Note that objects which have already been dereferenced, but which live in cycles

   and have not yet been collected by the garbage collector can be listed among the

   resulting referrers.  To get only currently live objects, call :func:`collect`

   before calling :func:`get_referrers`.

   Care must be taken when using objects returned by :func:`get_referrers` because

   some of them could still be under construction and hence in a temporarily

   invalid state. Avoid using :func:`get_referrers` for any purpose other than

   debugging.

   .. versionadded:: 2.2

.. function:: get_referents(*objs)

   Return a list of objects directly referred to by any of the arguments. The

   referents returned are those objects visited by the arguments' C-level

   :attr:`tp_traverse` methods (if any), and may not be all objects actually

   directly reachable.  :attr:`tp_traverse` methods are supported only by objects

   that support garbage collection, and are only required to visit objects that may

   be involved in a cycle.  So, for example, if an integer is directly reachable

   from an argument, that integer object may or may not appear in the result list.

   .. versionadded:: 2.3

The following variable is provided for read-only access (you can mutate its

value but should not rebind it):

.. data:: garbage

   A list of objects which the collector found to be unreachable but could not be

   freed (uncollectable objects).  By default, this list contains only objects with

   :meth:`__del__` methods. [#]_ Objects that have :meth:`__del__` methods and are

   part of a reference cycle cause the entire reference cycle to be uncollectable,

   including objects not necessarily in the cycle but reachable only from it.

   Python doesn't collect such cycles automatically because, in general, it isn't

   possible for Python to guess a safe order in which to run the :meth:`__del__`

   methods.  If you know a safe order, you can force the issue by examining the

   *garbage* list, and explicitly breaking cycles due to your objects within the

   list.  Note that these objects are kept alive even so by virtue of being in the

   *garbage* list, so they should be removed from *garbage* too.  For example,

   after breaking cycles, do ``del gc.garbage[:]`` to empty the list.  It's

   generally better to avoid the issue by not creating cycles containing objects

   with :meth:`__del__` methods, and *garbage* can be examined in that case to

   verify that no such cycles are being created.

   If :const:`DEBUG_SAVEALL` is set, then all unreachable objects will be added to

   this list rather than freed.

The following constants are provided for use with :func:`set_debug`:

.. data:: DEBUG_STATS

   Print statistics during collection.  This information can be useful when tuning

   the collection frequency.

.. data:: DEBUG_COLLECTABLE

   Print information on collectable objects found.

.. data:: DEBUG_UNCOLLECTABLE

   Print information of uncollectable objects found (objects which are not

   reachable but cannot be freed by the collector).  These objects will be added to

   the ``garbage`` list.

.. data:: DEBUG_INSTANCES

   When :const:`DEBUG_COLLECTABLE` or :const:`DEBUG_UNCOLLECTABLE` is set, print

   information about instance objects found.

.. data:: DEBUG_OBJECTS

   When :const:`DEBUG_COLLECTABLE` or :const:`DEBUG_UNCOLLECTABLE` is set, print

   information about objects other than instance objects found.

.. data:: DEBUG_SAVEALL

   When set, all unreachable objects found will be appended to *garbage* rather

   than being freed.  This can be useful for debugging a leaking program.

.. data:: DEBUG_LEAK

   The debugging flags necessary for the collector to print information about a

   leaking program (equal to ``DEBUG_COLLECTABLE | DEBUG_UNCOLLECTABLE |

   DEBUG_INSTANCES | DEBUG_OBJECTS | DEBUG_SAVEALL``).

.. rubric:: Footnotes

.. [#] Prior to Python 2.2, the list contained all instance objects in unreachable

   cycles,  not only those with :meth:`__del__` methods.