:mod:!gc --- Garbage Collector interface

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

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()

Return True if automatic collection is enabled.

.. function:: collect(generation=2)

Perform a 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 sum of collected objects and uncollectable objects is returned.

Calling gc.collect(0) will perform a GC collection on the young generation.

Calling gc.collect(1) will perform a GC collection on the young generation and an increment of the old generation.

Calling gc.collect(2) or gc.collect() performs a full collection

The free lists maintained for a number of built-in 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:float.

The effect of calling gc.collect() while the interpreter is already performing a collection is undefined.

.. versionchanged:: 3.14 generation=1 performs an increment of collection.

.. 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(generation=None)

Returns a list of all objects tracked by the collector, excluding the list returned. If generation is not None, return only the objects as follows:

• 0: All objects in the young generation
• 1: No objects, as there is no generation 1 (as of Python 3.14)
• 2: All objects in the old generation

.. versionchanged:: 3.8 New generation parameter.

.. versionchanged:: 3.14 Generation 1 is removed

.. audit-event:: gc.get_objects generation gc.get_objects

.. function:: get_stats()

Return a list of three per-generation dictionaries containing collection statistics since interpreter start. The number of keys may change in the future, but currently each dictionary will contain the following items:

• collections is the number of times this generation was collected;

• collected is the total number of objects collected inside this generation;

• uncollectable is the total number of objects which were found to be uncollectable (and were therefore moved to the :data:garbage list) inside this generation;

• candidates is the total number of objects in this generation which were considered for collection and traversed;

• duration is the total time in seconds spent in collections for this generation.

.. versionadded:: 3.4

.. versionchanged:: 3.15 Add duration and candidates.

.. 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 two generations depending on whether they have survived a collection. New objects are placed in the young generation. If an object survives a collection it is moved into the old generation.

In order to decide when to run, the collector keeps track of the number of object allocations and deallocations since the last collection. When the number of allocations minus the number of deallocations exceeds threshold0, collection starts. For each collection, all the objects in the young generation and some fraction of the old generation is collected.

In the free-threaded build, the increase in process memory usage is also checked before running the collector. If the memory usage has not increased by 10% since the last collection and the net number of object allocations has not exceeded 40 times threshold0, the collection is not run.

The fraction of the old generation that is collected is inversely proportional to threshold1. The larger threshold1 is, the slower objects in the old generation are collected. For the default value of 10, 1% of the old generation is scanned during each collection.

threshold2 is ignored.

See Garbage collector design <https://devguide.python.org/garbage_collector>_ for more information.

.. versionchanged:: 3.14 threshold2 is ignored

.. function:: get_count()

Return the current collection counts as a tuple of (count0, count1, count2).

.. 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.

.. warning:: 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.

.. audit-event:: gc.get_referrers objs gc.get_referrers

.. 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 :c:member:~PyTypeObject.tp_traverse methods (if any), and may not be all objects actually directly reachable. :c:member:~PyTypeObject.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.

.. audit-event:: gc.get_referents objs gc.get_referents

.. function:: is_tracked(obj)

Returns True if the object is currently tracked by the garbage collector, False otherwise. As a general rule, instances of atomic types aren't tracked and instances of non-atomic types (containers, user-defined objects...) are. However, some type-specific optimizations can be present in order to suppress the garbage collector footprint of simple instances (e.g. dicts containing only atomic keys and values)::

  >>> gc.is_tracked(0)
  False
  >>> gc.is_tracked("a")
  False
  >>> gc.is_tracked([])
  True
  >>> gc.is_tracked({})
  False
  >>> gc.is_tracked({"a": 1})
  True

.. versionadded:: 3.1

.. function:: is_finalized(obj)

Returns True if the given object has been finalized by the garbage collector, False otherwise. ::

  >>> x = None
  >>> class Lazarus:
  ...     def __del__(self):
  ...         global x
  ...         x = self
  ...
  >>> lazarus = Lazarus()
  >>> gc.is_finalized(lazarus)
  False
  >>> del lazarus
  >>> gc.is_finalized(x)
  True

.. versionadded:: 3.9

.. function:: freeze()

Freeze all the objects tracked by the garbage collector; move them to a permanent generation and ignore them in all the future collections.

If a process will fork() without exec(), avoiding unnecessary copy-on-write in child processes will maximize memory sharing and reduce overall memory usage. This requires both avoiding creation of freed "holes" in memory pages in the parent process and ensuring that GC collections in child processes won't touch the gc_refs counter of long-lived objects originating in the parent process. To accomplish both, call gc.disable() early in the parent process, gc.freeze() right before fork(), and gc.enable() early in child processes.

.. versionadded:: 3.7

.. function:: unfreeze()

Unfreeze the objects in the permanent generation, put them back into the oldest generation.

.. versionadded:: 3.7

.. function:: get_freeze_count()

Return the number of objects in the permanent generation.

.. versionadded:: 3.7

The following variables are provided for read-only access (you can mutate the values but should not rebind them):

.. data:: garbage

A list of objects which the collector found to be unreachable but could not be freed (uncollectable objects). Starting with Python 3.4, this list should be empty most of the time, except when using instances of C extension types with a non-NULL tp_del slot.

If :const:DEBUG_SAVEALL is set, then all unreachable objects will be added to this list rather than freed.

.. versionchanged:: 3.2 If this list is non-empty at :term:interpreter shutdown, a :exc:ResourceWarning is emitted, which is silent by default. If :const:DEBUG_UNCOLLECTABLE is set, in addition all uncollectable objects are printed.

.. versionchanged:: 3.4 Following :pep:442, objects with a :meth:~object.__del__ method don't end up in :data:gc.garbage anymore.

.. data:: callbacks

A list of callbacks that will be invoked by the garbage collector before and after collection. The callbacks will be called with two arguments, phase and info.

phase can be one of two values:

  "start": The garbage collection is about to start.

  "stop": The garbage collection has finished.

info is a dict providing more information for the callback. The following keys are currently defined:

  "generation": The oldest generation being collected.

  "collected": When *phase* is "stop", the number of objects
  successfully collected.

  "uncollectable": When *phase* is "stop", the number of objects
  that could not be collected and were put in :data:`garbage`.

  "candidates": When *phase* is "stop", the total number of objects in this
  generation which were considered for collection and traversed.

  "duration": When *phase* is "stop", the time in seconds spent in the
  collection.

Applications can add their own callbacks to this list. The primary use cases are:

  Gathering statistics about garbage collection, such as how often
  various generations are collected, and how long the collection
  takes.

  Allowing applications to identify and clear their own uncollectable
  types when they appear in :data:`garbage`.

.. versionadded:: 3.3

.. versionchanged:: 3.15 Add "duration" and "candidates".

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.

.. versionchanged:: 3.2 Also print the contents of the :data:garbage list at :term:interpreter shutdown, if it isn't empty.

.. 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_SAVEALL).