# 29.11. `gc` — Garbage Collector interface

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 `gc` module provides the following functions:

`gc.``enable`()

  Enable automatic garbage collection.

`gc.``disable`()

  Disable automatic garbage collection.

`gc.``isenabled`()

  Returns true if automatic collection is enabled.

`gc.``collect`(generation=2)

  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 `ValueError` is raised if the generation number is invalid. The number of unreachable objects found is returned.

  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 `float`.

`gc.``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.

`gc.``get_debug`()

  Return the debugging flags currently set.

`gc.``get_objects`()

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

`gc.``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 `garbage` list) inside this generation.

  New in version 3.4.

`gc.``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`.

`gc.``get_count`()

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

`gc.``get_threshold`()

  Return the current collection thresholds as a tuple of `(threshold0, threshold1, threshold2)`.

`gc.``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 `collect()` before calling `get_referrers()`.

  Care must be taken when using objects returned by `get_referrers()` because some of them could still be under construction and hence in a temporarily invalid state. Avoid using `get_referrers()` for any purpose other than debugging.

`gc.``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 `tp_traverse` methods (if any), and may not be all objects actually directly reachable. `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.

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

  New in version 3.1.

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

`gc.``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 `DEBUG_SAVEALL` is set, then all unreachable objects will be added to this list rather than freed.

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

  Changed in version 3.4: Following PEP 442, objects with a `__del__()` method don’t end up in `gc.garbage` anymore.

`gc.``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 `garbage`.

  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 `garbage`.

  New in version 3.3.

The following constants are provided for use with `set_debug()`:

`gc.``DEBUG_STATS`

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

`gc.``DEBUG_COLLECTABLE`

  Print information on collectable objects found.

`gc.``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.

  Changed in version 3.2: Also print the contents of the `garbage` list at interpreter shutdown, if it isn’t empty.

`gc.``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.

`gc.``DEBUG_LEAK`

  The debugging flags necessary for the collector to print information about a leaking program (equal to `DEBUG_COLLECTABLE | DEBUG_UNCOLLECTABLE | DEBUG_SAVEALL`).