29.11. "gc" — Garbage Collector interface
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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").
