Python 3.6.5 Documentation >  "gc" — Garbage Collector interface

"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").