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28.12. 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.
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
ValueError
is raised if the generation number is invalid. The number of unreachable objects found is returned.Changed in version 2.5: The optional generation argument was added.
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_objects
( )-
Returns a list of all objects tracked by the collector, excluding the list returned.
New in version 2.2.
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 generation2
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 generation0
is examined. If generation0
has been examined more than threshold1 times since generation1
has been examined, then generation1
is examined as well. Similarly, threshold2 controls the number of collections of generation1
before collecting generation2
.
gc.
get_count
( )-
Return the current collection counts as a tuple of
(count0, count1, count2)
.New in version 2.5.
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 callingget_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 usingget_referrers()
for any purpose other than debugging.New in version 2.2.
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.New in version 2.3.
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 2.7.
The following variable is provided for read-only access (you can mutate its value but should not rebind it):
gc.
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
__del__()
methods. 1 Objects that have__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__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, dodel gc.garbage[:]
to empty the list. It’s generally better to avoid the issue by not creating cycles containing objects with__del__()
methods, and garbage can be examined in that case to verify that no such cycles are being created.If
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 set_debug()
:
gc.
DEBUG_STATS
-
Print statistics during collection. This information can be useful when tuning the collection frequency.
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.
gc.
DEBUG_INSTANCES
-
When
DEBUG_COLLECTABLE
orDEBUG_UNCOLLECTABLE
is set, print information about instance objects found.
gc.
DEBUG_OBJECTS
-
When
DEBUG_COLLECTABLE
orDEBUG_UNCOLLECTABLE
is set, print information about objects other than instance objects found.
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_INSTANCES | DEBUG_OBJECTS | DEBUG_SAVEALL
).
Footnotes