:mod:`gc` --- Garbage Collector interface
=========================================
.. module:: gc
:synopsis: Interface to the cycle-detecting garbage collector.
.. moduleauthor:: Neil Schemenauer <nas@arctrix.com>
.. sectionauthor:: Neil Schemenauer <nas@arctrix.com>
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()
Returns true if automatic collection is enabled.
.. function:: 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
:exc:`ValueError` is raised if the generation number is invalid. The number of
unreachable objects found is returned.
.. versionchanged:: 2.5
The optional *generation* argument was added.
.. versionchanged:: 2.6
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:`int` and :class:`float`.
.. 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()
Returns a list of all objects tracked by the collector, excluding the list
returned.
.. versionadded:: 2.2
.. 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 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``.
.. function:: get_count()
Return the current collection counts as a tuple of ``(count0, count1,
count2)``.
.. versionadded:: 2.5
.. 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`.
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.
.. versionadded:: 2.2
.. 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.
.. versionadded:: 2.3
.. 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})
False
>>> gc.is_tracked({"a": []})
True
.. versionadded:: 2.7
The following variable is provided for read-only access (you can mutate its
value but should not rebind it):
.. data:: 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
:meth:`__del__` methods. [#]_ Objects that have :meth:`__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 :meth:`__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, do ``del gc.garbage[:]`` to empty the list. It's
generally better to avoid the issue by not creating cycles containing objects
with :meth:`__del__` methods, and *garbage* can be examined in that case to
verify that no such cycles are being created.
If :const:`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 :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.
.. data:: DEBUG_INSTANCES
When :const:`DEBUG_COLLECTABLE` or :const:`DEBUG_UNCOLLECTABLE` is set, print
information about instance objects found.
.. data:: DEBUG_OBJECTS
When :const:`DEBUG_COLLECTABLE` or :const:`DEBUG_UNCOLLECTABLE` is set, print
information about objects other than instance objects found.
.. 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_INSTANCES | DEBUG_OBJECTS | DEBUG_SAVEALL``).
.. rubric:: Footnotes
.. [#] Prior to Python 2.2, the list contained all instance objects in unreachable
cycles, not only those with :meth:`__del__` methods.