243 lines
8.8 KiB
ReStructuredText
243 lines
8.8 KiB
ReStructuredText
|
|
: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.
|
|
|