/Doc/library/gc.rst

  1
  2:mod:`gc` --- Garbage Collector interface
  3=========================================
  4
  5.. module:: gc
  6   :synopsis: Interface to the cycle-detecting garbage collector.
  7.. moduleauthor:: Neil Schemenauer <nas@arctrix.com>
  8.. sectionauthor:: Neil Schemenauer <nas@arctrix.com>
  9
 10
 11This module provides an interface to the optional garbage collector.  It
 12provides the ability to disable the collector, tune the collection frequency,
 13and set debugging options.  It also provides access to unreachable objects that
 14the collector found but cannot free.  Since the collector supplements the
 15reference counting already used in Python, you can disable the collector if you
 16are sure your program does not create reference cycles.  Automatic collection
 17can be disabled by calling ``gc.disable()``.  To debug a leaking program call
 18``gc.set_debug(gc.DEBUG_LEAK)``. Notice that this includes
 19``gc.DEBUG_SAVEALL``, causing garbage-collected objects to be saved in
 20gc.garbage for inspection.
 21
 22The :mod:`gc` module provides the following functions:
 23
 24
 25.. function:: enable()
 26
 27   Enable automatic garbage collection.
 28
 29
 30.. function:: disable()
 31
 32   Disable automatic garbage collection.
 33
 34
 35.. function:: isenabled()
 36
 37   Returns true if automatic collection is enabled.
 38
 39
 40.. function:: collect([generation])
 41
 42   With no arguments, run a full collection.  The optional argument *generation*
 43   may be an integer specifying which generation to collect (from 0 to 2).  A
 44   :exc:`ValueError` is raised if the generation number  is invalid. The number of
 45   unreachable objects found is returned.
 46
 47   .. versionchanged:: 2.5
 48      The optional *generation* argument was added.
 49
 50   .. versionchanged:: 2.6
 51      The free lists maintained for a number of builtin types are cleared
 52      whenever a full collection or collection of the highest generation (2)
 53      is run.  Not all items in some free lists may be freed due to the
 54      particular implementation, in particular :class:`int` and :class:`float`.
 55
 56
 57.. function:: set_debug(flags)
 58
 59   Set the garbage collection debugging flags. Debugging information will be
 60   written to ``sys.stderr``.  See below for a list of debugging flags which can be
 61   combined using bit operations to control debugging.
 62
 63
 64.. function:: get_debug()
 65
 66   Return the debugging flags currently set.
 67
 68
 69.. function:: get_objects()
 70
 71   Returns a list of all objects tracked by the collector, excluding the list
 72   returned.
 73
 74   .. versionadded:: 2.2
 75
 76
 77.. function:: set_threshold(threshold0[, threshold1[, threshold2]])
 78
 79   Set the garbage collection thresholds (the collection frequency). Setting
 80   *threshold0* to zero disables collection.
 81
 82   The GC classifies objects into three generations depending on how many
 83   collection sweeps they have survived.  New objects are placed in the youngest
 84   generation (generation ``0``).  If an object survives a collection it is moved
 85   into the next older generation.  Since generation ``2`` is the oldest
 86   generation, objects in that generation remain there after a collection.  In
 87   order to decide when to run, the collector keeps track of the number object
 88   allocations and deallocations since the last collection.  When the number of
 89   allocations minus the number of deallocations exceeds *threshold0*, collection
 90   starts.  Initially only generation ``0`` is examined.  If generation ``0`` has
 91   been examined more than *threshold1* times since generation ``1`` has been
 92   examined, then generation ``1`` is examined as well.  Similarly, *threshold2*
 93   controls the number of collections of generation ``1`` before collecting
 94   generation ``2``.
 95
 96
 97.. function:: get_count()
 98
 99   Return the current collection  counts as a tuple of ``(count0, count1,
100   count2)``.
101
102   .. versionadded:: 2.5
103
104
105.. function:: get_threshold()
106
107   Return the current collection thresholds as a tuple of ``(threshold0,
108   threshold1, threshold2)``.
109
110
111.. function:: get_referrers(*objs)
112
113   Return the list of objects that directly refer to any of objs. This function
114   will only locate those containers which support garbage collection; extension
115   types which do refer to other objects but do not support garbage collection will
116   not be found.
117
118   Note that objects which have already been dereferenced, but which live in cycles
119   and have not yet been collected by the garbage collector can be listed among the
120   resulting referrers.  To get only currently live objects, call :func:`collect`
121   before calling :func:`get_referrers`.
122
123   Care must be taken when using objects returned by :func:`get_referrers` because
124   some of them could still be under construction and hence in a temporarily
125   invalid state. Avoid using :func:`get_referrers` for any purpose other than
126   debugging.
127
128   .. versionadded:: 2.2
129
130
131.. function:: get_referents(*objs)
132
133   Return a list of objects directly referred to by any of the arguments. The
134   referents returned are those objects visited by the arguments' C-level
135   :attr:`tp_traverse` methods (if any), and may not be all objects actually
136   directly reachable.  :attr:`tp_traverse` methods are supported only by objects
137   that support garbage collection, and are only required to visit objects that may
138   be involved in a cycle.  So, for example, if an integer is directly reachable
139   from an argument, that integer object may or may not appear in the result list.
140
141   .. versionadded:: 2.3
142
143The following variable is provided for read-only access (you can mutate its
144value but should not rebind it):
145
146
147.. data:: garbage
148
149   A list of objects which the collector found to be unreachable but could not be
150   freed (uncollectable objects).  By default, this list contains only objects with
151   :meth:`__del__` methods. [#]_ Objects that have :meth:`__del__` methods and are
152   part of a reference cycle cause the entire reference cycle to be uncollectable,
153   including objects not necessarily in the cycle but reachable only from it.
154   Python doesn't collect such cycles automatically because, in general, it isn't
155   possible for Python to guess a safe order in which to run the :meth:`__del__`
156   methods.  If you know a safe order, you can force the issue by examining the
157   *garbage* list, and explicitly breaking cycles due to your objects within the
158   list.  Note that these objects are kept alive even so by virtue of being in the
159   *garbage* list, so they should be removed from *garbage* too.  For example,
160   after breaking cycles, do ``del gc.garbage[:]`` to empty the list.  It's
161   generally better to avoid the issue by not creating cycles containing objects
162   with :meth:`__del__` methods, and *garbage* can be examined in that case to
163   verify that no such cycles are being created.
164
165   If :const:`DEBUG_SAVEALL` is set, then all unreachable objects will be added to
166   this list rather than freed.
167
168The following constants are provided for use with :func:`set_debug`:
169
170
171.. data:: DEBUG_STATS
172
173   Print statistics during collection.  This information can be useful when tuning
174   the collection frequency.
175
176
177.. data:: DEBUG_COLLECTABLE
178
179   Print information on collectable objects found.
180
181
182.. data:: DEBUG_UNCOLLECTABLE
183
184   Print information of uncollectable objects found (objects which are not
185   reachable but cannot be freed by the collector).  These objects will be added to
186   the ``garbage`` list.
187
188
189.. data:: DEBUG_INSTANCES
190
191   When :const:`DEBUG_COLLECTABLE` or :const:`DEBUG_UNCOLLECTABLE` is set, print
192   information about instance objects found.
193
194
195.. data:: DEBUG_OBJECTS
196
197   When :const:`DEBUG_COLLECTABLE` or :const:`DEBUG_UNCOLLECTABLE` is set, print
198   information about objects other than instance objects found.
199
200
201.. data:: DEBUG_SAVEALL
202
203   When set, all unreachable objects found will be appended to *garbage* rather
204   than being freed.  This can be useful for debugging a leaking program.
205
206
207.. data:: DEBUG_LEAK
208
209   The debugging flags necessary for the collector to print information about a
210   leaking program (equal to ``DEBUG_COLLECTABLE | DEBUG_UNCOLLECTABLE |
211   DEBUG_INSTANCES | DEBUG_OBJECTS | DEBUG_SAVEALL``).
212
213.. rubric:: Footnotes
214
215.. [#] Prior to Python 2.2, the list contained all instance objects in unreachable
216   cycles,  not only those with :meth:`__del__` methods.
217