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 

    11 This module provides an interface to the optional garbage collector.  It

    12 provides the ability to disable the collector, tune the collection frequency,

    13 and set debugging options.  It also provides access to unreachable objects that

    14 the collector found but cannot free.  Since the collector supplements the

    15 reference counting already used in Python, you can disable the collector if you

    16 are sure your program does not create reference cycles.  Automatic collection

    17 can 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

    20 gc.garbage for inspection.

    21 

    22 The :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 

   143 The following variable is provided for read-only access (you can mutate its

   144 value 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 

   168 The 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