Doc/lib/libgc.tex

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