28.12. "gc" — Garbage Collector interface
*****************************************

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 "gc" module provides the following functions:

gc.enable()

   Enable automatic garbage collection.

gc.disable()

   Disable automatic garbage collection.

gc.isenabled()

   Returns true if automatic collection is enabled.

gc.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 "ValueError" is raised if the generation
   number  is invalid. The number of unreachable objects found is
   returned.

   在 2.5 版更改: The optional *generation* argument was added.

   在 2.6 版更改: 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
   "int" and "float".

gc.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.

gc.get_debug()

   Return the debugging flags currently set.

gc.get_objects()

   Returns a list of all objects tracked by the collector, excluding
   the list returned.

   2.2 新版功能.

gc.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".

gc.get_count()

   Return the current collection  counts as a tuple of "(count0,
   count1, count2)".

   2.5 新版功能.

gc.get_threshold()

   Return the current collection thresholds as a tuple of
   "(threshold0, threshold1, threshold2)".

gc.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 "collect()" before calling
   "get_referrers()".

   Care must be taken when using objects returned by "get_referrers()"
   because some of them could still be under construction and hence in
   a temporarily invalid state. Avoid using "get_referrers()" for any
   purpose other than debugging.

   2.2 新版功能.

gc.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 "tp_traverse" methods (if any), and may not be
   all objects actually directly reachable.  "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.

   2.3 新版功能.

gc.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

   2.7 新版功能.

The following variable is provided for read-only access (you can
mutate its value but should not rebind it):

gc.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 "__del__()" methods. [1] Objects that
   have "__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
   "__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 "__del__()" methods, and *garbage* can be
   examined in that case to verify that no such cycles are being
   created.

   If "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 "set_debug()":

gc.DEBUG_STATS

   Print statistics during collection.  This information can be useful
   when tuning the collection frequency.

gc.DEBUG_COLLECTABLE

   Print information on collectable objects found.

gc.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.

gc.DEBUG_INSTANCES

   When "DEBUG_COLLECTABLE" or "DEBUG_UNCOLLECTABLE" is set, print
   information about instance objects found.

gc.DEBUG_OBJECTS

   When "DEBUG_COLLECTABLE" or "DEBUG_UNCOLLECTABLE" is set, print
   information about objects other than instance objects found.

gc.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.

gc.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").

-[ Footnotes ]-

[1] Prior to Python 2.2, the list contained all instance objects
    in unreachable cycles,  not only those with "__del__()" methods.
