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Garbage Collector interface
===========================

Interface to the cycle-detecting garbage collector.  This module was
written by Neil Schemenauer <nas@arctrix.com>.
This manual section was written by Neil Schemenauer <nas@arctrix.com>.
The `gc' module is only available if the interpreter was built with the
optional cyclic garbage detector (enabled by default).  If this was not
enabled, an `ImportError' is raised by attempts to import this module.

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)'.

The `gc' module provides the following functions:

`enable()'
     Enable automatic garbage collection.

`disable()'
     Disable automatic garbage collection.

`isenabled()'
     Returns true if automatic collection is enabled.

`collect()'
     Run a full collection.  All generations are examined and the
     number of unreachable objects found is returned.

`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.

`get_debug()'
     Return the debugging flags currently set.

`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'.

`get_threshold()'
     Return the current collection thresholds as a tuple of
     `(THRESHOLD0, THRESHOLD1, THRESHOLD2)'.

The following variable is provided for read-only access:

`garbage'
     A list of objects which the collector found to be unreachable but
     could not be freed (uncollectable objects).  Objects that have
     `__del__()' methods and create part of a reference cycle cause the
     entire reference cycle to be uncollectable.  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()':

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

`DEBUG_COLLECTABLE'
     Print information on collectable objects found.

`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.

`DEBUG_INSTANCES'
     When `DEBUG_COLLECTABLE' or `DEBUG_UNCOLLECTABLE' is set, print
     information about instance objects found.

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

`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.

`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').