158 lines
6.4 KiB
TeX
158 lines
6.4 KiB
TeX
\section{\module{gc} ---
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Garbage Collector interface}
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\declaremodule{extension}{gc}
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\modulesynopsis{Interface to the cycle-detecting garbage collector.}
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\moduleauthor{Neil Schemenauer}{nas@arctrix.com}
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\sectionauthor{Neil Schemenauer}{nas@arctrix.com}
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The \module{gc} module is only available if the interpreter was built
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with the optional cyclic garbage detector (enabled by default). If
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this was not enabled, an \exception{ImportError} is raised by attempts
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to import this module.
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This module provides an interface to the optional garbage collector. It
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provides the ability to disable the collector, tune the collection
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frequency, and set debugging options. It also provides access to
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unreachable objects that the collector found but cannot free. Since the
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collector supplements the reference counting already used in Python, you
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can disable the collector if you are sure your program does not create
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reference cycles. Automatic collection can be disabled by calling
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\code{gc.disable()}. To debug a leaking program call
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\code{gc.set_debug(gc.DEBUG_LEAK)}.
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The \module{gc} module provides the following functions:
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\begin{funcdesc}{enable}{}
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Enable automatic garbage collection.
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\end{funcdesc}
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\begin{funcdesc}{disable}{}
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Disable automatic garbage collection.
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\end{funcdesc}
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\begin{funcdesc}{isenabled}{}
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Returns true if automatic collection is enabled.
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\end{funcdesc}
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\begin{funcdesc}{collect}{}
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Run a full collection. All generations are examined and the
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number of unreachable objects found is returned.
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\end{funcdesc}
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\begin{funcdesc}{set_debug}{flags}
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Set the garbage collection debugging flags.
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Debugging information will be written to \code{sys.stderr}. See below
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for a list of debugging flags which can be combined using bit
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operations to control debugging.
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\end{funcdesc}
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\begin{funcdesc}{get_debug}{}
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Return the debugging flags currently set.
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\end{funcdesc}
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\begin{funcdesc}{set_threshold}{threshold0\optional{,
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threshold1\optional{, threshold2}}}
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Set the garbage collection thresholds (the collection frequency).
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Setting \var{threshold0} to zero disables collection.
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The GC classifies objects into three generations depending on how many
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collection sweeps they have survived. New objects are placed in the
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youngest generation (generation \code{0}). If an object survives a
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collection it is moved into the next older generation. Since
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generation \code{2} is the oldest generation, objects in that
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generation remain there after a collection. In order to decide when
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to run, the collector keeps track of the number object allocations and
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deallocations since the last collection. When the number of
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allocations minus the number of deallocations exceeds
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\var{threshold0}, collection starts. Initially only generation
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\code{0} is examined. If generation \code{0} has been examined more
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than \var{threshold1} times since generation \code{1} has been
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examined, then generation \code{1} is examined as well. Similarly,
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\var{threshold2} controls the number of collections of generation
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\code{1} before collecting generation \code{2}.
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\end{funcdesc}
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\begin{funcdesc}{get_threshold}{}
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Return the current collection thresholds as a tuple of
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\code{(\var{threshold0}, \var{threshold1}, \var{threshold2})}.
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\end{funcdesc}
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\begin{funcdesc}{get_referrers}{*objs}
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Return the list of objects that directly refer to any of objs. This
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function will only locate those containers which support garbage
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collection; extension types which do refer to other objects but do not
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support garbage collection will not be found.
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\versionadded{2.2}
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\end{funcdesc}
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The following variable is provided for read-only access (you can
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mutate its value but should not rebind it):
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\begin{datadesc}{garbage}
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A list of objects which the collector found to be unreachable
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but could not be freed (uncollectable objects). By default, this list
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contains only objects with \method{__del__()} methods.\footnote{Prior to
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Python 2.2, the list contained all instance objects in unreachable
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cycles, not only those with \method{__del__()} methods.}
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Objects that have
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\method{__del__()} methods and are part of a reference cycle cause
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the entire reference cycle to be uncollectable, including objects
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not necessarily in the cycle but reachable only from it. Python doesn't
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collect such cycles automatically because, in general, it isn't possible
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for Python to guess a safe order in which to run the \method{__del__()}
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methods. If you know a safe order, you can force the issue by examining
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the \var{garbage} list, and explicitly breaking cycles due to your
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objects within the list. Note that these objects are kept alive even
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so by virtue of being in the \var{garbage} list, so they should be
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removed from \var{garbage} too. For example, after breaking cycles, do
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\code{del gc.garbage[:]} to empty the list. It's generally better
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to avoid the issue by not creating cycles containing objects with
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\method{__del__()} methods, and \var{garbage} can be examined in that
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case to verify that no such are being created.
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If \constant{DEBUG_SAVEALL} is set, then all unreachable objects will
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be added to this list rather than freed.
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\end{datadesc}
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The following constants are provided for use with
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\function{set_debug()}:
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\begin{datadesc}{DEBUG_STATS}
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Print statistics during collection. This information can
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be useful when tuning the collection frequency.
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\end{datadesc}
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\begin{datadesc}{DEBUG_COLLECTABLE}
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Print information on collectable objects found.
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\end{datadesc}
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\begin{datadesc}{DEBUG_UNCOLLECTABLE}
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Print information of uncollectable objects found (objects which are
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not reachable but cannot be freed by the collector). These objects
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will be added to the \code{garbage} list.
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\end{datadesc}
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\begin{datadesc}{DEBUG_INSTANCES}
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When \constant{DEBUG_COLLECTABLE} or \constant{DEBUG_UNCOLLECTABLE} is
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set, print information about instance objects found.
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\end{datadesc}
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\begin{datadesc}{DEBUG_OBJECTS}
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When \constant{DEBUG_COLLECTABLE} or \constant{DEBUG_UNCOLLECTABLE} is
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set, print information about objects other than instance objects found.
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\end{datadesc}
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\begin{datadesc}{DEBUG_SAVEALL}
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When set, all unreachable objects found will be appended to
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\var{garbage} rather than being freed. This can be useful for debugging
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a leaking program.
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\end{datadesc}
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\begin{datadesc}{DEBUG_LEAK}
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The debugging flags necessary for the collector to print
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information about a leaking program (equal to \code{DEBUG_COLLECTABLE |
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DEBUG_UNCOLLECTABLE | DEBUG_INSTANCES | DEBUG_OBJECTS | DEBUG_SAVEALL}).
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\end{datadesc}
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