\input{libgc}

\section{\module{gc} ---

Garbage Collector interface}

\declaremodule{extension}{gc}

\moduleauthor{Neil Schemenauer}{[email protected]}

\sectionauthor{Neil Schemenauer}{[email protected]}

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. The collector can be disabled by

calling \code{gc.set_threshold(0)}. To debug a leaking program call

\code{gc.set_debug(gc.DEBUG_LEAK)}.

The \module{gc} module provides the following functions:

\begin{funcdesc}{collect}{}

Run a full collection. All generations are examined and the

number of unreachable objects found is returned.

\end{funcdesc}

\begin{funcdesc}{set_debug}{flags}

Set the garbage collection debugging flags.

Debugging information will be written to \code{sys.stderr}. See below

for a list of debugging flags which can be combined using bit

operations to control debugging.

\end{funcdesc}

\begin{funcdesc}{get_debug}{}

Return the debugging flags currently set.

\end{funcdesc}

\begin{funcdesc}{set_threshold}{threshold0\optional{,

threshold1\optional{, threshold2}}}

Set the garbage collection thresholds (the collection frequency).

Setting \var{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 \code{0}). If an object survives a

collection it is moved into the next older generation. Since

generation \code{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

\var{threshold0}, collection starts. Initially only generation

\code{0} is examined. If generation \code{0} has been examined more

than \var{threshold1} times since generation \code{1} has been

examined, then generation \code{1} is examined as well. Similarly,

\var{threshold2} controls the number of collections of generation

\code{1} before collecting generation \code{2}.

\end{funcdesc}

\begin{funcdesc}{get_threshold}{}

Return the current collection thresholds as a tuple of

\code{(\var{threshold0}, \var{threshold1}, \var{threshold2})}.

\end{funcdesc}

The following variable is provided for read-only access:

\begin{datadesc}{garbage}

A list of objects which the collector found to be unreachable

but could not be freed (uncollectable objects). Objects that have

\method{__del__()} methods and create part of a reference cycle cause

the entire reference cycle to be uncollectable.

\end{datadesc}

The following constants are provided for use with

\function{set_debug()}:

\begin{datadesc}{DEBUG_STATS}

Print statistics during collection. This information can

be useful when tuning the collection frequency.

\end{datadesc}

\begin{datadesc}{DEBUG_COLLECTABLE}

Print information on collectable objects found.

\end{datadesc}

\begin{datadesc}{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 \code{garbage} list.

\end{datadesc}

\begin{datadesc}{DEBUG_INSTANCES}

When \constant{DEBUG_COLLECTABLE} or \constant{DEBUG_UNCOLLECTABLE} is

set, print information about instance objects found.

\end{datadesc}

\begin{datadesc}{DEBUG_OBJECTS}

When \constant{DEBUG_COLLECTABLE} or \constant{DEBUG_UNCOLLECTABLE} is

set, print information about objects other than instance objects found.

\end{datadesc}

\begin{datadesc}{DEBUG_LEAK}

The debugging flags necessary for the collector to print

information about a leaking program (equal to \code{DEBUG_COLLECTABLE |

DEBUG_UNCOLLECTABLE | DEBUG_INSTANCES | DEBUG_OBJECTS}).

\end{datadesc}