3.2 gc -- Garbage Collector interface
Keywords: lib

# 3.2 `gc` -- Garbage Collector interface

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`).