# 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)`.

`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. New in version 2.2.

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

`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.3.1Objects 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()`:

`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`).

#### Footnotes

... methods.3.1
  Prior to Python 2.2, the list contained all instance objects in unreachable cycles, not only those with `__del__()` methods.