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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 automatic garbage collection.
-
- Disable automatic garbage collection.
-
- Returns true if automatic collection is enabled.
-
- Run a full collection. All generations are examined and the number of unreachable
objects found is returned.
-
- 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.
-
- Return the debugging flags currently set.
-
- Returns a list of all objects tracked by the collector, excluding the list returned. New in version 2.2.
-
| 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.
-
- Return the current collection thresholds as a tuple of
(threshold0, threshold1,
threshold2).
-
- 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.
Note that objects which have already been dereferenced, but which live in cycles and
have not yet been collected by the garbage collector can be listed among the resulting
referrers. To get only currently live objects, call collect()
before calling get_referrers().
New in version 2.2.
-
- Return a list of objects directly referred to by any of the arguments. The referents
returned are those objects visited by the arguments' C-level tp_traverse
methods (if any), and may not be all objects actually directly reachable. tp_traverse methods are supported only by objects that support garbage
collection, and are only required to visit objects that may be involved in a cycle. So,
for example, if an integer is directly reachable from an argument, that integer object may
or may not appear in the result list.
New in version 2.3.
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.
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