1:mod:`gc` --- Garbage Collector interface 2========================================= 3 4.. module:: gc 5 :synopsis: Interface to the cycle-detecting garbage collector. 6 7.. moduleauthor:: Neil Schemenauer <nas@arctrix.com> 8.. sectionauthor:: Neil Schemenauer <nas@arctrix.com> 9 10-------------- 11 12This module provides an interface to the optional garbage collector. It 13provides the ability to disable the collector, tune the collection frequency, 14and set debugging options. It also provides access to unreachable objects that 15the collector found but cannot free. Since the collector supplements the 16reference counting already used in Python, you can disable the collector if you 17are sure your program does not create reference cycles. Automatic collection 18can be disabled by calling ``gc.disable()``. To debug a leaking program call 19``gc.set_debug(gc.DEBUG_LEAK)``. Notice that this includes 20``gc.DEBUG_SAVEALL``, causing garbage-collected objects to be saved in 21gc.garbage for inspection. 22 23The :mod:`gc` module provides the following functions: 24 25 26.. function:: enable() 27 28 Enable automatic garbage collection. 29 30 31.. function:: disable() 32 33 Disable automatic garbage collection. 34 35 36.. function:: isenabled() 37 38 Return ``True`` if automatic collection is enabled. 39 40 41.. function:: collect(generation=2) 42 43 With no arguments, run a full collection. The optional argument *generation* 44 may be an integer specifying which generation to collect (from 0 to 2). A 45 :exc:`ValueError` is raised if the generation number is invalid. The number of 46 unreachable objects found is returned. 47 48 The free lists maintained for a number of built-in types are cleared 49 whenever a full collection or collection of the highest generation (2) 50 is run. Not all items in some free lists may be freed due to the 51 particular implementation, in particular :class:`float`. 52 53 54.. function:: set_debug(flags) 55 56 Set the garbage collection debugging flags. Debugging information will be 57 written to ``sys.stderr``. See below for a list of debugging flags which can be 58 combined using bit operations to control debugging. 59 60 61.. function:: get_debug() 62 63 Return the debugging flags currently set. 64 65 66.. function:: get_objects(generation=None) 67 68 Returns a list of all objects tracked by the collector, excluding the list 69 returned. If *generation* is not None, return only the objects tracked by 70 the collector that are in that generation. 71 72 .. versionchanged:: 3.8 73 New *generation* parameter. 74 75.. function:: get_stats() 76 77 Return a list of three per-generation dictionaries containing collection 78 statistics since interpreter start. The number of keys may change 79 in the future, but currently each dictionary will contain the following 80 items: 81 82 * ``collections`` is the number of times this generation was collected; 83 84 * ``collected`` is the total number of objects collected inside this 85 generation; 86 87 * ``uncollectable`` is the total number of objects which were found 88 to be uncollectable (and were therefore moved to the :data:`garbage` 89 list) inside this generation. 90 91 .. versionadded:: 3.4 92 93 94.. function:: set_threshold(threshold0[, threshold1[, threshold2]]) 95 96 Set the garbage collection thresholds (the collection frequency). Setting 97 *threshold0* to zero disables collection. 98 99 The GC classifies objects into three generations depending on how many 100 collection sweeps they have survived. New objects are placed in the youngest 101 generation (generation ``0``). If an object survives a collection it is moved 102 into the next older generation. Since generation ``2`` is the oldest 103 generation, objects in that generation remain there after a collection. In 104 order to decide when to run, the collector keeps track of the number object 105 allocations and deallocations since the last collection. When the number of 106 allocations minus the number of deallocations exceeds *threshold0*, collection 107 starts. Initially only generation ``0`` is examined. If generation ``0`` has 108 been examined more than *threshold1* times since generation ``1`` has been 109 examined, then generation ``1`` is examined as well. 110 With the third generation, things are a bit more complicated, 111 see `Collecting the oldest generation <https://devguide.python.org/garbage_collector/#collecting-the-oldest-generation>`_ for more information. 112 113 114.. function:: get_count() 115 116 Return the current collection counts as a tuple of ``(count0, count1, 117 count2)``. 118 119 120.. function:: get_threshold() 121 122 Return the current collection thresholds as a tuple of ``(threshold0, 123 threshold1, threshold2)``. 124 125 126.. function:: get_referrers(*objs) 127 128 Return the list of objects that directly refer to any of objs. This function 129 will only locate those containers which support garbage collection; extension 130 types which do refer to other objects but do not support garbage collection will 131 not be found. 132 133 Note that objects which have already been dereferenced, but which live in cycles 134 and have not yet been collected by the garbage collector can be listed among the 135 resulting referrers. To get only currently live objects, call :func:`collect` 136 before calling :func:`get_referrers`. 137 138 Care must be taken when using objects returned by :func:`get_referrers` because 139 some of them could still be under construction and hence in a temporarily 140 invalid state. Avoid using :func:`get_referrers` for any purpose other than 141 debugging. 142 143 144.. function:: get_referents(*objs) 145 146 Return a list of objects directly referred to by any of the arguments. The 147 referents returned are those objects visited by the arguments' C-level 148 :c:member:`~PyTypeObject.tp_traverse` methods (if any), and may not be all objects actually 149 directly reachable. :c:member:`~PyTypeObject.tp_traverse` methods are supported only by objects 150 that support garbage collection, and are only required to visit objects that may 151 be involved in a cycle. So, for example, if an integer is directly reachable 152 from an argument, that integer object may or may not appear in the result list. 153 154 155.. function:: is_tracked(obj) 156 157 Returns ``True`` if the object is currently tracked by the garbage collector, 158 ``False`` otherwise. As a general rule, instances of atomic types aren't 159 tracked and instances of non-atomic types (containers, user-defined 160 objects...) are. However, some type-specific optimizations can be present 161 in order to suppress the garbage collector footprint of simple instances 162 (e.g. dicts containing only atomic keys and values):: 163 164 >>> gc.is_tracked(0) 165 False 166 >>> gc.is_tracked("a") 167 False 168 >>> gc.is_tracked([]) 169 True 170 >>> gc.is_tracked({}) 171 False 172 >>> gc.is_tracked({"a": 1}) 173 False 174 >>> gc.is_tracked({"a": []}) 175 True 176 177 .. versionadded:: 3.1 178 179 180.. function:: is_finalized(obj) 181 182 Returns ``True`` if the given object has been finalized by the 183 garbage collector, ``False`` otherwise. :: 184 185 >>> x = None 186 >>> class Lazarus: 187 ... def __del__(self): 188 ... global x 189 ... x = self 190 ... 191 >>> lazarus = Lazarus() 192 >>> gc.is_finalized(lazarus) 193 False 194 >>> del lazarus 195 >>> gc.is_finalized(x) 196 True 197 198 .. versionadded:: 3.9 199 200 201.. function:: freeze() 202 203 Freeze all the objects tracked by gc - move them to a permanent generation 204 and ignore all the future collections. This can be used before a POSIX 205 fork() call to make the gc copy-on-write friendly or to speed up collection. 206 Also collection before a POSIX fork() call may free pages for future 207 allocation which can cause copy-on-write too so it's advised to disable gc 208 in parent process and freeze before fork and enable gc in child process. 209 210 .. versionadded:: 3.7 211 212 213.. function:: unfreeze() 214 215 Unfreeze the objects in the permanent generation, put them back into the 216 oldest generation. 217 218 .. versionadded:: 3.7 219 220 221.. function:: get_freeze_count() 222 223 Return the number of objects in the permanent generation. 224 225 .. versionadded:: 3.7 226 227 228The following variables are provided for read-only access (you can mutate the 229values but should not rebind them): 230 231.. data:: garbage 232 233 A list of objects which the collector found to be unreachable but could 234 not be freed (uncollectable objects). Starting with Python 3.4, this 235 list should be empty most of the time, except when using instances of 236 C extension types with a non-``NULL`` ``tp_del`` slot. 237 238 If :const:`DEBUG_SAVEALL` is set, then all unreachable objects will be 239 added to this list rather than freed. 240 241 .. versionchanged:: 3.2 242 If this list is non-empty at :term:`interpreter shutdown`, a 243 :exc:`ResourceWarning` is emitted, which is silent by default. If 244 :const:`DEBUG_UNCOLLECTABLE` is set, in addition all uncollectable objects 245 are printed. 246 247 .. versionchanged:: 3.4 248 Following :pep:`442`, objects with a :meth:`__del__` method don't end 249 up in :attr:`gc.garbage` anymore. 250 251.. data:: callbacks 252 253 A list of callbacks that will be invoked by the garbage collector before and 254 after collection. The callbacks will be called with two arguments, 255 *phase* and *info*. 256 257 *phase* can be one of two values: 258 259 "start": The garbage collection is about to start. 260 261 "stop": The garbage collection has finished. 262 263 *info* is a dict providing more information for the callback. The following 264 keys are currently defined: 265 266 "generation": The oldest generation being collected. 267 268 "collected": When *phase* is "stop", the number of objects 269 successfully collected. 270 271 "uncollectable": When *phase* is "stop", the number of objects 272 that could not be collected and were put in :data:`garbage`. 273 274 Applications can add their own callbacks to this list. The primary 275 use cases are: 276 277 Gathering statistics about garbage collection, such as how often 278 various generations are collected, and how long the collection 279 takes. 280 281 Allowing applications to identify and clear their own uncollectable 282 types when they appear in :data:`garbage`. 283 284 .. versionadded:: 3.3 285 286 287The following constants are provided for use with :func:`set_debug`: 288 289 290.. data:: DEBUG_STATS 291 292 Print statistics during collection. This information can be useful when tuning 293 the collection frequency. 294 295 296.. data:: DEBUG_COLLECTABLE 297 298 Print information on collectable objects found. 299 300 301.. data:: DEBUG_UNCOLLECTABLE 302 303 Print information of uncollectable objects found (objects which are not 304 reachable but cannot be freed by the collector). These objects will be added 305 to the ``garbage`` list. 306 307 .. versionchanged:: 3.2 308 Also print the contents of the :data:`garbage` list at 309 :term:`interpreter shutdown`, if it isn't empty. 310 311.. data:: DEBUG_SAVEALL 312 313 When set, all unreachable objects found will be appended to *garbage* rather 314 than being freed. This can be useful for debugging a leaking program. 315 316 317.. data:: DEBUG_LEAK 318 319 The debugging flags necessary for the collector to print information about a 320 leaking program (equal to ``DEBUG_COLLECTABLE | DEBUG_UNCOLLECTABLE | 321 DEBUG_SAVEALL``). 322