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 (a] arctrix.com> 8 .. sectionauthor:: Neil Schemenauer <nas (a] arctrix.com> 9 10 -------------- 11 12 This module provides an interface to the optional garbage collector. It 13 provides the ability to disable the collector, tune the collection frequency, 14 and set debugging options. It also provides access to unreachable objects that 15 the collector found but cannot free. Since the collector supplements the 16 reference counting already used in Python, you can disable the collector if you 17 are sure your program does not create reference cycles. Automatic collection 18 can 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 21 gc.garbage for inspection. 22 23 The :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 Returns 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() 67 68 Returns a list of all objects tracked by the collector, excluding the list 69 returned. 70 71 72 .. function:: get_stats() 73 74 Return a list of three per-generation dictionaries containing collection 75 statistics since interpreter start. The number of keys may change 76 in the future, but currently each dictionary will contain the following 77 items: 78 79 * ``collections`` is the number of times this generation was collected; 80 81 * ``collected`` is the total number of objects collected inside this 82 generation; 83 84 * ``uncollectable`` is the total number of objects which were found 85 to be uncollectable (and were therefore moved to the :data:`garbage` 86 list) inside this generation. 87 88 .. versionadded:: 3.4 89 90 91 .. function:: set_threshold(threshold0[, threshold1[, threshold2]]) 92 93 Set the garbage collection thresholds (the collection frequency). Setting 94 *threshold0* to zero disables collection. 95 96 The GC classifies objects into three generations depending on how many 97 collection sweeps they have survived. New objects are placed in the youngest 98 generation (generation ``0``). If an object survives a collection it is moved 99 into the next older generation. Since generation ``2`` is the oldest 100 generation, objects in that generation remain there after a collection. In 101 order to decide when to run, the collector keeps track of the number object 102 allocations and deallocations since the last collection. When the number of 103 allocations minus the number of deallocations exceeds *threshold0*, collection 104 starts. Initially only generation ``0`` is examined. If generation ``0`` has 105 been examined more than *threshold1* times since generation ``1`` has been 106 examined, then generation ``1`` is examined as well. Similarly, *threshold2* 107 controls the number of collections of generation ``1`` before collecting 108 generation ``2``. 109 110 111 .. function:: get_count() 112 113 Return the current collection counts as a tuple of ``(count0, count1, 114 count2)``. 115 116 117 .. function:: get_threshold() 118 119 Return the current collection thresholds as a tuple of ``(threshold0, 120 threshold1, threshold2)``. 121 122 123 .. function:: get_referrers(*objs) 124 125 Return the list of objects that directly refer to any of objs. This function 126 will only locate those containers which support garbage collection; extension 127 types which do refer to other objects but do not support garbage collection will 128 not be found. 129 130 Note that objects which have already been dereferenced, but which live in cycles 131 and have not yet been collected by the garbage collector can be listed among the 132 resulting referrers. To get only currently live objects, call :func:`collect` 133 before calling :func:`get_referrers`. 134 135 Care must be taken when using objects returned by :func:`get_referrers` because 136 some of them could still be under construction and hence in a temporarily 137 invalid state. Avoid using :func:`get_referrers` for any purpose other than 138 debugging. 139 140 141 .. function:: get_referents(*objs) 142 143 Return a list of objects directly referred to by any of the arguments. The 144 referents returned are those objects visited by the arguments' C-level 145 :c:member:`~PyTypeObject.tp_traverse` methods (if any), and may not be all objects actually 146 directly reachable. :c:member:`~PyTypeObject.tp_traverse` methods are supported only by objects 147 that support garbage collection, and are only required to visit objects that may 148 be involved in a cycle. So, for example, if an integer is directly reachable 149 from an argument, that integer object may or may not appear in the result list. 150 151 152 .. function:: is_tracked(obj) 153 154 Returns ``True`` if the object is currently tracked by the garbage collector, 155 ``False`` otherwise. As a general rule, instances of atomic types aren't 156 tracked and instances of non-atomic types (containers, user-defined 157 objects...) are. However, some type-specific optimizations can be present 158 in order to suppress the garbage collector footprint of simple instances 159 (e.g. dicts containing only atomic keys and values):: 160 161 >>> gc.is_tracked(0) 162 False 163 >>> gc.is_tracked("a") 164 False 165 >>> gc.is_tracked([]) 166 True 167 >>> gc.is_tracked({}) 168 False 169 >>> gc.is_tracked({"a": 1}) 170 False 171 >>> gc.is_tracked({"a": []}) 172 True 173 174 .. versionadded:: 3.1 175 176 177 .. function:: freeze() 178 179 Freeze all the objects tracked by gc - move them to a permanent generation 180 and ignore all the future collections. This can be used before a POSIX 181 fork() call to make the gc copy-on-write friendly or to speed up collection. 182 Also collection before a POSIX fork() call may free pages for future 183 allocation which can cause copy-on-write too so it's advised to disable gc 184 in master process and freeze before fork and enable gc in child process. 185 186 .. versionadded:: 3.7 187 188 189 .. function:: unfreeze() 190 191 Unfreeze the objects in the permanent generation, put them back into the 192 oldest generation. 193 194 .. versionadded:: 3.7 195 196 197 .. function:: get_freeze_count() 198 199 Return the number of objects in the permanent generation. 200 201 .. versionadded:: 3.7 202 203 204 The following variables are provided for read-only access (you can mutate the 205 values but should not rebind them): 206 207 .. data:: garbage 208 209 A list of objects which the collector found to be unreachable but could 210 not be freed (uncollectable objects). Starting with Python 3.4, this 211 list should be empty most of the time, except when using instances of 212 C extension types with a non-NULL ``tp_del`` slot. 213 214 If :const:`DEBUG_SAVEALL` is set, then all unreachable objects will be 215 added to this list rather than freed. 216 217 .. versionchanged:: 3.2 218 If this list is non-empty at :term:`interpreter shutdown`, a 219 :exc:`ResourceWarning` is emitted, which is silent by default. If 220 :const:`DEBUG_UNCOLLECTABLE` is set, in addition all uncollectable objects 221 are printed. 222 223 .. versionchanged:: 3.4 224 Following :pep:`442`, objects with a :meth:`__del__` method don't end 225 up in :attr:`gc.garbage` anymore. 226 227 .. data:: callbacks 228 229 A list of callbacks that will be invoked by the garbage collector before and 230 after collection. The callbacks will be called with two arguments, 231 *phase* and *info*. 232 233 *phase* can be one of two values: 234 235 "start": The garbage collection is about to start. 236 237 "stop": The garbage collection has finished. 238 239 *info* is a dict providing more information for the callback. The following 240 keys are currently defined: 241 242 "generation": The oldest generation being collected. 243 244 "collected": When *phase* is "stop", the number of objects 245 successfully collected. 246 247 "uncollectable": When *phase* is "stop", the number of objects 248 that could not be collected and were put in :data:`garbage`. 249 250 Applications can add their own callbacks to this list. The primary 251 use cases are: 252 253 Gathering statistics about garbage collection, such as how often 254 various generations are collected, and how long the collection 255 takes. 256 257 Allowing applications to identify and clear their own uncollectable 258 types when they appear in :data:`garbage`. 259 260 .. versionadded:: 3.3 261 262 263 The following constants are provided for use with :func:`set_debug`: 264 265 266 .. data:: DEBUG_STATS 267 268 Print statistics during collection. This information can be useful when tuning 269 the collection frequency. 270 271 272 .. data:: DEBUG_COLLECTABLE 273 274 Print information on collectable objects found. 275 276 277 .. data:: DEBUG_UNCOLLECTABLE 278 279 Print information of uncollectable objects found (objects which are not 280 reachable but cannot be freed by the collector). These objects will be added 281 to the ``garbage`` list. 282 283 .. versionchanged:: 3.2 284 Also print the contents of the :data:`garbage` list at 285 :term:`interpreter shutdown`, if it isn't empty. 286 287 .. data:: DEBUG_SAVEALL 288 289 When set, all unreachable objects found will be appended to *garbage* rather 290 than being freed. This can be useful for debugging a leaking program. 291 292 293 .. data:: DEBUG_LEAK 294 295 The debugging flags necessary for the collector to print information about a 296 leaking program (equal to ``DEBUG_COLLECTABLE | DEBUG_UNCOLLECTABLE | 297 DEBUG_SAVEALL``). 298
