Antoine Pitrou | 64a467d | 2010-12-12 20:34:49 +0000 | [diff] [blame] | 1 | :mod:`threading` --- Thread-based parallelism |
| 2 | ============================================= |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 3 | |
| 4 | .. module:: threading |
Antoine Pitrou | 64a467d | 2010-12-12 20:34:49 +0000 | [diff] [blame] | 5 | :synopsis: Thread-based parallelism. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 6 | |
Raymond Hettinger | 1048094 | 2011-01-10 03:26:08 +0000 | [diff] [blame] | 7 | **Source code:** :source:`Lib/threading.py` |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 8 | |
Raymond Hettinger | 4f707fd | 2011-01-10 19:54:11 +0000 | [diff] [blame] | 9 | -------------- |
| 10 | |
Georg Brandl | 2067bfd | 2008-05-25 13:05:15 +0000 | [diff] [blame] | 11 | This module constructs higher-level threading interfaces on top of the lower |
| 12 | level :mod:`_thread` module. See also the :mod:`queue` module. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 13 | |
| 14 | The :mod:`dummy_threading` module is provided for situations where |
Georg Brandl | 2067bfd | 2008-05-25 13:05:15 +0000 | [diff] [blame] | 15 | :mod:`threading` cannot be used because :mod:`_thread` is missing. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 16 | |
Benjamin Peterson | 8bdd545 | 2008-08-18 22:38:41 +0000 | [diff] [blame] | 17 | .. note:: |
| 18 | |
Benjamin Peterson | b3085c9 | 2008-09-01 23:09:31 +0000 | [diff] [blame] | 19 | While they are not listed below, the ``camelCase`` names used for some |
| 20 | methods and functions in this module in the Python 2.x series are still |
| 21 | supported by this module. |
Benjamin Peterson | 8bdd545 | 2008-08-18 22:38:41 +0000 | [diff] [blame] | 22 | |
Antoine Pitrou | 0034281 | 2011-01-06 16:31:28 +0000 | [diff] [blame] | 23 | .. impl-detail:: |
| 24 | |
| 25 | Due to the :term:`Global Interpreter Lock`, in CPython only one thread |
| 26 | can execute Python code at once (even though certain performance-oriented |
| 27 | libraries might overcome this limitation). |
| 28 | If you want your application to make better of use of the computational |
| 29 | resources of multi-core machines, you are advised to use |
| 30 | :mod:`multiprocessing` or :class:`concurrent.futures.ProcessPoolExecutor`. |
| 31 | However, threading is still an appropriate model if you want to run |
| 32 | multiple I/O-bound tasks simultaneously. |
| 33 | |
Antoine Pitrou | 0034281 | 2011-01-06 16:31:28 +0000 | [diff] [blame] | 34 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 35 | This module defines the following functions and objects: |
| 36 | |
| 37 | |
Benjamin Peterson | 672b803 | 2008-06-11 19:14:14 +0000 | [diff] [blame] | 38 | .. function:: active_count() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 39 | |
| 40 | Return the number of :class:`Thread` objects currently alive. The returned |
Benjamin Peterson | 4ac9ce4 | 2009-10-04 14:49:41 +0000 | [diff] [blame] | 41 | count is equal to the length of the list returned by :func:`.enumerate`. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 42 | |
| 43 | |
| 44 | .. function:: Condition() |
| 45 | :noindex: |
| 46 | |
| 47 | A factory function that returns a new condition variable object. A condition |
| 48 | variable allows one or more threads to wait until they are notified by another |
| 49 | thread. |
| 50 | |
Georg Brandl | 179249f | 2010-08-26 14:30:15 +0000 | [diff] [blame] | 51 | See :ref:`condition-objects`. |
| 52 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 53 | |
Benjamin Peterson | 672b803 | 2008-06-11 19:14:14 +0000 | [diff] [blame] | 54 | .. function:: current_thread() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 55 | |
| 56 | Return the current :class:`Thread` object, corresponding to the caller's thread |
| 57 | of control. If the caller's thread of control was not created through the |
| 58 | :mod:`threading` module, a dummy thread object with limited functionality is |
| 59 | returned. |
| 60 | |
| 61 | |
| 62 | .. function:: enumerate() |
| 63 | |
Benjamin Peterson | 672b803 | 2008-06-11 19:14:14 +0000 | [diff] [blame] | 64 | Return a list of all :class:`Thread` objects currently alive. The list |
| 65 | includes daemonic threads, dummy thread objects created by |
| 66 | :func:`current_thread`, and the main thread. It excludes terminated threads |
| 67 | and threads that have not yet been started. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 68 | |
| 69 | |
| 70 | .. function:: Event() |
| 71 | :noindex: |
| 72 | |
| 73 | A factory function that returns a new event object. An event manages a flag |
Georg Brandl | 502d9a5 | 2009-07-26 15:02:41 +0000 | [diff] [blame] | 74 | that can be set to true with the :meth:`~Event.set` method and reset to false |
| 75 | with the :meth:`clear` method. The :meth:`wait` method blocks until the flag |
| 76 | is true. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 77 | |
Georg Brandl | 179249f | 2010-08-26 14:30:15 +0000 | [diff] [blame] | 78 | See :ref:`event-objects`. |
| 79 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 80 | |
| 81 | .. class:: local |
| 82 | |
| 83 | A class that represents thread-local data. Thread-local data are data whose |
| 84 | values are thread specific. To manage thread-local data, just create an |
| 85 | instance of :class:`local` (or a subclass) and store attributes on it:: |
| 86 | |
| 87 | mydata = threading.local() |
| 88 | mydata.x = 1 |
| 89 | |
| 90 | The instance's values will be different for separate threads. |
| 91 | |
| 92 | For more details and extensive examples, see the documentation string of the |
| 93 | :mod:`_threading_local` module. |
| 94 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 95 | |
| 96 | .. function:: Lock() |
| 97 | |
| 98 | A factory function that returns a new primitive lock object. Once a thread has |
| 99 | acquired it, subsequent attempts to acquire it block, until it is released; any |
| 100 | thread may release it. |
| 101 | |
Georg Brandl | 179249f | 2010-08-26 14:30:15 +0000 | [diff] [blame] | 102 | See :ref:`lock-objects`. |
| 103 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 104 | |
| 105 | .. function:: RLock() |
| 106 | |
| 107 | A factory function that returns a new reentrant lock object. A reentrant lock |
| 108 | must be released by the thread that acquired it. Once a thread has acquired a |
| 109 | reentrant lock, the same thread may acquire it again without blocking; the |
| 110 | thread must release it once for each time it has acquired it. |
| 111 | |
Georg Brandl | 179249f | 2010-08-26 14:30:15 +0000 | [diff] [blame] | 112 | See :ref:`rlock-objects`. |
| 113 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 114 | |
Georg Brandl | 7f01a13 | 2009-09-16 15:58:14 +0000 | [diff] [blame] | 115 | .. function:: Semaphore(value=1) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 116 | :noindex: |
| 117 | |
| 118 | A factory function that returns a new semaphore object. A semaphore manages a |
| 119 | counter representing the number of :meth:`release` calls minus the number of |
| 120 | :meth:`acquire` calls, plus an initial value. The :meth:`acquire` method blocks |
| 121 | if necessary until it can return without making the counter negative. If not |
| 122 | given, *value* defaults to 1. |
| 123 | |
Georg Brandl | 179249f | 2010-08-26 14:30:15 +0000 | [diff] [blame] | 124 | See :ref:`semaphore-objects`. |
| 125 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 126 | |
Georg Brandl | 7f01a13 | 2009-09-16 15:58:14 +0000 | [diff] [blame] | 127 | .. function:: BoundedSemaphore(value=1) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 128 | |
| 129 | A factory function that returns a new bounded semaphore object. A bounded |
| 130 | semaphore checks to make sure its current value doesn't exceed its initial |
| 131 | value. If it does, :exc:`ValueError` is raised. In most situations semaphores |
| 132 | are used to guard resources with limited capacity. If the semaphore is released |
| 133 | too many times it's a sign of a bug. If not given, *value* defaults to 1. |
| 134 | |
| 135 | |
| 136 | .. class:: Thread |
Georg Brandl | 179249f | 2010-08-26 14:30:15 +0000 | [diff] [blame] | 137 | :noindex: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 138 | |
| 139 | A class that represents a thread of control. This class can be safely |
| 140 | subclassed in a limited fashion. |
| 141 | |
Georg Brandl | 179249f | 2010-08-26 14:30:15 +0000 | [diff] [blame] | 142 | See :ref:`thread-objects`. |
| 143 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 144 | |
| 145 | .. class:: Timer |
Georg Brandl | 179249f | 2010-08-26 14:30:15 +0000 | [diff] [blame] | 146 | :noindex: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 147 | |
| 148 | A thread that executes a function after a specified interval has passed. |
| 149 | |
Georg Brandl | 179249f | 2010-08-26 14:30:15 +0000 | [diff] [blame] | 150 | See :ref:`timer-objects`. |
| 151 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 152 | |
| 153 | .. function:: settrace(func) |
| 154 | |
| 155 | .. index:: single: trace function |
| 156 | |
| 157 | Set a trace function for all threads started from the :mod:`threading` module. |
| 158 | The *func* will be passed to :func:`sys.settrace` for each thread, before its |
| 159 | :meth:`run` method is called. |
| 160 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 161 | |
| 162 | .. function:: setprofile(func) |
| 163 | |
| 164 | .. index:: single: profile function |
| 165 | |
| 166 | Set a profile function for all threads started from the :mod:`threading` module. |
| 167 | The *func* will be passed to :func:`sys.setprofile` for each thread, before its |
| 168 | :meth:`run` method is called. |
| 169 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 170 | |
| 171 | .. function:: stack_size([size]) |
| 172 | |
| 173 | Return the thread stack size used when creating new threads. The optional |
| 174 | *size* argument specifies the stack size to be used for subsequently created |
| 175 | threads, and must be 0 (use platform or configured default) or a positive |
| 176 | integer value of at least 32,768 (32kB). If changing the thread stack size is |
| 177 | unsupported, a :exc:`ThreadError` is raised. If the specified stack size is |
| 178 | invalid, a :exc:`ValueError` is raised and the stack size is unmodified. 32kB |
| 179 | is currently the minimum supported stack size value to guarantee sufficient |
| 180 | stack space for the interpreter itself. Note that some platforms may have |
| 181 | particular restrictions on values for the stack size, such as requiring a |
| 182 | minimum stack size > 32kB or requiring allocation in multiples of the system |
| 183 | memory page size - platform documentation should be referred to for more |
| 184 | information (4kB pages are common; using multiples of 4096 for the stack size is |
| 185 | the suggested approach in the absence of more specific information). |
| 186 | Availability: Windows, systems with POSIX threads. |
| 187 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 188 | |
Antoine Pitrou | 7c3e577 | 2010-04-14 15:44:10 +0000 | [diff] [blame] | 189 | This module also defines the following constant: |
| 190 | |
| 191 | .. data:: TIMEOUT_MAX |
| 192 | |
| 193 | The maximum value allowed for the *timeout* parameter of blocking functions |
| 194 | (:meth:`Lock.acquire`, :meth:`RLock.acquire`, :meth:`Condition.wait`, etc.). |
Georg Brandl | 6faee4e | 2010-09-21 14:48:28 +0000 | [diff] [blame] | 195 | Specifying a timeout greater than this value will raise an |
Antoine Pitrou | 7c3e577 | 2010-04-14 15:44:10 +0000 | [diff] [blame] | 196 | :exc:`OverflowError`. |
| 197 | |
Antoine Pitrou | adbc009 | 2010-04-19 14:05:51 +0000 | [diff] [blame] | 198 | .. versionadded:: 3.2 |
Antoine Pitrou | 7c3e577 | 2010-04-14 15:44:10 +0000 | [diff] [blame] | 199 | |
Georg Brandl | 67b21b7 | 2010-08-17 15:07:14 +0000 | [diff] [blame] | 200 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 201 | Detailed interfaces for the objects are documented below. |
| 202 | |
| 203 | The design of this module is loosely based on Java's threading model. However, |
| 204 | where Java makes locks and condition variables basic behavior of every object, |
| 205 | they are separate objects in Python. Python's :class:`Thread` class supports a |
| 206 | subset of the behavior of Java's Thread class; currently, there are no |
| 207 | priorities, no thread groups, and threads cannot be destroyed, stopped, |
| 208 | suspended, resumed, or interrupted. The static methods of Java's Thread class, |
| 209 | when implemented, are mapped to module-level functions. |
| 210 | |
| 211 | All of the methods described below are executed atomically. |
| 212 | |
| 213 | |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 214 | .. _thread-objects: |
| 215 | |
| 216 | Thread Objects |
| 217 | -------------- |
| 218 | |
| 219 | This class represents an activity that is run in a separate thread of control. |
| 220 | There are two ways to specify the activity: by passing a callable object to the |
| 221 | constructor, or by overriding the :meth:`run` method in a subclass. No other |
| 222 | methods (except for the constructor) should be overridden in a subclass. In |
| 223 | other words, *only* override the :meth:`__init__` and :meth:`run` methods of |
| 224 | this class. |
| 225 | |
| 226 | Once a thread object is created, its activity must be started by calling the |
| 227 | thread's :meth:`start` method. This invokes the :meth:`run` method in a |
| 228 | separate thread of control. |
| 229 | |
| 230 | Once the thread's activity is started, the thread is considered 'alive'. It |
| 231 | stops being alive when its :meth:`run` method terminates -- either normally, or |
| 232 | by raising an unhandled exception. The :meth:`is_alive` method tests whether the |
| 233 | thread is alive. |
| 234 | |
| 235 | Other threads can call a thread's :meth:`join` method. This blocks the calling |
| 236 | thread until the thread whose :meth:`join` method is called is terminated. |
| 237 | |
| 238 | A thread has a name. The name can be passed to the constructor, and read or |
| 239 | changed through the :attr:`name` attribute. |
| 240 | |
| 241 | A thread can be flagged as a "daemon thread". The significance of this flag is |
| 242 | that the entire Python program exits when only daemon threads are left. The |
| 243 | initial value is inherited from the creating thread. The flag can be set |
Antoine Pitrou | 0bd4deb | 2011-02-25 22:07:43 +0000 | [diff] [blame^] | 244 | through the :attr:`daemon` property or the *daemon* constructor argument. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 245 | |
| 246 | There is a "main thread" object; this corresponds to the initial thread of |
| 247 | control in the Python program. It is not a daemon thread. |
| 248 | |
| 249 | There is the possibility that "dummy thread objects" are created. These are |
| 250 | thread objects corresponding to "alien threads", which are threads of control |
| 251 | started outside the threading module, such as directly from C code. Dummy |
| 252 | thread objects have limited functionality; they are always considered alive and |
| 253 | daemonic, and cannot be :meth:`join`\ ed. They are never deleted, since it is |
| 254 | impossible to detect the termination of alien threads. |
| 255 | |
| 256 | |
Antoine Pitrou | 0bd4deb | 2011-02-25 22:07:43 +0000 | [diff] [blame^] | 257 | .. class:: Thread(group=None, target=None, name=None, args=(), kwargs={}, |
| 258 | verbose=None, *, daemon=None) |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 259 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 260 | This constructor should always be called with keyword arguments. Arguments |
| 261 | are: |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 262 | |
| 263 | *group* should be ``None``; reserved for future extension when a |
| 264 | :class:`ThreadGroup` class is implemented. |
| 265 | |
| 266 | *target* is the callable object to be invoked by the :meth:`run` method. |
| 267 | Defaults to ``None``, meaning nothing is called. |
| 268 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 269 | *name* is the thread name. By default, a unique name is constructed of the |
| 270 | form "Thread-*N*" where *N* is a small decimal number. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 271 | |
| 272 | *args* is the argument tuple for the target invocation. Defaults to ``()``. |
| 273 | |
| 274 | *kwargs* is a dictionary of keyword arguments for the target invocation. |
| 275 | Defaults to ``{}``. |
| 276 | |
Antoine Pitrou | 0bd4deb | 2011-02-25 22:07:43 +0000 | [diff] [blame^] | 277 | *verbose* is a flag used for debugging messages. |
| 278 | |
| 279 | If not ``None``, *daemon* explicitly sets whether the thread is daemonic. |
| 280 | If ``None`` (the default), the daemonic property is inherited from the |
| 281 | current thread. |
| 282 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 283 | If the subclass overrides the constructor, it must make sure to invoke the |
| 284 | base class constructor (``Thread.__init__()``) before doing anything else to |
| 285 | the thread. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 286 | |
Antoine Pitrou | 0bd4deb | 2011-02-25 22:07:43 +0000 | [diff] [blame^] | 287 | .. versionchanged:: 3.3 |
| 288 | Added the *daemon* argument. |
| 289 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 290 | .. method:: start() |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 291 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 292 | Start the thread's activity. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 293 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 294 | It must be called at most once per thread object. It arranges for the |
| 295 | object's :meth:`run` method to be invoked in a separate thread of control. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 296 | |
Brian Curtin | bd0c897 | 2011-01-31 19:35:02 +0000 | [diff] [blame] | 297 | This method will raise a :exc:`RuntimeError` if called more than once |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 298 | on the same thread object. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 299 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 300 | .. method:: run() |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 301 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 302 | Method representing the thread's activity. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 303 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 304 | You may override this method in a subclass. The standard :meth:`run` |
| 305 | method invokes the callable object passed to the object's constructor as |
| 306 | the *target* argument, if any, with sequential and keyword arguments taken |
| 307 | from the *args* and *kwargs* arguments, respectively. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 308 | |
Georg Brandl | 7f01a13 | 2009-09-16 15:58:14 +0000 | [diff] [blame] | 309 | .. method:: join(timeout=None) |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 310 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 311 | Wait until the thread terminates. This blocks the calling thread until the |
| 312 | thread whose :meth:`join` method is called terminates -- either normally |
| 313 | or through an unhandled exception -- or until the optional timeout occurs. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 314 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 315 | When the *timeout* argument is present and not ``None``, it should be a |
| 316 | floating point number specifying a timeout for the operation in seconds |
| 317 | (or fractions thereof). As :meth:`join` always returns ``None``, you must |
| 318 | call :meth:`is_alive` after :meth:`join` to decide whether a timeout |
| 319 | happened -- if the thread is still alive, the :meth:`join` call timed out. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 320 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 321 | When the *timeout* argument is not present or ``None``, the operation will |
| 322 | block until the thread terminates. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 323 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 324 | A thread can be :meth:`join`\ ed many times. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 325 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 326 | :meth:`join` raises a :exc:`RuntimeError` if an attempt is made to join |
| 327 | the current thread as that would cause a deadlock. It is also an error to |
| 328 | :meth:`join` a thread before it has been started and attempts to do so |
| 329 | raises the same exception. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 330 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 331 | .. attribute:: name |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 332 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 333 | A string used for identification purposes only. It has no semantics. |
| 334 | Multiple threads may be given the same name. The initial name is set by |
| 335 | the constructor. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 336 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 337 | .. method:: getName() |
| 338 | setName() |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 339 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 340 | Old getter/setter API for :attr:`~Thread.name`; use it directly as a |
| 341 | property instead. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 342 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 343 | .. attribute:: ident |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 344 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 345 | The 'thread identifier' of this thread or ``None`` if the thread has not |
| 346 | been started. This is a nonzero integer. See the |
| 347 | :func:`thread.get_ident()` function. Thread identifiers may be recycled |
| 348 | when a thread exits and another thread is created. The identifier is |
| 349 | available even after the thread has exited. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 350 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 351 | .. method:: is_alive() |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 352 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 353 | Return whether the thread is alive. |
Georg Brandl | 770b0be | 2009-01-02 20:10:05 +0000 | [diff] [blame] | 354 | |
Brett Cannon | a57edd0 | 2010-07-23 12:26:35 +0000 | [diff] [blame] | 355 | This method returns ``True`` just before the :meth:`run` method starts |
| 356 | until just after the :meth:`run` method terminates. The module function |
Benjamin Peterson | 4ac9ce4 | 2009-10-04 14:49:41 +0000 | [diff] [blame] | 357 | :func:`.enumerate` returns a list of all alive threads. |
Georg Brandl | 770b0be | 2009-01-02 20:10:05 +0000 | [diff] [blame] | 358 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 359 | .. attribute:: daemon |
Georg Brandl | 770b0be | 2009-01-02 20:10:05 +0000 | [diff] [blame] | 360 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 361 | A boolean value indicating whether this thread is a daemon thread (True) |
| 362 | or not (False). This must be set before :meth:`start` is called, |
| 363 | otherwise :exc:`RuntimeError` is raised. Its initial value is inherited |
| 364 | from the creating thread; the main thread is not a daemon thread and |
| 365 | therefore all threads created in the main thread default to :attr:`daemon` |
| 366 | = ``False``. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 367 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 368 | The entire Python program exits when no alive non-daemon threads are left. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 369 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 370 | .. method:: isDaemon() |
| 371 | setDaemon() |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 372 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 373 | Old getter/setter API for :attr:`~Thread.daemon`; use it directly as a |
| 374 | property instead. |
Georg Brandl | 770b0be | 2009-01-02 20:10:05 +0000 | [diff] [blame] | 375 | |
| 376 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 377 | .. _lock-objects: |
| 378 | |
| 379 | Lock Objects |
| 380 | ------------ |
| 381 | |
| 382 | A primitive lock is a synchronization primitive that is not owned by a |
| 383 | particular thread when locked. In Python, it is currently the lowest level |
Georg Brandl | 2067bfd | 2008-05-25 13:05:15 +0000 | [diff] [blame] | 384 | synchronization primitive available, implemented directly by the :mod:`_thread` |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 385 | extension module. |
| 386 | |
| 387 | A primitive lock is in one of two states, "locked" or "unlocked". It is created |
| 388 | in the unlocked state. It has two basic methods, :meth:`acquire` and |
| 389 | :meth:`release`. When the state is unlocked, :meth:`acquire` changes the state |
| 390 | to locked and returns immediately. When the state is locked, :meth:`acquire` |
| 391 | blocks until a call to :meth:`release` in another thread changes it to unlocked, |
| 392 | then the :meth:`acquire` call resets it to locked and returns. The |
| 393 | :meth:`release` method should only be called in the locked state; it changes the |
| 394 | state to unlocked and returns immediately. If an attempt is made to release an |
| 395 | unlocked lock, a :exc:`RuntimeError` will be raised. |
| 396 | |
| 397 | When more than one thread is blocked in :meth:`acquire` waiting for the state to |
| 398 | turn to unlocked, only one thread proceeds when a :meth:`release` call resets |
| 399 | the state to unlocked; which one of the waiting threads proceeds is not defined, |
| 400 | and may vary across implementations. |
| 401 | |
| 402 | All methods are executed atomically. |
| 403 | |
| 404 | |
Antoine Pitrou | 7c3e577 | 2010-04-14 15:44:10 +0000 | [diff] [blame] | 405 | .. method:: Lock.acquire(blocking=True, timeout=-1) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 406 | |
| 407 | Acquire a lock, blocking or non-blocking. |
| 408 | |
| 409 | When invoked without arguments, block until the lock is unlocked, then set it to |
| 410 | locked, and return true. |
| 411 | |
| 412 | When invoked with the *blocking* argument set to true, do the same thing as when |
| 413 | called without arguments, and return true. |
| 414 | |
| 415 | When invoked with the *blocking* argument set to false, do not block. If a call |
| 416 | without an argument would block, return false immediately; otherwise, do the |
| 417 | same thing as when called without arguments, and return true. |
| 418 | |
Antoine Pitrou | 7c3e577 | 2010-04-14 15:44:10 +0000 | [diff] [blame] | 419 | When invoked with the floating-point *timeout* argument set to a positive |
| 420 | value, block for at most the number of seconds specified by *timeout* |
| 421 | and as long as the lock cannot be acquired. A negative *timeout* argument |
| 422 | specifies an unbounded wait. It is forbidden to specify a *timeout* |
| 423 | when *blocking* is false. |
| 424 | |
| 425 | The return value is ``True`` if the lock is acquired successfully, |
| 426 | ``False`` if not (for example if the *timeout* expired). |
| 427 | |
Antoine Pitrou | adbc009 | 2010-04-19 14:05:51 +0000 | [diff] [blame] | 428 | .. versionchanged:: 3.2 |
| 429 | The *timeout* parameter is new. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 430 | |
Antoine Pitrou | 810023d | 2010-12-15 22:59:16 +0000 | [diff] [blame] | 431 | .. versionchanged:: 3.2 |
| 432 | Lock acquires can now be interrupted by signals on POSIX. |
| 433 | |
Georg Brandl | 67b21b7 | 2010-08-17 15:07:14 +0000 | [diff] [blame] | 434 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 435 | .. method:: Lock.release() |
| 436 | |
| 437 | Release a lock. |
| 438 | |
| 439 | When the lock is locked, reset it to unlocked, and return. If any other threads |
| 440 | are blocked waiting for the lock to become unlocked, allow exactly one of them |
| 441 | to proceed. |
| 442 | |
| 443 | Do not call this method when the lock is unlocked. |
| 444 | |
| 445 | There is no return value. |
| 446 | |
| 447 | |
| 448 | .. _rlock-objects: |
| 449 | |
| 450 | RLock Objects |
| 451 | ------------- |
| 452 | |
| 453 | A reentrant lock is a synchronization primitive that may be acquired multiple |
| 454 | times by the same thread. Internally, it uses the concepts of "owning thread" |
| 455 | and "recursion level" in addition to the locked/unlocked state used by primitive |
| 456 | locks. In the locked state, some thread owns the lock; in the unlocked state, |
| 457 | no thread owns it. |
| 458 | |
| 459 | To lock the lock, a thread calls its :meth:`acquire` method; this returns once |
| 460 | the thread owns the lock. To unlock the lock, a thread calls its |
| 461 | :meth:`release` method. :meth:`acquire`/:meth:`release` call pairs may be |
| 462 | nested; only the final :meth:`release` (the :meth:`release` of the outermost |
| 463 | pair) resets the lock to unlocked and allows another thread blocked in |
| 464 | :meth:`acquire` to proceed. |
| 465 | |
| 466 | |
Antoine Pitrou | 7c3e577 | 2010-04-14 15:44:10 +0000 | [diff] [blame] | 467 | .. method:: RLock.acquire(blocking=True, timeout=-1) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 468 | |
| 469 | Acquire a lock, blocking or non-blocking. |
| 470 | |
| 471 | When invoked without arguments: if this thread already owns the lock, increment |
| 472 | the recursion level by one, and return immediately. Otherwise, if another |
| 473 | thread owns the lock, block until the lock is unlocked. Once the lock is |
| 474 | unlocked (not owned by any thread), then grab ownership, set the recursion level |
| 475 | to one, and return. If more than one thread is blocked waiting until the lock |
| 476 | is unlocked, only one at a time will be able to grab ownership of the lock. |
| 477 | There is no return value in this case. |
| 478 | |
| 479 | When invoked with the *blocking* argument set to true, do the same thing as when |
| 480 | called without arguments, and return true. |
| 481 | |
| 482 | When invoked with the *blocking* argument set to false, do not block. If a call |
| 483 | without an argument would block, return false immediately; otherwise, do the |
| 484 | same thing as when called without arguments, and return true. |
| 485 | |
Antoine Pitrou | 7c3e577 | 2010-04-14 15:44:10 +0000 | [diff] [blame] | 486 | When invoked with the floating-point *timeout* argument set to a positive |
| 487 | value, block for at most the number of seconds specified by *timeout* |
| 488 | and as long as the lock cannot be acquired. Return true if the lock has |
| 489 | been acquired, false if the timeout has elapsed. |
| 490 | |
Antoine Pitrou | adbc009 | 2010-04-19 14:05:51 +0000 | [diff] [blame] | 491 | .. versionchanged:: 3.2 |
| 492 | The *timeout* parameter is new. |
| 493 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 494 | |
| 495 | .. method:: RLock.release() |
| 496 | |
| 497 | Release a lock, decrementing the recursion level. If after the decrement it is |
| 498 | zero, reset the lock to unlocked (not owned by any thread), and if any other |
| 499 | threads are blocked waiting for the lock to become unlocked, allow exactly one |
| 500 | of them to proceed. If after the decrement the recursion level is still |
| 501 | nonzero, the lock remains locked and owned by the calling thread. |
| 502 | |
| 503 | Only call this method when the calling thread owns the lock. A |
| 504 | :exc:`RuntimeError` is raised if this method is called when the lock is |
| 505 | unlocked. |
| 506 | |
| 507 | There is no return value. |
| 508 | |
| 509 | |
| 510 | .. _condition-objects: |
| 511 | |
| 512 | Condition Objects |
| 513 | ----------------- |
| 514 | |
| 515 | A condition variable is always associated with some kind of lock; this can be |
| 516 | passed in or one will be created by default. (Passing one in is useful when |
| 517 | several condition variables must share the same lock.) |
| 518 | |
| 519 | A condition variable has :meth:`acquire` and :meth:`release` methods that call |
| 520 | the corresponding methods of the associated lock. It also has a :meth:`wait` |
Georg Brandl | f992640 | 2008-06-13 06:32:25 +0000 | [diff] [blame] | 521 | method, and :meth:`notify` and :meth:`notify_all` methods. These three must only |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 522 | be called when the calling thread has acquired the lock, otherwise a |
| 523 | :exc:`RuntimeError` is raised. |
| 524 | |
| 525 | The :meth:`wait` method releases the lock, and then blocks until it is awakened |
Georg Brandl | f992640 | 2008-06-13 06:32:25 +0000 | [diff] [blame] | 526 | by a :meth:`notify` or :meth:`notify_all` call for the same condition variable in |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 527 | another thread. Once awakened, it re-acquires the lock and returns. It is also |
| 528 | possible to specify a timeout. |
| 529 | |
| 530 | The :meth:`notify` method wakes up one of the threads waiting for the condition |
Georg Brandl | f992640 | 2008-06-13 06:32:25 +0000 | [diff] [blame] | 531 | variable, if any are waiting. The :meth:`notify_all` method wakes up all threads |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 532 | waiting for the condition variable. |
| 533 | |
Georg Brandl | f992640 | 2008-06-13 06:32:25 +0000 | [diff] [blame] | 534 | Note: the :meth:`notify` and :meth:`notify_all` methods don't release the lock; |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 535 | this means that the thread or threads awakened will not return from their |
| 536 | :meth:`wait` call immediately, but only when the thread that called |
Georg Brandl | f992640 | 2008-06-13 06:32:25 +0000 | [diff] [blame] | 537 | :meth:`notify` or :meth:`notify_all` finally relinquishes ownership of the lock. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 538 | |
| 539 | Tip: the typical programming style using condition variables uses the lock to |
| 540 | synchronize access to some shared state; threads that are interested in a |
| 541 | particular change of state call :meth:`wait` repeatedly until they see the |
| 542 | desired state, while threads that modify the state call :meth:`notify` or |
Georg Brandl | f992640 | 2008-06-13 06:32:25 +0000 | [diff] [blame] | 543 | :meth:`notify_all` when they change the state in such a way that it could |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 544 | possibly be a desired state for one of the waiters. For example, the following |
| 545 | code is a generic producer-consumer situation with unlimited buffer capacity:: |
| 546 | |
| 547 | # Consume one item |
| 548 | cv.acquire() |
| 549 | while not an_item_is_available(): |
| 550 | cv.wait() |
| 551 | get_an_available_item() |
| 552 | cv.release() |
| 553 | |
| 554 | # Produce one item |
| 555 | cv.acquire() |
| 556 | make_an_item_available() |
| 557 | cv.notify() |
| 558 | cv.release() |
| 559 | |
Georg Brandl | f992640 | 2008-06-13 06:32:25 +0000 | [diff] [blame] | 560 | To choose between :meth:`notify` and :meth:`notify_all`, consider whether one |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 561 | state change can be interesting for only one or several waiting threads. E.g. |
| 562 | in a typical producer-consumer situation, adding one item to the buffer only |
| 563 | needs to wake up one consumer thread. |
| 564 | |
Kristján Valur Jónsson | 6331520 | 2010-11-18 12:46:39 +0000 | [diff] [blame] | 565 | Note: Condition variables can be, depending on the implementation, subject |
| 566 | to both spurious wakeups (when :meth:`wait` returns without a :meth:`notify` |
| 567 | call) and stolen wakeups (when another thread acquires the lock before the |
| 568 | awoken thread.) For this reason, it is always necessary to verify the state |
| 569 | the thread is waiting for when :meth:`wait` returns and optionally repeat |
| 570 | the call as often as necessary. |
| 571 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 572 | |
Georg Brandl | 7f01a13 | 2009-09-16 15:58:14 +0000 | [diff] [blame] | 573 | .. class:: Condition(lock=None) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 574 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 575 | If the *lock* argument is given and not ``None``, it must be a :class:`Lock` |
| 576 | or :class:`RLock` object, and it is used as the underlying lock. Otherwise, |
| 577 | a new :class:`RLock` object is created and used as the underlying lock. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 578 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 579 | .. method:: acquire(*args) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 580 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 581 | Acquire the underlying lock. This method calls the corresponding method on |
| 582 | the underlying lock; the return value is whatever that method returns. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 583 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 584 | .. method:: release() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 585 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 586 | Release the underlying lock. This method calls the corresponding method on |
| 587 | the underlying lock; there is no return value. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 588 | |
Georg Brandl | 7f01a13 | 2009-09-16 15:58:14 +0000 | [diff] [blame] | 589 | .. method:: wait(timeout=None) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 590 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 591 | Wait until notified or until a timeout occurs. If the calling thread has |
| 592 | not acquired the lock when this method is called, a :exc:`RuntimeError` is |
| 593 | raised. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 594 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 595 | This method releases the underlying lock, and then blocks until it is |
| 596 | awakened by a :meth:`notify` or :meth:`notify_all` call for the same |
| 597 | condition variable in another thread, or until the optional timeout |
| 598 | occurs. Once awakened or timed out, it re-acquires the lock and returns. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 599 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 600 | When the *timeout* argument is present and not ``None``, it should be a |
| 601 | floating point number specifying a timeout for the operation in seconds |
| 602 | (or fractions thereof). |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 603 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 604 | When the underlying lock is an :class:`RLock`, it is not released using |
| 605 | its :meth:`release` method, since this may not actually unlock the lock |
| 606 | when it was acquired multiple times recursively. Instead, an internal |
| 607 | interface of the :class:`RLock` class is used, which really unlocks it |
| 608 | even when it has been recursively acquired several times. Another internal |
| 609 | interface is then used to restore the recursion level when the lock is |
| 610 | reacquired. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 611 | |
Georg Brandl | b9a4391 | 2010-10-28 09:03:20 +0000 | [diff] [blame] | 612 | The return value is ``True`` unless a given *timeout* expired, in which |
| 613 | case it is ``False``. |
| 614 | |
| 615 | .. versionchanged:: 3.2 |
| 616 | Previously, the method always returned ``None``. |
| 617 | |
Kristján Valur Jónsson | 6331520 | 2010-11-18 12:46:39 +0000 | [diff] [blame] | 618 | .. method:: wait_for(predicate, timeout=None) |
| 619 | |
| 620 | Wait until a condition evaluates to True. *predicate* should be a |
| 621 | callable which result will be interpreted as a boolean value. |
| 622 | A *timeout* may be provided giving the maximum time to wait. |
| 623 | |
| 624 | This utility method may call :meth:`wait` repeatedly until the predicate |
| 625 | is satisfied, or until a timeout occurs. The return value is |
| 626 | the last return value of the predicate and will evaluate to |
| 627 | ``False`` if the method timed out. |
| 628 | |
| 629 | Ignoring the timeout feature, calling this method is roughly equivalent to |
| 630 | writing:: |
| 631 | |
| 632 | while not predicate(): |
| 633 | cv.wait() |
| 634 | |
| 635 | Therefore, the same rules apply as with :meth:`wait`: The lock must be |
| 636 | held when called and is re-aquired on return. The predicate is evaluated |
| 637 | with the lock held. |
| 638 | |
| 639 | Using this method, the consumer example above can be written thus:: |
| 640 | |
| 641 | with cv: |
| 642 | cv.wait_for(an_item_is_available) |
| 643 | get_an_available_item() |
| 644 | |
| 645 | .. versionadded:: 3.2 |
| 646 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 647 | .. method:: notify() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 648 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 649 | Wake up a thread waiting on this condition, if any. If the calling thread |
| 650 | has not acquired the lock when this method is called, a |
| 651 | :exc:`RuntimeError` is raised. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 652 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 653 | This method wakes up one of the threads waiting for the condition |
| 654 | variable, if any are waiting; it is a no-op if no threads are waiting. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 655 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 656 | The current implementation wakes up exactly one thread, if any are |
| 657 | waiting. However, it's not safe to rely on this behavior. A future, |
| 658 | optimized implementation may occasionally wake up more than one thread. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 659 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 660 | Note: the awakened thread does not actually return from its :meth:`wait` |
| 661 | call until it can reacquire the lock. Since :meth:`notify` does not |
| 662 | release the lock, its caller should. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 663 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 664 | .. method:: notify_all() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 665 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 666 | Wake up all threads waiting on this condition. This method acts like |
| 667 | :meth:`notify`, but wakes up all waiting threads instead of one. If the |
| 668 | calling thread has not acquired the lock when this method is called, a |
| 669 | :exc:`RuntimeError` is raised. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 670 | |
| 671 | |
| 672 | .. _semaphore-objects: |
| 673 | |
| 674 | Semaphore Objects |
| 675 | ----------------- |
| 676 | |
| 677 | This is one of the oldest synchronization primitives in the history of computer |
| 678 | science, invented by the early Dutch computer scientist Edsger W. Dijkstra (he |
| 679 | used :meth:`P` and :meth:`V` instead of :meth:`acquire` and :meth:`release`). |
| 680 | |
| 681 | A semaphore manages an internal counter which is decremented by each |
| 682 | :meth:`acquire` call and incremented by each :meth:`release` call. The counter |
| 683 | can never go below zero; when :meth:`acquire` finds that it is zero, it blocks, |
| 684 | waiting until some other thread calls :meth:`release`. |
| 685 | |
| 686 | |
Georg Brandl | 7f01a13 | 2009-09-16 15:58:14 +0000 | [diff] [blame] | 687 | .. class:: Semaphore(value=1) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 688 | |
| 689 | The optional argument gives the initial *value* for the internal counter; it |
| 690 | defaults to ``1``. If the *value* given is less than 0, :exc:`ValueError` is |
| 691 | raised. |
| 692 | |
Antoine Pitrou | 0454af9 | 2010-04-17 23:51:58 +0000 | [diff] [blame] | 693 | .. method:: acquire(blocking=True, timeout=None) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 694 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 695 | Acquire a semaphore. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 696 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 697 | When invoked without arguments: if the internal counter is larger than |
| 698 | zero on entry, decrement it by one and return immediately. If it is zero |
| 699 | on entry, block, waiting until some other thread has called |
| 700 | :meth:`release` to make it larger than zero. This is done with proper |
| 701 | interlocking so that if multiple :meth:`acquire` calls are blocked, |
| 702 | :meth:`release` will wake exactly one of them up. The implementation may |
| 703 | pick one at random, so the order in which blocked threads are awakened |
Antoine Pitrou | 0454af9 | 2010-04-17 23:51:58 +0000 | [diff] [blame] | 704 | should not be relied on. Returns true (or blocks indefinitely). |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 705 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 706 | When invoked with *blocking* set to false, do not block. If a call |
Antoine Pitrou | 0454af9 | 2010-04-17 23:51:58 +0000 | [diff] [blame] | 707 | without an argument would block, return false immediately; otherwise, |
| 708 | do the same thing as when called without arguments, and return true. |
| 709 | |
| 710 | When invoked with a *timeout* other than None, it will block for at |
| 711 | most *timeout* seconds. If acquire does not complete successfully in |
| 712 | that interval, return false. Return true otherwise. |
| 713 | |
| 714 | .. versionchanged:: 3.2 |
| 715 | The *timeout* parameter is new. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 716 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 717 | .. method:: release() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 718 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 719 | Release a semaphore, incrementing the internal counter by one. When it |
| 720 | was zero on entry and another thread is waiting for it to become larger |
| 721 | than zero again, wake up that thread. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 722 | |
| 723 | |
| 724 | .. _semaphore-examples: |
| 725 | |
| 726 | :class:`Semaphore` Example |
| 727 | ^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| 728 | |
| 729 | Semaphores are often used to guard resources with limited capacity, for example, |
Georg Brandl | a572476 | 2011-01-06 19:28:18 +0000 | [diff] [blame] | 730 | a database server. In any situation where the size of the resource is fixed, |
| 731 | you should use a bounded semaphore. Before spawning any worker threads, your |
| 732 | main thread would initialize the semaphore:: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 733 | |
| 734 | maxconnections = 5 |
| 735 | ... |
| 736 | pool_sema = BoundedSemaphore(value=maxconnections) |
| 737 | |
| 738 | Once spawned, worker threads call the semaphore's acquire and release methods |
| 739 | when they need to connect to the server:: |
| 740 | |
| 741 | pool_sema.acquire() |
| 742 | conn = connectdb() |
| 743 | ... use connection ... |
| 744 | conn.close() |
| 745 | pool_sema.release() |
| 746 | |
| 747 | The use of a bounded semaphore reduces the chance that a programming error which |
| 748 | causes the semaphore to be released more than it's acquired will go undetected. |
| 749 | |
| 750 | |
| 751 | .. _event-objects: |
| 752 | |
| 753 | Event Objects |
| 754 | ------------- |
| 755 | |
| 756 | This is one of the simplest mechanisms for communication between threads: one |
| 757 | thread signals an event and other threads wait for it. |
| 758 | |
| 759 | An event object manages an internal flag that can be set to true with the |
Georg Brandl | 502d9a5 | 2009-07-26 15:02:41 +0000 | [diff] [blame] | 760 | :meth:`~Event.set` method and reset to false with the :meth:`clear` method. The |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 761 | :meth:`wait` method blocks until the flag is true. |
| 762 | |
| 763 | |
| 764 | .. class:: Event() |
| 765 | |
| 766 | The internal flag is initially false. |
| 767 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 768 | .. method:: is_set() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 769 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 770 | Return true if and only if the internal flag is true. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 771 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 772 | .. method:: set() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 773 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 774 | Set the internal flag to true. All threads waiting for it to become true |
| 775 | are awakened. Threads that call :meth:`wait` once the flag is true will |
| 776 | not block at all. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 777 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 778 | .. method:: clear() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 779 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 780 | Reset the internal flag to false. Subsequently, threads calling |
Georg Brandl | 502d9a5 | 2009-07-26 15:02:41 +0000 | [diff] [blame] | 781 | :meth:`wait` will block until :meth:`.set` is called to set the internal |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 782 | flag to true again. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 783 | |
Georg Brandl | 7f01a13 | 2009-09-16 15:58:14 +0000 | [diff] [blame] | 784 | .. method:: wait(timeout=None) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 785 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 786 | Block until the internal flag is true. If the internal flag is true on |
| 787 | entry, return immediately. Otherwise, block until another thread calls |
| 788 | :meth:`set` to set the flag to true, or until the optional timeout occurs. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 789 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 790 | When the timeout argument is present and not ``None``, it should be a |
| 791 | floating point number specifying a timeout for the operation in seconds |
| 792 | (or fractions thereof). |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 793 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 794 | This method returns the internal flag on exit, so it will always return |
| 795 | ``True`` except if a timeout is given and the operation times out. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 796 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 797 | .. versionchanged:: 3.1 |
| 798 | Previously, the method always returned ``None``. |
Benjamin Peterson | d23f822 | 2009-04-05 19:13:16 +0000 | [diff] [blame] | 799 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 800 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 801 | .. _timer-objects: |
| 802 | |
| 803 | Timer Objects |
| 804 | ------------- |
| 805 | |
| 806 | This class represents an action that should be run only after a certain amount |
| 807 | of time has passed --- a timer. :class:`Timer` is a subclass of :class:`Thread` |
| 808 | and as such also functions as an example of creating custom threads. |
| 809 | |
| 810 | Timers are started, as with threads, by calling their :meth:`start` method. The |
| 811 | timer can be stopped (before its action has begun) by calling the :meth:`cancel` |
| 812 | method. The interval the timer will wait before executing its action may not be |
| 813 | exactly the same as the interval specified by the user. |
| 814 | |
| 815 | For example:: |
| 816 | |
| 817 | def hello(): |
Collin Winter | c79461b | 2007-09-01 23:34:30 +0000 | [diff] [blame] | 818 | print("hello, world") |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 819 | |
| 820 | t = Timer(30.0, hello) |
| 821 | t.start() # after 30 seconds, "hello, world" will be printed |
| 822 | |
| 823 | |
| 824 | .. class:: Timer(interval, function, args=[], kwargs={}) |
| 825 | |
| 826 | Create a timer that will run *function* with arguments *args* and keyword |
| 827 | arguments *kwargs*, after *interval* seconds have passed. |
| 828 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 829 | .. method:: cancel() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 830 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 831 | Stop the timer, and cancel the execution of the timer's action. This will |
| 832 | only work if the timer is still in its waiting stage. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 833 | |
| 834 | |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 835 | Barrier Objects |
| 836 | --------------- |
| 837 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 838 | .. versionadded:: 3.2 |
| 839 | |
| 840 | This class provides a simple synchronization primitive for use by a fixed number |
| 841 | of threads that need to wait for each other. Each of the threads tries to pass |
| 842 | the barrier by calling the :meth:`wait` method and will block until all of the |
| 843 | threads have made the call. At this points, the threads are released |
| 844 | simultanously. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 845 | |
| 846 | The barrier can be reused any number of times for the same number of threads. |
| 847 | |
| 848 | As an example, here is a simple way to synchronize a client and server thread:: |
| 849 | |
| 850 | b = Barrier(2, timeout=5) |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 851 | |
| 852 | def server(): |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 853 | start_server() |
| 854 | b.wait() |
| 855 | while True: |
| 856 | connection = accept_connection() |
| 857 | process_server_connection(connection) |
| 858 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 859 | def client(): |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 860 | b.wait() |
| 861 | while True: |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 862 | connection = make_connection() |
| 863 | process_client_connection(connection) |
| 864 | |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 865 | |
| 866 | .. class:: Barrier(parties, action=None, timeout=None) |
| 867 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 868 | Create a barrier object for *parties* number of threads. An *action*, when |
| 869 | provided, is a callable to be called by one of the threads when they are |
| 870 | released. *timeout* is the default timeout value if none is specified for |
| 871 | the :meth:`wait` method. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 872 | |
| 873 | .. method:: wait(timeout=None) |
| 874 | |
| 875 | Pass the barrier. When all the threads party to the barrier have called |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 876 | this function, they are all released simultaneously. If a *timeout* is |
| 877 | provided, is is used in preference to any that was supplied to the class |
| 878 | constructor. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 879 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 880 | The return value is an integer in the range 0 to *parties* -- 1, different |
Raymond Hettinger | 5cee47f | 2011-01-11 19:59:46 +0000 | [diff] [blame] | 881 | for each thread. This can be used to select a thread to do some special |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 882 | housekeeping, e.g.:: |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 883 | |
| 884 | i = barrier.wait() |
| 885 | if i == 0: |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 886 | # Only one thread needs to print this |
| 887 | print("passed the barrier") |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 888 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 889 | If an *action* was provided to the constructor, one of the threads will |
| 890 | have called it prior to being released. Should this call raise an error, |
| 891 | the barrier is put into the broken state. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 892 | |
| 893 | If the call times out, the barrier is put into the broken state. |
| 894 | |
| 895 | This method may raise a :class:`BrokenBarrierError` exception if the |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 896 | barrier is broken or reset while a thread is waiting. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 897 | |
| 898 | .. method:: reset() |
| 899 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 900 | Return the barrier to the default, empty state. Any threads waiting on it |
| 901 | will receive the :class:`BrokenBarrierError` exception. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 902 | |
| 903 | Note that using this function may can require some external |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 904 | synchronization if there are other threads whose state is unknown. If a |
| 905 | barrier is broken it may be better to just leave it and create a new one. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 906 | |
| 907 | .. method:: abort() |
| 908 | |
| 909 | Put the barrier into a broken state. This causes any active or future |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 910 | calls to :meth:`wait` to fail with the :class:`BrokenBarrierError`. Use |
| 911 | this for example if one of the needs to abort, to avoid deadlocking the |
| 912 | application. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 913 | |
| 914 | It may be preferable to simply create the barrier with a sensible |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 915 | *timeout* value to automatically guard against one of the threads going |
| 916 | awry. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 917 | |
| 918 | .. attribute:: parties |
| 919 | |
| 920 | The number of threads required to pass the barrier. |
| 921 | |
| 922 | .. attribute:: n_waiting |
| 923 | |
| 924 | The number of threads currently waiting in the barrier. |
| 925 | |
| 926 | .. attribute:: broken |
| 927 | |
| 928 | A boolean that is ``True`` if the barrier is in the broken state. |
| 929 | |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 930 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 931 | .. exception:: BrokenBarrierError |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 932 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 933 | This exception, a subclass of :exc:`RuntimeError`, is raised when the |
| 934 | :class:`Barrier` object is reset or broken. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 935 | |
| 936 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 937 | .. _with-locks: |
| 938 | |
| 939 | Using locks, conditions, and semaphores in the :keyword:`with` statement |
| 940 | ------------------------------------------------------------------------ |
| 941 | |
| 942 | All of the objects provided by this module that have :meth:`acquire` and |
| 943 | :meth:`release` methods can be used as context managers for a :keyword:`with` |
| 944 | statement. The :meth:`acquire` method will be called when the block is entered, |
| 945 | and :meth:`release` will be called when the block is exited. |
| 946 | |
| 947 | Currently, :class:`Lock`, :class:`RLock`, :class:`Condition`, |
| 948 | :class:`Semaphore`, and :class:`BoundedSemaphore` objects may be used as |
| 949 | :keyword:`with` statement context managers. For example:: |
| 950 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 951 | import threading |
| 952 | |
| 953 | some_rlock = threading.RLock() |
| 954 | |
| 955 | with some_rlock: |
Collin Winter | c79461b | 2007-09-01 23:34:30 +0000 | [diff] [blame] | 956 | print("some_rlock is locked while this executes") |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 957 | |
Christian Heimes | dd15f6c | 2008-03-16 00:07:10 +0000 | [diff] [blame] | 958 | |
| 959 | .. _threaded-imports: |
| 960 | |
| 961 | Importing in threaded code |
| 962 | -------------------------- |
| 963 | |
Georg Brandl | f285bcc | 2010-10-19 21:07:16 +0000 | [diff] [blame] | 964 | While the import machinery is thread-safe, there are two key restrictions on |
| 965 | threaded imports due to inherent limitations in the way that thread-safety is |
| 966 | provided: |
Christian Heimes | dd15f6c | 2008-03-16 00:07:10 +0000 | [diff] [blame] | 967 | |
| 968 | * Firstly, other than in the main module, an import should not have the |
| 969 | side effect of spawning a new thread and then waiting for that thread in |
| 970 | any way. Failing to abide by this restriction can lead to a deadlock if |
| 971 | the spawned thread directly or indirectly attempts to import a module. |
| 972 | * Secondly, all import attempts must be completed before the interpreter |
| 973 | starts shutting itself down. This can be most easily achieved by only |
| 974 | performing imports from non-daemon threads created through the threading |
| 975 | module. Daemon threads and threads created directly with the thread |
| 976 | module will require some other form of synchronization to ensure they do |
| 977 | not attempt imports after system shutdown has commenced. Failure to |
| 978 | abide by this restriction will lead to intermittent exceptions and |
| 979 | crashes during interpreter shutdown (as the late imports attempt to |
| 980 | access machinery which is no longer in a valid state). |