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 | |
Antoine Pitrou | b43c4ca | 2017-09-18 22:04:20 +0200 | [diff] [blame] | 14 | .. versionchanged:: 3.7 |
| 15 | This module used to be optional, it is now always available. |
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 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 24 | This module defines the following functions: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 25 | |
| 26 | |
Benjamin Peterson | 672b803 | 2008-06-11 19:14:14 +0000 | [diff] [blame] | 27 | .. function:: active_count() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 28 | |
| 29 | Return the number of :class:`Thread` objects currently alive. The returned |
Benjamin Peterson | 4ac9ce4 | 2009-10-04 14:49:41 +0000 | [diff] [blame] | 30 | 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] | 31 | |
| 32 | |
Benjamin Peterson | 672b803 | 2008-06-11 19:14:14 +0000 | [diff] [blame] | 33 | .. function:: current_thread() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 34 | |
| 35 | Return the current :class:`Thread` object, corresponding to the caller's thread |
| 36 | of control. If the caller's thread of control was not created through the |
| 37 | :mod:`threading` module, a dummy thread object with limited functionality is |
| 38 | returned. |
| 39 | |
| 40 | |
Victor Stinner | cd590a7 | 2019-05-28 00:39:52 +0200 | [diff] [blame] | 41 | .. function:: excepthook(args, /) |
| 42 | |
| 43 | Handle uncaught exception raised by :func:`Thread.run`. |
| 44 | |
| 45 | The *args* argument has the following attributes: |
| 46 | |
| 47 | * *exc_type*: Exception type. |
| 48 | * *exc_value*: Exception value, can be ``None``. |
| 49 | * *exc_traceback*: Exception traceback, can be ``None``. |
| 50 | * *thread*: Thread which raised the exception, can be ``None``. |
| 51 | |
| 52 | If *exc_type* is :exc:`SystemExit`, the exception is silently ignored. |
| 53 | Otherwise, the exception is printed out on :data:`sys.stderr`. |
| 54 | |
| 55 | If this function raises an exception, :func:`sys.excepthook` is called to |
| 56 | handle it. |
| 57 | |
| 58 | :func:`threading.excepthook` can be overridden to control how uncaught |
| 59 | exceptions raised by :func:`Thread.run` are handled. |
| 60 | |
Victor Stinner | 212646c | 2019-06-14 18:03:22 +0200 | [diff] [blame] | 61 | Storing *exc_value* using a custom hook can create a reference cycle. It |
| 62 | should be cleared explicitly to break the reference cycle when the |
| 63 | exception is no longer needed. |
| 64 | |
| 65 | Storing *object* using a custom hook can resurrect it if it is set to an |
| 66 | object which is being finalized. Avoid storing *object* after the custom |
| 67 | hook completes to avoid resurrecting objects. |
| 68 | |
Victor Stinner | cd590a7 | 2019-05-28 00:39:52 +0200 | [diff] [blame] | 69 | .. seealso:: |
| 70 | :func:`sys.excepthook` handles uncaught exceptions. |
| 71 | |
| 72 | .. versionadded:: 3.8 |
| 73 | |
| 74 | |
Victor Stinner | 2a12974 | 2011-05-30 23:02:52 +0200 | [diff] [blame] | 75 | .. function:: get_ident() |
| 76 | |
| 77 | Return the 'thread identifier' of the current thread. This is a nonzero |
| 78 | integer. Its value has no direct meaning; it is intended as a magic cookie |
| 79 | to be used e.g. to index a dictionary of thread-specific data. Thread |
| 80 | identifiers may be recycled when a thread exits and another thread is |
| 81 | created. |
| 82 | |
| 83 | .. versionadded:: 3.3 |
| 84 | |
| 85 | |
Jake Tesler | b121f63 | 2019-05-22 08:43:17 -0700 | [diff] [blame] | 86 | .. function:: get_native_id() |
| 87 | |
| 88 | Return the native integral Thread ID of the current thread assigned by the kernel. |
| 89 | This is a non-negative integer. |
| 90 | Its value may be used to uniquely identify this particular thread system-wide |
| 91 | (until the thread terminates, after which the value may be recycled by the OS). |
| 92 | |
Michael Felt | d0eeb93 | 2019-06-14 00:34:46 +0200 | [diff] [blame] | 93 | .. availability:: Windows, FreeBSD, Linux, macOS, OpenBSD, NetBSD, AIX. |
Jake Tesler | b121f63 | 2019-05-22 08:43:17 -0700 | [diff] [blame] | 94 | |
| 95 | .. versionadded:: 3.8 |
| 96 | |
| 97 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 98 | .. function:: enumerate() |
| 99 | |
Benjamin Peterson | 672b803 | 2008-06-11 19:14:14 +0000 | [diff] [blame] | 100 | Return a list of all :class:`Thread` objects currently alive. The list |
| 101 | includes daemonic threads, dummy thread objects created by |
| 102 | :func:`current_thread`, and the main thread. It excludes terminated threads |
| 103 | and threads that have not yet been started. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 104 | |
| 105 | |
Andrew Svetlov | 58b5c5a | 2013-09-04 07:01:07 +0300 | [diff] [blame] | 106 | .. function:: main_thread() |
| 107 | |
| 108 | Return the main :class:`Thread` object. In normal conditions, the |
| 109 | main thread is the thread from which the Python interpreter was |
| 110 | started. |
| 111 | |
| 112 | .. versionadded:: 3.4 |
| 113 | |
| 114 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 115 | .. function:: settrace(func) |
| 116 | |
| 117 | .. index:: single: trace function |
| 118 | |
| 119 | Set a trace function for all threads started from the :mod:`threading` module. |
| 120 | The *func* will be passed to :func:`sys.settrace` for each thread, before its |
Serhiy Storchaka | 9e0ae53 | 2013-08-24 00:23:38 +0300 | [diff] [blame] | 121 | :meth:`~Thread.run` method is called. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 122 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 123 | |
| 124 | .. function:: setprofile(func) |
| 125 | |
| 126 | .. index:: single: profile function |
| 127 | |
| 128 | Set a profile function for all threads started from the :mod:`threading` module. |
| 129 | The *func* will be passed to :func:`sys.setprofile` for each thread, before its |
Serhiy Storchaka | 9e0ae53 | 2013-08-24 00:23:38 +0300 | [diff] [blame] | 130 | :meth:`~Thread.run` method is called. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 131 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 132 | |
| 133 | .. function:: stack_size([size]) |
| 134 | |
| 135 | Return the thread stack size used when creating new threads. The optional |
| 136 | *size* argument specifies the stack size to be used for subsequently created |
| 137 | threads, and must be 0 (use platform or configured default) or a positive |
Martin Panter | 31e7f50 | 2015-08-31 03:15:52 +0000 | [diff] [blame] | 138 | integer value of at least 32,768 (32 KiB). If *size* is not specified, |
| 139 | 0 is used. If changing the thread stack size is |
Georg Brandl | 9a13b43 | 2012-04-05 09:53:04 +0200 | [diff] [blame] | 140 | unsupported, a :exc:`RuntimeError` is raised. If the specified stack size is |
Serhiy Storchaka | f8def28 | 2013-02-16 17:29:56 +0200 | [diff] [blame] | 141 | invalid, a :exc:`ValueError` is raised and the stack size is unmodified. 32 KiB |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 142 | is currently the minimum supported stack size value to guarantee sufficient |
| 143 | stack space for the interpreter itself. Note that some platforms may have |
| 144 | particular restrictions on values for the stack size, such as requiring a |
Serhiy Storchaka | f8def28 | 2013-02-16 17:29:56 +0200 | [diff] [blame] | 145 | minimum stack size > 32 KiB or requiring allocation in multiples of the system |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 146 | memory page size - platform documentation should be referred to for more |
Serhiy Storchaka | f8def28 | 2013-02-16 17:29:56 +0200 | [diff] [blame] | 147 | information (4 KiB pages are common; using multiples of 4096 for the stack size is |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 148 | the suggested approach in the absence of more specific information). |
Cheryl Sabella | 2d6097d | 2018-10-12 10:55:20 -0400 | [diff] [blame] | 149 | |
| 150 | .. availability:: Windows, systems with POSIX threads. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 151 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 152 | |
Antoine Pitrou | 7c3e577 | 2010-04-14 15:44:10 +0000 | [diff] [blame] | 153 | This module also defines the following constant: |
| 154 | |
| 155 | .. data:: TIMEOUT_MAX |
| 156 | |
| 157 | The maximum value allowed for the *timeout* parameter of blocking functions |
| 158 | (:meth:`Lock.acquire`, :meth:`RLock.acquire`, :meth:`Condition.wait`, etc.). |
Georg Brandl | 6faee4e | 2010-09-21 14:48:28 +0000 | [diff] [blame] | 159 | Specifying a timeout greater than this value will raise an |
Antoine Pitrou | 7c3e577 | 2010-04-14 15:44:10 +0000 | [diff] [blame] | 160 | :exc:`OverflowError`. |
| 161 | |
Antoine Pitrou | adbc009 | 2010-04-19 14:05:51 +0000 | [diff] [blame] | 162 | .. versionadded:: 3.2 |
Antoine Pitrou | 7c3e577 | 2010-04-14 15:44:10 +0000 | [diff] [blame] | 163 | |
Georg Brandl | 67b21b7 | 2010-08-17 15:07:14 +0000 | [diff] [blame] | 164 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 165 | This module defines a number of classes, which are detailed in the sections |
| 166 | below. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 167 | |
| 168 | The design of this module is loosely based on Java's threading model. However, |
| 169 | where Java makes locks and condition variables basic behavior of every object, |
| 170 | they are separate objects in Python. Python's :class:`Thread` class supports a |
| 171 | subset of the behavior of Java's Thread class; currently, there are no |
| 172 | priorities, no thread groups, and threads cannot be destroyed, stopped, |
| 173 | suspended, resumed, or interrupted. The static methods of Java's Thread class, |
| 174 | when implemented, are mapped to module-level functions. |
| 175 | |
| 176 | All of the methods described below are executed atomically. |
| 177 | |
| 178 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 179 | Thread-Local Data |
| 180 | ----------------- |
| 181 | |
| 182 | Thread-local data is data whose values are thread specific. To manage |
| 183 | thread-local data, just create an instance of :class:`local` (or a |
| 184 | subclass) and store attributes on it:: |
| 185 | |
| 186 | mydata = threading.local() |
| 187 | mydata.x = 1 |
| 188 | |
| 189 | The instance's values will be different for separate threads. |
| 190 | |
| 191 | |
| 192 | .. class:: local() |
| 193 | |
| 194 | A class that represents thread-local data. |
| 195 | |
| 196 | For more details and extensive examples, see the documentation string of the |
| 197 | :mod:`_threading_local` module. |
| 198 | |
| 199 | |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 200 | .. _thread-objects: |
| 201 | |
| 202 | Thread Objects |
| 203 | -------------- |
| 204 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 205 | The :class:`Thread` class represents an activity that is run in a separate |
| 206 | thread of control. There are two ways to specify the activity: by passing a |
| 207 | callable object to the constructor, or by overriding the :meth:`~Thread.run` |
| 208 | method in a subclass. No other methods (except for the constructor) should be |
| 209 | overridden in a subclass. In other words, *only* override the |
| 210 | :meth:`~Thread.__init__` and :meth:`~Thread.run` methods of this class. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 211 | |
| 212 | Once a thread object is created, its activity must be started by calling the |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 213 | thread's :meth:`~Thread.start` method. This invokes the :meth:`~Thread.run` |
| 214 | method in a separate thread of control. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 215 | |
| 216 | Once the thread's activity is started, the thread is considered 'alive'. It |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 217 | stops being alive when its :meth:`~Thread.run` method terminates -- either |
| 218 | normally, or by raising an unhandled exception. The :meth:`~Thread.is_alive` |
| 219 | method tests whether the thread is alive. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 220 | |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 221 | Other threads can call a thread's :meth:`~Thread.join` method. This blocks |
| 222 | the calling thread until the thread whose :meth:`~Thread.join` method is |
| 223 | called is terminated. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 224 | |
| 225 | A thread has a name. The name can be passed to the constructor, and read or |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 226 | changed through the :attr:`~Thread.name` attribute. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 227 | |
Victor Stinner | cd590a7 | 2019-05-28 00:39:52 +0200 | [diff] [blame] | 228 | If the :meth:`~Thread.run` method raises an exception, |
| 229 | :func:`threading.excepthook` is called to handle it. By default, |
| 230 | :func:`threading.excepthook` ignores silently :exc:`SystemExit`. |
| 231 | |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 232 | A thread can be flagged as a "daemon thread". The significance of this flag is |
| 233 | that the entire Python program exits when only daemon threads are left. The |
| 234 | initial value is inherited from the creating thread. The flag can be set |
Antoine Pitrou | 61d85ba | 2012-04-10 22:51:26 +0200 | [diff] [blame] | 235 | through the :attr:`~Thread.daemon` property or the *daemon* constructor |
| 236 | argument. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 237 | |
Antoine Pitrou | 38b8254 | 2013-02-15 21:27:18 +0100 | [diff] [blame] | 238 | .. note:: |
| 239 | Daemon threads are abruptly stopped at shutdown. Their resources (such |
| 240 | as open files, database transactions, etc.) may not be released properly. |
| 241 | If you want your threads to stop gracefully, make them non-daemonic and |
| 242 | use a suitable signalling mechanism such as an :class:`Event`. |
| 243 | |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 244 | There is a "main thread" object; this corresponds to the initial thread of |
| 245 | control in the Python program. It is not a daemon thread. |
| 246 | |
| 247 | There is the possibility that "dummy thread objects" are created. These are |
| 248 | thread objects corresponding to "alien threads", which are threads of control |
| 249 | started outside the threading module, such as directly from C code. Dummy |
| 250 | thread objects have limited functionality; they are always considered alive and |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 251 | daemonic, and cannot be :meth:`~Thread.join`\ ed. They are never deleted, |
| 252 | since it is impossible to detect the termination of alien threads. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 253 | |
| 254 | |
Ezio Melotti | 8b61611 | 2012-09-08 20:49:18 +0300 | [diff] [blame] | 255 | .. class:: Thread(group=None, target=None, name=None, args=(), kwargs={}, *, \ |
| 256 | daemon=None) |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 257 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 258 | This constructor should always be called with keyword arguments. Arguments |
| 259 | are: |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 260 | |
| 261 | *group* should be ``None``; reserved for future extension when a |
| 262 | :class:`ThreadGroup` class is implemented. |
| 263 | |
| 264 | *target* is the callable object to be invoked by the :meth:`run` method. |
| 265 | Defaults to ``None``, meaning nothing is called. |
| 266 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 267 | *name* is the thread name. By default, a unique name is constructed of the |
| 268 | form "Thread-*N*" where *N* is a small decimal number. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 269 | |
| 270 | *args* is the argument tuple for the target invocation. Defaults to ``()``. |
| 271 | |
| 272 | *kwargs* is a dictionary of keyword arguments for the target invocation. |
| 273 | Defaults to ``{}``. |
| 274 | |
Antoine Pitrou | 0bd4deb | 2011-02-25 22:07:43 +0000 | [diff] [blame] | 275 | If not ``None``, *daemon* explicitly sets whether the thread is daemonic. |
| 276 | If ``None`` (the default), the daemonic property is inherited from the |
| 277 | current thread. |
| 278 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 279 | If the subclass overrides the constructor, it must make sure to invoke the |
| 280 | base class constructor (``Thread.__init__()``) before doing anything else to |
| 281 | the thread. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 282 | |
Victor Stinner | 066e5b1 | 2019-06-14 18:55:22 +0200 | [diff] [blame^] | 283 | Daemon threads must not be used in subinterpreters. |
| 284 | |
Antoine Pitrou | 0bd4deb | 2011-02-25 22:07:43 +0000 | [diff] [blame] | 285 | .. versionchanged:: 3.3 |
| 286 | Added the *daemon* argument. |
| 287 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 288 | .. method:: start() |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 289 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 290 | Start the thread's activity. |
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 | It must be called at most once per thread object. It arranges for the |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 293 | object's :meth:`~Thread.run` method to be invoked in a separate thread |
| 294 | of control. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 295 | |
Brian Curtin | bd0c897 | 2011-01-31 19:35:02 +0000 | [diff] [blame] | 296 | This method will raise a :exc:`RuntimeError` if called more than once |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 297 | on the same thread object. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 298 | |
Victor Stinner | 066e5b1 | 2019-06-14 18:55:22 +0200 | [diff] [blame^] | 299 | Raise a :exc:`RuntimeError` if the thread is a daemon thread and the |
| 300 | method is called from a subinterpreter. |
| 301 | |
| 302 | .. versionchanged:: 3.9 |
| 303 | In a subinterpreter, spawning a daemon thread now raises an exception. |
| 304 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 305 | .. method:: run() |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 306 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 307 | Method representing the thread's activity. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 308 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 309 | You may override this method in a subclass. The standard :meth:`run` |
| 310 | method invokes the callable object passed to the object's constructor as |
Mathieu Dupuy | 29d018a | 2019-04-23 15:01:09 +0200 | [diff] [blame] | 311 | the *target* argument, if any, with positional and keyword arguments taken |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 312 | from the *args* and *kwargs* arguments, respectively. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 313 | |
Georg Brandl | 7f01a13 | 2009-09-16 15:58:14 +0000 | [diff] [blame] | 314 | .. method:: join(timeout=None) |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 315 | |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 316 | Wait until the thread terminates. This blocks the calling thread until |
| 317 | the thread whose :meth:`~Thread.join` method is called terminates -- either |
Martin Panter | 972e04e | 2016-12-24 07:28:26 +0000 | [diff] [blame] | 318 | normally or through an unhandled exception -- or until the optional |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 319 | timeout occurs. |
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 present and not ``None``, it should be a |
| 322 | floating point number specifying a timeout for the operation in seconds |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 323 | (or fractions thereof). As :meth:`~Thread.join` always returns ``None``, |
| 324 | you must call :meth:`~Thread.is_alive` after :meth:`~Thread.join` to |
| 325 | decide whether a timeout happened -- if the thread is still alive, the |
| 326 | :meth:`~Thread.join` call timed out. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 327 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 328 | When the *timeout* argument is not present or ``None``, the operation will |
| 329 | block until the thread terminates. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 330 | |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 331 | A thread can be :meth:`~Thread.join`\ ed many times. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 332 | |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 333 | :meth:`~Thread.join` raises a :exc:`RuntimeError` if an attempt is made |
| 334 | to join the current thread as that would cause a deadlock. It is also |
| 335 | an error to :meth:`~Thread.join` a thread before it has been started |
| 336 | and attempts to do so raise the same exception. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 337 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 338 | .. attribute:: name |
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 | A string used for identification purposes only. It has no semantics. |
| 341 | Multiple threads may be given the same name. The initial name is set by |
| 342 | the constructor. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 343 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 344 | .. method:: getName() |
| 345 | setName() |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 346 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 347 | Old getter/setter API for :attr:`~Thread.name`; use it directly as a |
| 348 | property instead. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 349 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 350 | .. attribute:: ident |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 351 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 352 | The 'thread identifier' of this thread or ``None`` if the thread has not |
Benjamin Peterson | 236329e | 2017-09-26 23:13:15 -0700 | [diff] [blame] | 353 | been started. This is a nonzero integer. See the :func:`get_ident` |
| 354 | function. Thread identifiers may be recycled when a thread exits and |
| 355 | another thread is created. The identifier is available even after the |
| 356 | thread has exited. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 357 | |
Jake Tesler | b121f63 | 2019-05-22 08:43:17 -0700 | [diff] [blame] | 358 | .. attribute:: native_id |
| 359 | |
| 360 | The native integral thread ID of this thread. |
| 361 | This is a non-negative integer, or ``None`` if the thread has not |
| 362 | been started. See the :func:`get_native_id` function. |
| 363 | This represents the Thread ID (``TID``) as assigned to the |
| 364 | thread by the OS (kernel). Its value may be used to uniquely identify |
| 365 | this particular thread system-wide (until the thread terminates, |
| 366 | after which the value may be recycled by the OS). |
| 367 | |
| 368 | .. note:: |
| 369 | |
| 370 | Similar to Process IDs, Thread IDs are only valid (guaranteed unique |
| 371 | system-wide) from the time the thread is created until the thread |
| 372 | has been terminated. |
| 373 | |
David Carlier | 0b9956e | 2019-06-03 16:43:33 +0100 | [diff] [blame] | 374 | .. availability:: Require :func:`get_native_id` function. |
Jake Tesler | b121f63 | 2019-05-22 08:43:17 -0700 | [diff] [blame] | 375 | |
| 376 | .. versionadded:: 3.8 |
| 377 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 378 | .. method:: is_alive() |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 379 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 380 | Return whether the thread is alive. |
Georg Brandl | 770b0be | 2009-01-02 20:10:05 +0000 | [diff] [blame] | 381 | |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 382 | This method returns ``True`` just before the :meth:`~Thread.run` method |
| 383 | starts until just after the :meth:`~Thread.run` method terminates. The |
| 384 | module function :func:`.enumerate` returns a list of all alive threads. |
Georg Brandl | 770b0be | 2009-01-02 20:10:05 +0000 | [diff] [blame] | 385 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 386 | .. attribute:: daemon |
Georg Brandl | 770b0be | 2009-01-02 20:10:05 +0000 | [diff] [blame] | 387 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 388 | A boolean value indicating whether this thread is a daemon thread (True) |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 389 | or not (False). This must be set before :meth:`~Thread.start` is called, |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 390 | otherwise :exc:`RuntimeError` is raised. Its initial value is inherited |
| 391 | from the creating thread; the main thread is not a daemon thread and |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 392 | therefore all threads created in the main thread default to |
| 393 | :attr:`~Thread.daemon` = ``False``. |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 394 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 395 | 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] | 396 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 397 | .. method:: isDaemon() |
| 398 | setDaemon() |
Georg Brandl | a971c65 | 2008-11-07 09:39:56 +0000 | [diff] [blame] | 399 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 400 | Old getter/setter API for :attr:`~Thread.daemon`; use it directly as a |
| 401 | property instead. |
Georg Brandl | 770b0be | 2009-01-02 20:10:05 +0000 | [diff] [blame] | 402 | |
| 403 | |
Antoine Pitrou | d6d17c5 | 2011-02-28 22:04:51 +0000 | [diff] [blame] | 404 | .. impl-detail:: |
| 405 | |
Ezio Melotti | 6d043fc | 2013-01-18 19:58:47 +0200 | [diff] [blame] | 406 | In CPython, due to the :term:`Global Interpreter Lock`, only one thread |
Antoine Pitrou | d6d17c5 | 2011-02-28 22:04:51 +0000 | [diff] [blame] | 407 | can execute Python code at once (even though certain performance-oriented |
| 408 | libraries might overcome this limitation). |
Ezio Melotti | 6d043fc | 2013-01-18 19:58:47 +0200 | [diff] [blame] | 409 | If you want your application to make better use of the computational |
Antoine Pitrou | d6d17c5 | 2011-02-28 22:04:51 +0000 | [diff] [blame] | 410 | resources of multi-core machines, you are advised to use |
| 411 | :mod:`multiprocessing` or :class:`concurrent.futures.ProcessPoolExecutor`. |
| 412 | However, threading is still an appropriate model if you want to run |
| 413 | multiple I/O-bound tasks simultaneously. |
| 414 | |
| 415 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 416 | .. _lock-objects: |
| 417 | |
| 418 | Lock Objects |
| 419 | ------------ |
| 420 | |
| 421 | A primitive lock is a synchronization primitive that is not owned by a |
| 422 | particular thread when locked. In Python, it is currently the lowest level |
Georg Brandl | 2067bfd | 2008-05-25 13:05:15 +0000 | [diff] [blame] | 423 | synchronization primitive available, implemented directly by the :mod:`_thread` |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 424 | extension module. |
| 425 | |
| 426 | A primitive lock is in one of two states, "locked" or "unlocked". It is created |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 427 | in the unlocked state. It has two basic methods, :meth:`~Lock.acquire` and |
| 428 | :meth:`~Lock.release`. When the state is unlocked, :meth:`~Lock.acquire` |
| 429 | changes the state to locked and returns immediately. When the state is locked, |
| 430 | :meth:`~Lock.acquire` blocks until a call to :meth:`~Lock.release` in another |
| 431 | thread changes it to unlocked, then the :meth:`~Lock.acquire` call resets it |
| 432 | to locked and returns. The :meth:`~Lock.release` method should only be |
| 433 | called in the locked state; it changes the state to unlocked and returns |
| 434 | immediately. If an attempt is made to release an unlocked lock, a |
| 435 | :exc:`RuntimeError` will be raised. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 436 | |
Serhiy Storchaka | 1486799 | 2014-09-10 23:43:41 +0300 | [diff] [blame] | 437 | Locks also support the :ref:`context management protocol <with-locks>`. |
Antoine Pitrou | b96a354 | 2012-04-10 22:47:55 +0200 | [diff] [blame] | 438 | |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 439 | When more than one thread is blocked in :meth:`~Lock.acquire` waiting for the |
| 440 | state to turn to unlocked, only one thread proceeds when a :meth:`~Lock.release` |
| 441 | call resets the state to unlocked; which one of the waiting threads proceeds |
| 442 | is not defined, and may vary across implementations. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 443 | |
| 444 | All methods are executed atomically. |
| 445 | |
| 446 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 447 | .. class:: Lock() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 448 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 449 | The class implementing primitive lock objects. Once a thread has acquired a |
| 450 | lock, subsequent attempts to acquire it block, until it is released; any |
| 451 | thread may release it. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 452 | |
csabella | 56ddfd2 | 2017-05-31 20:14:19 -0400 | [diff] [blame] | 453 | Note that ``Lock`` is actually a factory function which returns an instance |
| 454 | of the most efficient version of the concrete Lock class that is supported |
| 455 | by the platform. |
Antoine Pitrou | 810023d | 2010-12-15 22:59:16 +0000 | [diff] [blame] | 456 | |
Georg Brandl | 67b21b7 | 2010-08-17 15:07:14 +0000 | [diff] [blame] | 457 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 458 | .. method:: acquire(blocking=True, timeout=-1) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 459 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 460 | Acquire a lock, blocking or non-blocking. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 461 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 462 | When invoked with the *blocking* argument set to ``True`` (the default), |
| 463 | block until the lock is unlocked, then set it to locked and return ``True``. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 464 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 465 | When invoked with the *blocking* argument set to ``False``, do not block. |
| 466 | If a call with *blocking* set to ``True`` would block, return ``False`` |
| 467 | immediately; otherwise, set the lock to locked and return ``True``. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 468 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 469 | When invoked with the floating-point *timeout* argument set to a positive |
| 470 | value, block for at most the number of seconds specified by *timeout* |
Georg Brandl | b19ef18 | 2013-10-06 10:48:08 +0200 | [diff] [blame] | 471 | and as long as the lock cannot be acquired. A *timeout* argument of ``-1`` |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 472 | specifies an unbounded wait. It is forbidden to specify a *timeout* |
| 473 | when *blocking* is false. |
| 474 | |
| 475 | The return value is ``True`` if the lock is acquired successfully, |
| 476 | ``False`` if not (for example if the *timeout* expired). |
| 477 | |
| 478 | .. versionchanged:: 3.2 |
| 479 | The *timeout* parameter is new. |
| 480 | |
| 481 | .. versionchanged:: 3.2 |
Benjamin Peterson | 5b10d51 | 2018-09-12 13:48:03 -0700 | [diff] [blame] | 482 | Lock acquisition can now be interrupted by signals on POSIX if the |
| 483 | underlying threading implementation supports it. |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 484 | |
| 485 | |
| 486 | .. method:: release() |
| 487 | |
| 488 | Release a lock. This can be called from any thread, not only the thread |
| 489 | which has acquired the lock. |
| 490 | |
| 491 | When the lock is locked, reset it to unlocked, and return. If any other threads |
| 492 | are blocked waiting for the lock to become unlocked, allow exactly one of them |
| 493 | to proceed. |
| 494 | |
| 495 | When invoked on an unlocked lock, a :exc:`RuntimeError` is raised. |
| 496 | |
| 497 | There is no return value. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 498 | |
| 499 | |
| 500 | .. _rlock-objects: |
| 501 | |
| 502 | RLock Objects |
| 503 | ------------- |
| 504 | |
| 505 | A reentrant lock is a synchronization primitive that may be acquired multiple |
| 506 | times by the same thread. Internally, it uses the concepts of "owning thread" |
| 507 | and "recursion level" in addition to the locked/unlocked state used by primitive |
| 508 | locks. In the locked state, some thread owns the lock; in the unlocked state, |
| 509 | no thread owns it. |
| 510 | |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 511 | To lock the lock, a thread calls its :meth:`~RLock.acquire` method; this |
| 512 | returns once the thread owns the lock. To unlock the lock, a thread calls |
| 513 | its :meth:`~Lock.release` method. :meth:`~Lock.acquire`/:meth:`~Lock.release` |
| 514 | call pairs may be nested; only the final :meth:`~Lock.release` (the |
| 515 | :meth:`~Lock.release` of the outermost pair) resets the lock to unlocked and |
| 516 | allows another thread blocked in :meth:`~Lock.acquire` to proceed. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 517 | |
Serhiy Storchaka | 1486799 | 2014-09-10 23:43:41 +0300 | [diff] [blame] | 518 | Reentrant locks also support the :ref:`context management protocol <with-locks>`. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 519 | |
| 520 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 521 | .. class:: RLock() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 522 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 523 | This class implements reentrant lock objects. A reentrant lock must be |
| 524 | released by the thread that acquired it. Once a thread has acquired a |
| 525 | reentrant lock, the same thread may acquire it again without blocking; the |
| 526 | thread must release it once for each time it has acquired it. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 527 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 528 | Note that ``RLock`` is actually a factory function which returns an instance |
| 529 | of the most efficient version of the concrete RLock class that is supported |
| 530 | by the platform. |
Antoine Pitrou | adbc009 | 2010-04-19 14:05:51 +0000 | [diff] [blame] | 531 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 532 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 533 | .. method:: acquire(blocking=True, timeout=-1) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 534 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 535 | Acquire a lock, blocking or non-blocking. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 536 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 537 | When invoked without arguments: if this thread already owns the lock, increment |
| 538 | the recursion level by one, and return immediately. Otherwise, if another |
| 539 | thread owns the lock, block until the lock is unlocked. Once the lock is |
| 540 | unlocked (not owned by any thread), then grab ownership, set the recursion level |
| 541 | to one, and return. If more than one thread is blocked waiting until the lock |
| 542 | is unlocked, only one at a time will be able to grab ownership of the lock. |
| 543 | There is no return value in this case. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 544 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 545 | When invoked with the *blocking* argument set to true, do the same thing as when |
| 546 | called without arguments, and return true. |
| 547 | |
| 548 | When invoked with the *blocking* argument set to false, do not block. If a call |
| 549 | without an argument would block, return false immediately; otherwise, do the |
| 550 | same thing as when called without arguments, and return true. |
| 551 | |
| 552 | When invoked with the floating-point *timeout* argument set to a positive |
| 553 | value, block for at most the number of seconds specified by *timeout* |
| 554 | and as long as the lock cannot be acquired. Return true if the lock has |
| 555 | been acquired, false if the timeout has elapsed. |
| 556 | |
| 557 | .. versionchanged:: 3.2 |
| 558 | The *timeout* parameter is new. |
| 559 | |
| 560 | |
| 561 | .. method:: release() |
| 562 | |
| 563 | Release a lock, decrementing the recursion level. If after the decrement it is |
| 564 | zero, reset the lock to unlocked (not owned by any thread), and if any other |
| 565 | threads are blocked waiting for the lock to become unlocked, allow exactly one |
| 566 | of them to proceed. If after the decrement the recursion level is still |
| 567 | nonzero, the lock remains locked and owned by the calling thread. |
| 568 | |
| 569 | Only call this method when the calling thread owns the lock. A |
| 570 | :exc:`RuntimeError` is raised if this method is called when the lock is |
| 571 | unlocked. |
| 572 | |
| 573 | There is no return value. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 574 | |
| 575 | |
| 576 | .. _condition-objects: |
| 577 | |
| 578 | Condition Objects |
| 579 | ----------------- |
| 580 | |
| 581 | A condition variable is always associated with some kind of lock; this can be |
Antoine Pitrou | 126aef7 | 2012-04-10 22:24:05 +0200 | [diff] [blame] | 582 | passed in or one will be created by default. Passing one in is useful when |
| 583 | several condition variables must share the same lock. The lock is part of |
| 584 | the condition object: you don't have to track it separately. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 585 | |
Serhiy Storchaka | 1486799 | 2014-09-10 23:43:41 +0300 | [diff] [blame] | 586 | A condition variable obeys the :ref:`context management protocol <with-locks>`: |
Antoine Pitrou | b96a354 | 2012-04-10 22:47:55 +0200 | [diff] [blame] | 587 | using the ``with`` statement acquires the associated lock for the duration of |
| 588 | the enclosed block. The :meth:`~Condition.acquire` and |
| 589 | :meth:`~Condition.release` methods also call the corresponding methods of |
| 590 | the associated lock. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 591 | |
Antoine Pitrou | 126aef7 | 2012-04-10 22:24:05 +0200 | [diff] [blame] | 592 | Other methods must be called with the associated lock held. The |
| 593 | :meth:`~Condition.wait` method releases the lock, and then blocks until |
| 594 | another thread awakens it by calling :meth:`~Condition.notify` or |
| 595 | :meth:`~Condition.notify_all`. Once awakened, :meth:`~Condition.wait` |
| 596 | re-acquires the lock and returns. It is also possible to specify a timeout. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 597 | |
Antoine Pitrou | 126aef7 | 2012-04-10 22:24:05 +0200 | [diff] [blame] | 598 | The :meth:`~Condition.notify` method wakes up one of the threads waiting for |
| 599 | the condition variable, if any are waiting. The :meth:`~Condition.notify_all` |
| 600 | method wakes up all threads waiting for the condition variable. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 601 | |
Antoine Pitrou | 126aef7 | 2012-04-10 22:24:05 +0200 | [diff] [blame] | 602 | Note: the :meth:`~Condition.notify` and :meth:`~Condition.notify_all` methods |
| 603 | don't release the lock; this means that the thread or threads awakened will |
| 604 | not return from their :meth:`~Condition.wait` call immediately, but only when |
| 605 | the thread that called :meth:`~Condition.notify` or :meth:`~Condition.notify_all` |
| 606 | finally relinquishes ownership of the lock. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 607 | |
Antoine Pitrou | 126aef7 | 2012-04-10 22:24:05 +0200 | [diff] [blame] | 608 | The typical programming style using condition variables uses the lock to |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 609 | synchronize access to some shared state; threads that are interested in a |
Antoine Pitrou | 126aef7 | 2012-04-10 22:24:05 +0200 | [diff] [blame] | 610 | particular change of state call :meth:`~Condition.wait` repeatedly until they |
| 611 | see the desired state, while threads that modify the state call |
| 612 | :meth:`~Condition.notify` or :meth:`~Condition.notify_all` when they change |
| 613 | the state in such a way that it could possibly be a desired state for one |
| 614 | of the waiters. For example, the following code is a generic |
| 615 | producer-consumer situation with unlimited buffer capacity:: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 616 | |
| 617 | # Consume one item |
Antoine Pitrou | 126aef7 | 2012-04-10 22:24:05 +0200 | [diff] [blame] | 618 | with cv: |
| 619 | while not an_item_is_available(): |
| 620 | cv.wait() |
| 621 | get_an_available_item() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 622 | |
| 623 | # Produce one item |
Antoine Pitrou | 126aef7 | 2012-04-10 22:24:05 +0200 | [diff] [blame] | 624 | with cv: |
| 625 | make_an_item_available() |
Antoine Pitrou | f6cd9b2 | 2012-04-11 19:37:56 +0200 | [diff] [blame] | 626 | cv.notify() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 627 | |
Antoine Pitrou | 126aef7 | 2012-04-10 22:24:05 +0200 | [diff] [blame] | 628 | The ``while`` loop checking for the application's condition is necessary |
| 629 | because :meth:`~Condition.wait` can return after an arbitrary long time, |
Antoine Pitrou | f6cd9b2 | 2012-04-11 19:37:56 +0200 | [diff] [blame] | 630 | and the condition which prompted the :meth:`~Condition.notify` call may |
| 631 | no longer hold true. This is inherent to multi-threaded programming. The |
| 632 | :meth:`~Condition.wait_for` method can be used to automate the condition |
| 633 | checking, and eases the computation of timeouts:: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 634 | |
Antoine Pitrou | 126aef7 | 2012-04-10 22:24:05 +0200 | [diff] [blame] | 635 | # Consume an item |
| 636 | with cv: |
| 637 | cv.wait_for(an_item_is_available) |
| 638 | get_an_available_item() |
Kristján Valur Jónsson | 6331520 | 2010-11-18 12:46:39 +0000 | [diff] [blame] | 639 | |
Antoine Pitrou | 126aef7 | 2012-04-10 22:24:05 +0200 | [diff] [blame] | 640 | To choose between :meth:`~Condition.notify` and :meth:`~Condition.notify_all`, |
| 641 | consider whether one state change can be interesting for only one or several |
| 642 | waiting threads. E.g. in a typical producer-consumer situation, adding one |
| 643 | item to the buffer only needs to wake up one consumer thread. |
| 644 | |
| 645 | |
Georg Brandl | 7f01a13 | 2009-09-16 15:58:14 +0000 | [diff] [blame] | 646 | .. class:: Condition(lock=None) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 647 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 648 | This class implements condition variable objects. A condition variable |
| 649 | allows one or more threads to wait until they are notified by another thread. |
| 650 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 651 | If the *lock* argument is given and not ``None``, it must be a :class:`Lock` |
| 652 | or :class:`RLock` object, and it is used as the underlying lock. Otherwise, |
| 653 | 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] | 654 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 655 | .. versionchanged:: 3.3 |
| 656 | changed from a factory function to a class. |
| 657 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 658 | .. method:: acquire(*args) |
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 | Acquire the underlying lock. This method calls the corresponding method on |
| 661 | the underlying lock; the return value is whatever that method returns. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 662 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 663 | .. method:: release() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 664 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 665 | Release the underlying lock. This method calls the corresponding method on |
| 666 | the underlying lock; there is no return value. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 667 | |
Georg Brandl | 7f01a13 | 2009-09-16 15:58:14 +0000 | [diff] [blame] | 668 | .. method:: wait(timeout=None) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 669 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 670 | Wait until notified or until a timeout occurs. If the calling thread has |
| 671 | not acquired the lock when this method is called, a :exc:`RuntimeError` is |
| 672 | raised. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 673 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 674 | This method releases the underlying lock, and then blocks until it is |
| 675 | awakened by a :meth:`notify` or :meth:`notify_all` call for the same |
| 676 | condition variable in another thread, or until the optional timeout |
| 677 | 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] | 678 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 679 | When the *timeout* argument is present and not ``None``, it should be a |
| 680 | floating point number specifying a timeout for the operation in seconds |
| 681 | (or fractions thereof). |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 682 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 683 | When the underlying lock is an :class:`RLock`, it is not released using |
| 684 | its :meth:`release` method, since this may not actually unlock the lock |
| 685 | when it was acquired multiple times recursively. Instead, an internal |
| 686 | interface of the :class:`RLock` class is used, which really unlocks it |
| 687 | even when it has been recursively acquired several times. Another internal |
| 688 | interface is then used to restore the recursion level when the lock is |
| 689 | reacquired. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 690 | |
Georg Brandl | b9a4391 | 2010-10-28 09:03:20 +0000 | [diff] [blame] | 691 | The return value is ``True`` unless a given *timeout* expired, in which |
| 692 | case it is ``False``. |
| 693 | |
| 694 | .. versionchanged:: 3.2 |
| 695 | Previously, the method always returned ``None``. |
| 696 | |
Kristján Valur Jónsson | 6331520 | 2010-11-18 12:46:39 +0000 | [diff] [blame] | 697 | .. method:: wait_for(predicate, timeout=None) |
| 698 | |
Serhiy Storchaka | 4adf01c | 2016-10-19 18:30:05 +0300 | [diff] [blame] | 699 | Wait until a condition evaluates to true. *predicate* should be a |
Kristján Valur Jónsson | 6331520 | 2010-11-18 12:46:39 +0000 | [diff] [blame] | 700 | callable which result will be interpreted as a boolean value. |
| 701 | A *timeout* may be provided giving the maximum time to wait. |
| 702 | |
| 703 | This utility method may call :meth:`wait` repeatedly until the predicate |
| 704 | is satisfied, or until a timeout occurs. The return value is |
| 705 | the last return value of the predicate and will evaluate to |
| 706 | ``False`` if the method timed out. |
| 707 | |
| 708 | Ignoring the timeout feature, calling this method is roughly equivalent to |
| 709 | writing:: |
| 710 | |
| 711 | while not predicate(): |
| 712 | cv.wait() |
| 713 | |
| 714 | Therefore, the same rules apply as with :meth:`wait`: The lock must be |
Senthil Kumaran | b4760ef | 2015-06-14 17:35:37 -0700 | [diff] [blame] | 715 | held when called and is re-acquired on return. The predicate is evaluated |
Kristján Valur Jónsson | 6331520 | 2010-11-18 12:46:39 +0000 | [diff] [blame] | 716 | with the lock held. |
| 717 | |
Kristján Valur Jónsson | 6331520 | 2010-11-18 12:46:39 +0000 | [diff] [blame] | 718 | .. versionadded:: 3.2 |
| 719 | |
Eli Bendersky | d44af82 | 2011-11-12 20:44:25 +0200 | [diff] [blame] | 720 | .. method:: notify(n=1) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 721 | |
Eli Bendersky | d44af82 | 2011-11-12 20:44:25 +0200 | [diff] [blame] | 722 | By default, wake up one thread waiting on this condition, if any. If the |
| 723 | calling thread has not acquired the lock when this method is called, a |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 724 | :exc:`RuntimeError` is raised. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 725 | |
Eli Bendersky | d44af82 | 2011-11-12 20:44:25 +0200 | [diff] [blame] | 726 | This method wakes up at most *n* of the threads waiting for the condition |
| 727 | variable; it is a no-op if no threads are waiting. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 728 | |
Eli Bendersky | d44af82 | 2011-11-12 20:44:25 +0200 | [diff] [blame] | 729 | The current implementation wakes up exactly *n* threads, if at least *n* |
| 730 | threads are waiting. However, it's not safe to rely on this behavior. |
| 731 | A future, optimized implementation may occasionally wake up more than |
| 732 | *n* threads. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 733 | |
Eli Bendersky | d44af82 | 2011-11-12 20:44:25 +0200 | [diff] [blame] | 734 | Note: an awakened thread does not actually return from its :meth:`wait` |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 735 | call until it can reacquire the lock. Since :meth:`notify` does not |
| 736 | release the lock, its caller should. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 737 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 738 | .. method:: notify_all() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 739 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 740 | Wake up all threads waiting on this condition. This method acts like |
| 741 | :meth:`notify`, but wakes up all waiting threads instead of one. If the |
| 742 | calling thread has not acquired the lock when this method is called, a |
| 743 | :exc:`RuntimeError` is raised. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 744 | |
| 745 | |
| 746 | .. _semaphore-objects: |
| 747 | |
| 748 | Semaphore Objects |
| 749 | ----------------- |
| 750 | |
| 751 | This is one of the oldest synchronization primitives in the history of computer |
| 752 | science, invented by the early Dutch computer scientist Edsger W. Dijkstra (he |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 753 | used the names ``P()`` and ``V()`` instead of :meth:`~Semaphore.acquire` and |
| 754 | :meth:`~Semaphore.release`). |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 755 | |
| 756 | A semaphore manages an internal counter which is decremented by each |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 757 | :meth:`~Semaphore.acquire` call and incremented by each :meth:`~Semaphore.release` |
| 758 | call. The counter can never go below zero; when :meth:`~Semaphore.acquire` |
| 759 | finds that it is zero, it blocks, waiting until some other thread calls |
| 760 | :meth:`~Semaphore.release`. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 761 | |
Serhiy Storchaka | 1486799 | 2014-09-10 23:43:41 +0300 | [diff] [blame] | 762 | Semaphores also support the :ref:`context management protocol <with-locks>`. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 763 | |
| 764 | |
Georg Brandl | 7f01a13 | 2009-09-16 15:58:14 +0000 | [diff] [blame] | 765 | .. class:: Semaphore(value=1) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 766 | |
Garrett Berg | a0374dd | 2017-12-07 11:04:26 -0700 | [diff] [blame] | 767 | This class implements semaphore objects. A semaphore manages an atomic |
| 768 | counter representing the number of :meth:`release` calls minus the number of |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 769 | :meth:`acquire` calls, plus an initial value. The :meth:`acquire` method |
| 770 | blocks if necessary until it can return without making the counter negative. |
| 771 | If not given, *value* defaults to 1. |
| 772 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 773 | The optional argument gives the initial *value* for the internal counter; it |
| 774 | defaults to ``1``. If the *value* given is less than 0, :exc:`ValueError` is |
| 775 | raised. |
| 776 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 777 | .. versionchanged:: 3.3 |
| 778 | changed from a factory function to a class. |
| 779 | |
Antoine Pitrou | 0454af9 | 2010-04-17 23:51:58 +0000 | [diff] [blame] | 780 | .. method:: acquire(blocking=True, timeout=None) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 781 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 782 | Acquire a semaphore. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 783 | |
Garrett Berg | a0374dd | 2017-12-07 11:04:26 -0700 | [diff] [blame] | 784 | When invoked without arguments: |
| 785 | |
| 786 | * If the internal counter is larger than zero on entry, decrement it by |
| 787 | one and return true immediately. |
| 788 | * If the internal counter is zero on entry, block until awoken by a call to |
| 789 | :meth:`~Semaphore.release`. Once awoken (and the counter is greater |
| 790 | than 0), decrement the counter by 1 and return true. Exactly one |
| 791 | thread will be awoken by each call to :meth:`~Semaphore.release`. The |
| 792 | order in which threads are awoken should not be relied on. |
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 | When invoked with *blocking* set to false, do not block. If a call |
Garrett Berg | a0374dd | 2017-12-07 11:04:26 -0700 | [diff] [blame] | 795 | without an argument would block, return false immediately; otherwise, do |
| 796 | the same thing as when called without arguments, and return true. |
Antoine Pitrou | 0454af9 | 2010-04-17 23:51:58 +0000 | [diff] [blame] | 797 | |
Serhiy Storchaka | ecf41da | 2016-10-19 16:29:26 +0300 | [diff] [blame] | 798 | When invoked with a *timeout* other than ``None``, it will block for at |
Antoine Pitrou | 0454af9 | 2010-04-17 23:51:58 +0000 | [diff] [blame] | 799 | most *timeout* seconds. If acquire does not complete successfully in |
| 800 | that interval, return false. Return true otherwise. |
| 801 | |
| 802 | .. versionchanged:: 3.2 |
| 803 | The *timeout* parameter is new. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 804 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 805 | .. method:: release() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 806 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 807 | Release a semaphore, incrementing the internal counter by one. When it |
| 808 | was zero on entry and another thread is waiting for it to become larger |
| 809 | than zero again, wake up that thread. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 810 | |
| 811 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 812 | .. class:: BoundedSemaphore(value=1) |
| 813 | |
| 814 | Class implementing bounded semaphore objects. A bounded semaphore checks to |
| 815 | make sure its current value doesn't exceed its initial value. If it does, |
| 816 | :exc:`ValueError` is raised. In most situations semaphores are used to guard |
| 817 | resources with limited capacity. If the semaphore is released too many times |
| 818 | it's a sign of a bug. If not given, *value* defaults to 1. |
| 819 | |
| 820 | .. versionchanged:: 3.3 |
| 821 | changed from a factory function to a class. |
| 822 | |
| 823 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 824 | .. _semaphore-examples: |
| 825 | |
| 826 | :class:`Semaphore` Example |
| 827 | ^^^^^^^^^^^^^^^^^^^^^^^^^^ |
| 828 | |
| 829 | Semaphores are often used to guard resources with limited capacity, for example, |
Georg Brandl | a572476 | 2011-01-06 19:28:18 +0000 | [diff] [blame] | 830 | a database server. In any situation where the size of the resource is fixed, |
| 831 | you should use a bounded semaphore. Before spawning any worker threads, your |
| 832 | main thread would initialize the semaphore:: |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 833 | |
| 834 | maxconnections = 5 |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 835 | # ... |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 836 | pool_sema = BoundedSemaphore(value=maxconnections) |
| 837 | |
| 838 | Once spawned, worker threads call the semaphore's acquire and release methods |
| 839 | when they need to connect to the server:: |
| 840 | |
Antoine Pitrou | b96a354 | 2012-04-10 22:47:55 +0200 | [diff] [blame] | 841 | with pool_sema: |
| 842 | conn = connectdb() |
| 843 | try: |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 844 | # ... use connection ... |
Antoine Pitrou | b96a354 | 2012-04-10 22:47:55 +0200 | [diff] [blame] | 845 | finally: |
| 846 | conn.close() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 847 | |
| 848 | The use of a bounded semaphore reduces the chance that a programming error which |
| 849 | causes the semaphore to be released more than it's acquired will go undetected. |
| 850 | |
| 851 | |
| 852 | .. _event-objects: |
| 853 | |
| 854 | Event Objects |
| 855 | ------------- |
| 856 | |
| 857 | This is one of the simplest mechanisms for communication between threads: one |
| 858 | thread signals an event and other threads wait for it. |
| 859 | |
| 860 | An event object manages an internal flag that can be set to true with the |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 861 | :meth:`~Event.set` method and reset to false with the :meth:`~Event.clear` |
| 862 | method. The :meth:`~Event.wait` method blocks until the flag is true. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 863 | |
| 864 | |
| 865 | .. class:: Event() |
| 866 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 867 | Class implementing event objects. An event manages a flag that can be set to |
| 868 | true with the :meth:`~Event.set` method and reset to false with the |
| 869 | :meth:`clear` method. The :meth:`wait` method blocks until the flag is true. |
| 870 | The flag is initially false. |
| 871 | |
| 872 | .. versionchanged:: 3.3 |
| 873 | changed from a factory function to a class. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 874 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 875 | .. method:: is_set() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 876 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 877 | Return true if and only if the internal flag is true. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 878 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 879 | .. method:: set() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 880 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 881 | Set the internal flag to true. All threads waiting for it to become true |
| 882 | are awakened. Threads that call :meth:`wait` once the flag is true will |
| 883 | not block at all. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 884 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 885 | .. method:: clear() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 886 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 887 | Reset the internal flag to false. Subsequently, threads calling |
Georg Brandl | 502d9a5 | 2009-07-26 15:02:41 +0000 | [diff] [blame] | 888 | :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] | 889 | flag to true again. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 890 | |
Georg Brandl | 7f01a13 | 2009-09-16 15:58:14 +0000 | [diff] [blame] | 891 | .. method:: wait(timeout=None) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 892 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 893 | Block until the internal flag is true. If the internal flag is true on |
| 894 | entry, return immediately. Otherwise, block until another thread calls |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 895 | :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] | 896 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 897 | When the timeout argument is present and not ``None``, it should be a |
| 898 | floating point number specifying a timeout for the operation in seconds |
| 899 | (or fractions thereof). |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 900 | |
Charles-François Natali | ded0348 | 2012-01-07 18:24:56 +0100 | [diff] [blame] | 901 | This method returns true if and only if the internal flag has been set to |
| 902 | true, either before the wait call or after the wait starts, so it will |
| 903 | always return ``True`` except if a timeout is given and the operation |
| 904 | times out. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 905 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 906 | .. versionchanged:: 3.1 |
| 907 | Previously, the method always returned ``None``. |
Benjamin Peterson | d23f822 | 2009-04-05 19:13:16 +0000 | [diff] [blame] | 908 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 909 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 910 | .. _timer-objects: |
| 911 | |
| 912 | Timer Objects |
| 913 | ------------- |
| 914 | |
| 915 | This class represents an action that should be run only after a certain amount |
| 916 | of time has passed --- a timer. :class:`Timer` is a subclass of :class:`Thread` |
| 917 | and as such also functions as an example of creating custom threads. |
| 918 | |
Serhiy Storchaka | 9e0ae53 | 2013-08-24 00:23:38 +0300 | [diff] [blame] | 919 | Timers are started, as with threads, by calling their :meth:`~Timer.start` |
| 920 | method. The timer can be stopped (before its action has begun) by calling the |
| 921 | :meth:`~Timer.cancel` method. The interval the timer will wait before |
| 922 | executing its action may not be exactly the same as the interval specified by |
| 923 | the user. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 924 | |
| 925 | For example:: |
| 926 | |
| 927 | def hello(): |
Collin Winter | c79461b | 2007-09-01 23:34:30 +0000 | [diff] [blame] | 928 | print("hello, world") |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 929 | |
| 930 | t = Timer(30.0, hello) |
Serhiy Storchaka | dba9039 | 2016-05-10 12:01:23 +0300 | [diff] [blame] | 931 | t.start() # after 30 seconds, "hello, world" will be printed |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 932 | |
| 933 | |
R David Murray | 19aeb43 | 2013-03-30 17:19:38 -0400 | [diff] [blame] | 934 | .. class:: Timer(interval, function, args=None, kwargs=None) |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 935 | |
| 936 | Create a timer that will run *function* with arguments *args* and keyword |
| 937 | arguments *kwargs*, after *interval* seconds have passed. |
Serhiy Storchaka | ecf41da | 2016-10-19 16:29:26 +0300 | [diff] [blame] | 938 | If *args* is ``None`` (the default) then an empty list will be used. |
| 939 | If *kwargs* is ``None`` (the default) then an empty dict will be used. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 940 | |
R David Murray | ef4d286 | 2012-10-06 14:35:35 -0400 | [diff] [blame] | 941 | .. versionchanged:: 3.3 |
| 942 | changed from a factory function to a class. |
| 943 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 944 | .. method:: cancel() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 945 | |
Georg Brandl | 7a72b3a | 2009-07-26 14:48:09 +0000 | [diff] [blame] | 946 | Stop the timer, and cancel the execution of the timer's action. This will |
| 947 | only work if the timer is still in its waiting stage. |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 948 | |
| 949 | |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 950 | Barrier Objects |
| 951 | --------------- |
| 952 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 953 | .. versionadded:: 3.2 |
| 954 | |
| 955 | This class provides a simple synchronization primitive for use by a fixed number |
| 956 | of threads that need to wait for each other. Each of the threads tries to pass |
Antoine Pitrou | 2c9f104 | 2012-04-10 22:35:53 +0200 | [diff] [blame] | 957 | the barrier by calling the :meth:`~Barrier.wait` method and will block until |
Saurabh Chaturvedi | 143be36 | 2017-08-15 00:24:53 +0530 | [diff] [blame] | 958 | all of the threads have made their :meth:`~Barrier.wait` calls. At this point, |
| 959 | the threads are released simultaneously. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 960 | |
| 961 | The barrier can be reused any number of times for the same number of threads. |
| 962 | |
| 963 | As an example, here is a simple way to synchronize a client and server thread:: |
| 964 | |
| 965 | b = Barrier(2, timeout=5) |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 966 | |
| 967 | def server(): |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 968 | start_server() |
| 969 | b.wait() |
| 970 | while True: |
| 971 | connection = accept_connection() |
| 972 | process_server_connection(connection) |
| 973 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 974 | def client(): |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 975 | b.wait() |
| 976 | while True: |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 977 | connection = make_connection() |
| 978 | process_client_connection(connection) |
| 979 | |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 980 | |
| 981 | .. class:: Barrier(parties, action=None, timeout=None) |
| 982 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 983 | Create a barrier object for *parties* number of threads. An *action*, when |
| 984 | provided, is a callable to be called by one of the threads when they are |
| 985 | released. *timeout* is the default timeout value if none is specified for |
| 986 | the :meth:`wait` method. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 987 | |
| 988 | .. method:: wait(timeout=None) |
| 989 | |
| 990 | 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] | 991 | this function, they are all released simultaneously. If a *timeout* is |
Ezio Melotti | e130a52 | 2011-10-19 10:58:56 +0300 | [diff] [blame] | 992 | provided, it is used in preference to any that was supplied to the class |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 993 | constructor. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 994 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 995 | 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] | 996 | 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] | 997 | housekeeping, e.g.:: |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 998 | |
| 999 | i = barrier.wait() |
| 1000 | if i == 0: |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 1001 | # Only one thread needs to print this |
| 1002 | print("passed the barrier") |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 1003 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 1004 | If an *action* was provided to the constructor, one of the threads will |
| 1005 | have called it prior to being released. Should this call raise an error, |
| 1006 | the barrier is put into the broken state. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 1007 | |
| 1008 | If the call times out, the barrier is put into the broken state. |
| 1009 | |
| 1010 | This method may raise a :class:`BrokenBarrierError` exception if the |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 1011 | barrier is broken or reset while a thread is waiting. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 1012 | |
| 1013 | .. method:: reset() |
| 1014 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 1015 | Return the barrier to the default, empty state. Any threads waiting on it |
| 1016 | will receive the :class:`BrokenBarrierError` exception. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 1017 | |
Géry Ogam | 51a860e | 2019-05-18 00:44:57 +0200 | [diff] [blame] | 1018 | Note that using this function may require some external |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 1019 | synchronization if there are other threads whose state is unknown. If a |
| 1020 | 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] | 1021 | |
| 1022 | .. method:: abort() |
| 1023 | |
| 1024 | 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] | 1025 | calls to :meth:`wait` to fail with the :class:`BrokenBarrierError`. Use |
Géry Ogam | 51a860e | 2019-05-18 00:44:57 +0200 | [diff] [blame] | 1026 | this for example if one of the threads needs to abort, to avoid deadlocking the |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 1027 | application. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 1028 | |
| 1029 | It may be preferable to simply create the barrier with a sensible |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 1030 | *timeout* value to automatically guard against one of the threads going |
| 1031 | awry. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 1032 | |
| 1033 | .. attribute:: parties |
| 1034 | |
| 1035 | The number of threads required to pass the barrier. |
| 1036 | |
| 1037 | .. attribute:: n_waiting |
| 1038 | |
| 1039 | The number of threads currently waiting in the barrier. |
| 1040 | |
| 1041 | .. attribute:: broken |
| 1042 | |
| 1043 | A boolean that is ``True`` if the barrier is in the broken state. |
| 1044 | |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 1045 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 1046 | .. exception:: BrokenBarrierError |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 1047 | |
Georg Brandl | 5bc1686 | 2010-10-28 13:07:50 +0000 | [diff] [blame] | 1048 | This exception, a subclass of :exc:`RuntimeError`, is raised when the |
| 1049 | :class:`Barrier` object is reset or broken. |
Kristján Valur Jónsson | 3be0003 | 2010-10-28 09:43:10 +0000 | [diff] [blame] | 1050 | |
| 1051 | |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 1052 | .. _with-locks: |
| 1053 | |
Serhiy Storchaka | 2b57c43 | 2018-12-19 08:09:46 +0200 | [diff] [blame] | 1054 | Using locks, conditions, and semaphores in the :keyword:`!with` statement |
| 1055 | ------------------------------------------------------------------------- |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 1056 | |
| 1057 | All of the objects provided by this module that have :meth:`acquire` and |
| 1058 | :meth:`release` methods can be used as context managers for a :keyword:`with` |
Antoine Pitrou | b96a354 | 2012-04-10 22:47:55 +0200 | [diff] [blame] | 1059 | statement. The :meth:`acquire` method will be called when the block is |
| 1060 | entered, and :meth:`release` will be called when the block is exited. Hence, |
| 1061 | the following snippet:: |
| 1062 | |
| 1063 | with some_lock: |
| 1064 | # do something... |
| 1065 | |
| 1066 | is equivalent to:: |
| 1067 | |
| 1068 | some_lock.acquire() |
| 1069 | try: |
| 1070 | # do something... |
| 1071 | finally: |
| 1072 | some_lock.release() |
Georg Brandl | 116aa62 | 2007-08-15 14:28:22 +0000 | [diff] [blame] | 1073 | |
| 1074 | Currently, :class:`Lock`, :class:`RLock`, :class:`Condition`, |
| 1075 | :class:`Semaphore`, and :class:`BoundedSemaphore` objects may be used as |
Antoine Pitrou | b96a354 | 2012-04-10 22:47:55 +0200 | [diff] [blame] | 1076 | :keyword:`with` statement context managers. |