1:mod:`_thread` --- Low-level threading API 2========================================== 3 4.. module:: _thread 5 :synopsis: Low-level threading API. 6 7.. index:: 8 single: light-weight processes 9 single: processes, light-weight 10 single: binary semaphores 11 single: semaphores, binary 12 13-------------- 14 15This module provides low-level primitives for working with multiple threads 16(also called :dfn:`light-weight processes` or :dfn:`tasks`) --- multiple threads of 17control sharing their global data space. For synchronization, simple locks 18(also called :dfn:`mutexes` or :dfn:`binary semaphores`) are provided. 19The :mod:`threading` module provides an easier to use and higher-level 20threading API built on top of this module. 21 22.. index:: 23 single: pthreads 24 pair: threads; POSIX 25 26.. versionchanged:: 3.7 27 This module used to be optional, it is now always available. 28 29This module defines the following constants and functions: 30 31.. exception:: error 32 33 Raised on thread-specific errors. 34 35 .. versionchanged:: 3.3 36 This is now a synonym of the built-in :exc:`RuntimeError`. 37 38 39.. data:: LockType 40 41 This is the type of lock objects. 42 43 44.. function:: start_new_thread(function, args[, kwargs]) 45 46 Start a new thread and return its identifier. The thread executes the 47 function *function* with the argument list *args* (which must be a tuple). 48 The optional *kwargs* argument specifies a dictionary of keyword arguments. 49 50 When the function returns, the thread silently exits. 51 52 When the function terminates with an unhandled exception, 53 :func:`sys.unraisablehook` is called to handle the exception. The *object* 54 attribute of the hook argument is *function*. By default, a stack trace is 55 printed and then the thread exits (but other threads continue to run). 56 57 When the function raises a :exc:`SystemExit` exception, it is silently 58 ignored. 59 60 .. versionchanged:: 3.8 61 :func:`sys.unraisablehook` is now used to handle unhandled exceptions. 62 63 64.. function:: interrupt_main(signum=signal.SIGINT, /) 65 66 Simulate the effect of a signal arriving in the main thread. 67 A thread can use this function to interrupt the main thread, though 68 there is no guarantee that the interruption will happen immediately. 69 70 If given, *signum* is the number of the signal to simulate. 71 If *signum* is not given, :data:`signal.SIGINT` is simulated. 72 73 If the given signal isn't handled by Python (it was set to 74 :data:`signal.SIG_DFL` or :data:`signal.SIG_IGN`), this function does 75 nothing. 76 77 .. versionchanged:: 3.10 78 The *signum* argument is added to customize the signal number. 79 80 .. note:: 81 This does not emit the corresponding signal but schedules a call to 82 the associated handler (if it exists). 83 If you want to truly emit the signal, use :func:`signal.raise_signal`. 84 85 86.. function:: exit() 87 88 Raise the :exc:`SystemExit` exception. When not caught, this will cause the 89 thread to exit silently. 90 91.. 92 function:: exit_prog(status) 93 94 Exit all threads and report the value of the integer argument 95 *status* as the exit status of the entire program. 96 **Caveat:** code in pending :keyword:`finally` clauses, in this thread 97 or in other threads, is not executed. 98 99 100.. function:: allocate_lock() 101 102 Return a new lock object. Methods of locks are described below. The lock is 103 initially unlocked. 104 105 106.. function:: get_ident() 107 108 Return the 'thread identifier' of the current thread. This is a nonzero 109 integer. Its value has no direct meaning; it is intended as a magic cookie to 110 be used e.g. to index a dictionary of thread-specific data. Thread identifiers 111 may be recycled when a thread exits and another thread is created. 112 113 114.. function:: get_native_id() 115 116 Return the native integral Thread ID of the current thread assigned by the kernel. 117 This is a non-negative integer. 118 Its value may be used to uniquely identify this particular thread system-wide 119 (until the thread terminates, after which the value may be recycled by the OS). 120 121 .. availability:: Windows, FreeBSD, Linux, macOS, OpenBSD, NetBSD, AIX. 122 123 .. versionadded:: 3.8 124 125 126.. function:: stack_size([size]) 127 128 Return the thread stack size used when creating new threads. The optional 129 *size* argument specifies the stack size to be used for subsequently created 130 threads, and must be 0 (use platform or configured default) or a positive 131 integer value of at least 32,768 (32 KiB). If *size* is not specified, 132 0 is used. If changing the thread stack size is 133 unsupported, a :exc:`RuntimeError` is raised. If the specified stack size is 134 invalid, a :exc:`ValueError` is raised and the stack size is unmodified. 32 KiB 135 is currently the minimum supported stack size value to guarantee sufficient 136 stack space for the interpreter itself. Note that some platforms may have 137 particular restrictions on values for the stack size, such as requiring a 138 minimum stack size > 32 KiB or requiring allocation in multiples of the system 139 memory page size - platform documentation should be referred to for more 140 information (4 KiB pages are common; using multiples of 4096 for the stack size is 141 the suggested approach in the absence of more specific information). 142 143 .. availability:: Windows, pthreads. 144 145 Unix platforms with POSIX threads support. 146 147 148.. data:: TIMEOUT_MAX 149 150 The maximum value allowed for the *timeout* parameter of 151 :meth:`Lock.acquire`. Specifying a timeout greater than this value will 152 raise an :exc:`OverflowError`. 153 154 .. versionadded:: 3.2 155 156 157Lock objects have the following methods: 158 159 160.. method:: lock.acquire(blocking=True, timeout=-1) 161 162 Without any optional argument, this method acquires the lock unconditionally, if 163 necessary waiting until it is released by another thread (only one thread at a 164 time can acquire a lock --- that's their reason for existence). 165 166 If the *blocking* argument is present, the action depends on its 167 value: if it is False, the lock is only acquired if it can be acquired 168 immediately without waiting, while if it is True, the lock is acquired 169 unconditionally as above. 170 171 If the floating-point *timeout* argument is present and positive, it 172 specifies the maximum wait time in seconds before returning. A negative 173 *timeout* argument specifies an unbounded wait. You cannot specify 174 a *timeout* if *blocking* is False. 175 176 The return value is ``True`` if the lock is acquired successfully, 177 ``False`` if not. 178 179 .. versionchanged:: 3.2 180 The *timeout* parameter is new. 181 182 .. versionchanged:: 3.2 183 Lock acquires can now be interrupted by signals on POSIX. 184 185 186.. method:: lock.release() 187 188 Releases the lock. The lock must have been acquired earlier, but not 189 necessarily by the same thread. 190 191 192.. method:: lock.locked() 193 194 Return the status of the lock: ``True`` if it has been acquired by some thread, 195 ``False`` if not. 196 197In addition to these methods, lock objects can also be used via the 198:keyword:`with` statement, e.g.:: 199 200 import _thread 201 202 a_lock = _thread.allocate_lock() 203 204 with a_lock: 205 print("a_lock is locked while this executes") 206 207**Caveats:** 208 209 .. index:: pair: module; signal 210 211* Threads interact strangely with interrupts: the :exc:`KeyboardInterrupt` 212 exception will be received by an arbitrary thread. (When the :mod:`signal` 213 module is available, interrupts always go to the main thread.) 214 215* Calling :func:`sys.exit` or raising the :exc:`SystemExit` exception is 216 equivalent to calling :func:`_thread.exit`. 217 218* It is not possible to interrupt the :meth:`acquire` method on a lock --- the 219 :exc:`KeyboardInterrupt` exception will happen after the lock has been acquired. 220 221* When the main thread exits, it is system defined whether the other threads 222 survive. On most systems, they are killed without executing 223 :keyword:`try` ... :keyword:`finally` clauses or executing object 224 destructors. 225 226* When the main thread exits, it does not do any of its usual cleanup (except 227 that :keyword:`try` ... :keyword:`finally` clauses are honored), and the 228 standard I/O files are not flushed. 229 230