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() 65 66 Simulate the effect of a :data:`signal.SIGINT` signal arriving in the main 67 thread. A thread can use this function to interrupt the main thread. 68 69 If :data:`signal.SIGINT` isn't handled by Python (it was set to 70 :data:`signal.SIG_DFL` or :data:`signal.SIG_IGN`), this function does 71 nothing. 72 73 74.. function:: exit() 75 76 Raise the :exc:`SystemExit` exception. When not caught, this will cause the 77 thread to exit silently. 78 79.. 80 function:: exit_prog(status) 81 82 Exit all threads and report the value of the integer argument 83 *status* as the exit status of the entire program. 84 **Caveat:** code in pending :keyword:`finally` clauses, in this thread 85 or in other threads, is not executed. 86 87 88.. function:: allocate_lock() 89 90 Return a new lock object. Methods of locks are described below. The lock is 91 initially unlocked. 92 93 94.. function:: get_ident() 95 96 Return the 'thread identifier' of the current thread. This is a nonzero 97 integer. Its value has no direct meaning; it is intended as a magic cookie to 98 be used e.g. to index a dictionary of thread-specific data. Thread identifiers 99 may be recycled when a thread exits and another thread is created. 100 101 102.. function:: get_native_id() 103 104 Return the native integral Thread ID of the current thread assigned by the kernel. 105 This is a non-negative integer. 106 Its value may be used to uniquely identify this particular thread system-wide 107 (until the thread terminates, after which the value may be recycled by the OS). 108 109 .. availability:: Windows, FreeBSD, Linux, macOS, OpenBSD, NetBSD, AIX. 110 111 .. versionadded:: 3.8 112 113 114.. function:: stack_size([size]) 115 116 Return the thread stack size used when creating new threads. The optional 117 *size* argument specifies the stack size to be used for subsequently created 118 threads, and must be 0 (use platform or configured default) or a positive 119 integer value of at least 32,768 (32 KiB). If *size* is not specified, 120 0 is used. If changing the thread stack size is 121 unsupported, a :exc:`RuntimeError` is raised. If the specified stack size is 122 invalid, a :exc:`ValueError` is raised and the stack size is unmodified. 32 KiB 123 is currently the minimum supported stack size value to guarantee sufficient 124 stack space for the interpreter itself. Note that some platforms may have 125 particular restrictions on values for the stack size, such as requiring a 126 minimum stack size > 32 KiB or requiring allocation in multiples of the system 127 memory page size - platform documentation should be referred to for more 128 information (4 KiB pages are common; using multiples of 4096 for the stack size is 129 the suggested approach in the absence of more specific information). 130 131 .. availability:: Windows, systems with POSIX threads. 132 133 134.. data:: TIMEOUT_MAX 135 136 The maximum value allowed for the *timeout* parameter of 137 :meth:`Lock.acquire`. Specifying a timeout greater than this value will 138 raise an :exc:`OverflowError`. 139 140 .. versionadded:: 3.2 141 142 143Lock objects have the following methods: 144 145 146.. method:: lock.acquire(waitflag=1, timeout=-1) 147 148 Without any optional argument, this method acquires the lock unconditionally, if 149 necessary waiting until it is released by another thread (only one thread at a 150 time can acquire a lock --- that's their reason for existence). 151 152 If the integer *waitflag* argument is present, the action depends on its 153 value: if it is zero, the lock is only acquired if it can be acquired 154 immediately without waiting, while if it is nonzero, the lock is acquired 155 unconditionally as above. 156 157 If the floating-point *timeout* argument is present and positive, it 158 specifies the maximum wait time in seconds before returning. A negative 159 *timeout* argument specifies an unbounded wait. You cannot specify 160 a *timeout* if *waitflag* is zero. 161 162 The return value is ``True`` if the lock is acquired successfully, 163 ``False`` if not. 164 165 .. versionchanged:: 3.2 166 The *timeout* parameter is new. 167 168 .. versionchanged:: 3.2 169 Lock acquires can now be interrupted by signals on POSIX. 170 171 172.. method:: lock.release() 173 174 Releases the lock. The lock must have been acquired earlier, but not 175 necessarily by the same thread. 176 177 178.. method:: lock.locked() 179 180 Return the status of the lock: ``True`` if it has been acquired by some thread, 181 ``False`` if not. 182 183In addition to these methods, lock objects can also be used via the 184:keyword:`with` statement, e.g.:: 185 186 import _thread 187 188 a_lock = _thread.allocate_lock() 189 190 with a_lock: 191 print("a_lock is locked while this executes") 192 193**Caveats:** 194 195 .. index:: module: signal 196 197* Threads interact strangely with interrupts: the :exc:`KeyboardInterrupt` 198 exception will be received by an arbitrary thread. (When the :mod:`signal` 199 module is available, interrupts always go to the main thread.) 200 201* Calling :func:`sys.exit` or raising the :exc:`SystemExit` exception is 202 equivalent to calling :func:`_thread.exit`. 203 204* It is not possible to interrupt the :meth:`acquire` method on a lock --- the 205 :exc:`KeyboardInterrupt` exception will happen after the lock has been acquired. 206 207* When the main thread exits, it is system defined whether the other threads 208 survive. On most systems, they are killed without executing 209 :keyword:`try` ... :keyword:`finally` clauses or executing object 210 destructors. 211 212* When the main thread exits, it does not do any of its usual cleanup (except 213 that :keyword:`try` ... :keyword:`finally` clauses are honored), and the 214 standard I/O files are not flushed. 215 216