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 15 This module provides low-level primitives for working with multiple threads 16 (also called :dfn:`light-weight processes` or :dfn:`tasks`) --- multiple threads of 17 control sharing their global data space. For synchronization, simple locks 18 (also called :dfn:`mutexes` or :dfn:`binary semaphores`) are provided. 19 The :mod:`threading` module provides an easier to use and higher-level 20 threading 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 29 This 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 function 47 *function* with the argument list *args* (which must be a tuple). The optional 48 *kwargs* argument specifies a dictionary of keyword arguments. When the function 49 returns, the thread silently exits. When the function terminates with an 50 unhandled exception, a stack trace is printed and then the thread exits (but 51 other threads continue to run). 52 53 54 .. function:: interrupt_main() 55 56 Raise a :exc:`KeyboardInterrupt` exception in the main thread. A subthread can 57 use this function to interrupt the main thread. 58 59 60 .. function:: exit() 61 62 Raise the :exc:`SystemExit` exception. When not caught, this will cause the 63 thread to exit silently. 64 65 .. 66 function:: exit_prog(status) 67 68 Exit all threads and report the value of the integer argument 69 *status* as the exit status of the entire program. 70 **Caveat:** code in pending :keyword:`finally` clauses, in this thread 71 or in other threads, is not executed. 72 73 74 .. function:: allocate_lock() 75 76 Return a new lock object. Methods of locks are described below. The lock is 77 initially unlocked. 78 79 80 .. function:: get_ident() 81 82 Return the 'thread identifier' of the current thread. This is a nonzero 83 integer. Its value has no direct meaning; it is intended as a magic cookie to 84 be used e.g. to index a dictionary of thread-specific data. Thread identifiers 85 may be recycled when a thread exits and another thread is created. 86 87 88 .. function:: stack_size([size]) 89 90 Return the thread stack size used when creating new threads. The optional 91 *size* argument specifies the stack size to be used for subsequently created 92 threads, and must be 0 (use platform or configured default) or a positive 93 integer value of at least 32,768 (32 KiB). If *size* is not specified, 94 0 is used. If changing the thread stack size is 95 unsupported, a :exc:`RuntimeError` is raised. If the specified stack size is 96 invalid, a :exc:`ValueError` is raised and the stack size is unmodified. 32 KiB 97 is currently the minimum supported stack size value to guarantee sufficient 98 stack space for the interpreter itself. Note that some platforms may have 99 particular restrictions on values for the stack size, such as requiring a 100 minimum stack size > 32 KiB or requiring allocation in multiples of the system 101 memory page size - platform documentation should be referred to for more 102 information (4 KiB pages are common; using multiples of 4096 for the stack size is 103 the suggested approach in the absence of more specific information). 104 105 .. availability:: Windows, systems with POSIX threads. 106 107 108 .. data:: TIMEOUT_MAX 109 110 The maximum value allowed for the *timeout* parameter of 111 :meth:`Lock.acquire`. Specifying a timeout greater than this value will 112 raise an :exc:`OverflowError`. 113 114 .. versionadded:: 3.2 115 116 117 Lock objects have the following methods: 118 119 120 .. method:: lock.acquire(waitflag=1, timeout=-1) 121 122 Without any optional argument, this method acquires the lock unconditionally, if 123 necessary waiting until it is released by another thread (only one thread at a 124 time can acquire a lock --- that's their reason for existence). 125 126 If the integer *waitflag* argument is present, the action depends on its 127 value: if it is zero, the lock is only acquired if it can be acquired 128 immediately without waiting, while if it is nonzero, the lock is acquired 129 unconditionally as above. 130 131 If the floating-point *timeout* argument is present and positive, it 132 specifies the maximum wait time in seconds before returning. A negative 133 *timeout* argument specifies an unbounded wait. You cannot specify 134 a *timeout* if *waitflag* is zero. 135 136 The return value is ``True`` if the lock is acquired successfully, 137 ``False`` if not. 138 139 .. versionchanged:: 3.2 140 The *timeout* parameter is new. 141 142 .. versionchanged:: 3.2 143 Lock acquires can now be interrupted by signals on POSIX. 144 145 146 .. method:: lock.release() 147 148 Releases the lock. The lock must have been acquired earlier, but not 149 necessarily by the same thread. 150 151 152 .. method:: lock.locked() 153 154 Return the status of the lock: ``True`` if it has been acquired by some thread, 155 ``False`` if not. 156 157 In addition to these methods, lock objects can also be used via the 158 :keyword:`with` statement, e.g.:: 159 160 import _thread 161 162 a_lock = _thread.allocate_lock() 163 164 with a_lock: 165 print("a_lock is locked while this executes") 166 167 **Caveats:** 168 169 .. index:: module: signal 170 171 * Threads interact strangely with interrupts: the :exc:`KeyboardInterrupt` 172 exception will be received by an arbitrary thread. (When the :mod:`signal` 173 module is available, interrupts always go to the main thread.) 174 175 * Calling :func:`sys.exit` or raising the :exc:`SystemExit` exception is 176 equivalent to calling :func:`_thread.exit`. 177 178 * It is not possible to interrupt the :meth:`acquire` method on a lock --- the 179 :exc:`KeyboardInterrupt` exception will happen after the lock has been acquired. 180 181 * When the main thread exits, it is system defined whether the other threads 182 survive. On most systems, they are killed without executing 183 :keyword:`try` ... :keyword:`finally` clauses or executing object 184 destructors. 185 186 * When the main thread exits, it does not do any of its usual cleanup (except 187 that :keyword:`try` ... :keyword:`finally` clauses are honored), and the 188 standard I/O files are not flushed. 189 190
