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| 55 | <H1><A NAME="SECTION009400000000000000000"> |
| 56 | 7.4 <tt class="module">thread</tt> -- |
| 57 | Multiple threads of control</A> |
| 58 | </H1> |
| 59 | |
| 60 | <P> |
| 61 | <A NAME="module-thread"></A> |
| 62 | |
| 63 | <P> |
| 64 | This module provides low-level primitives for working with multiple |
| 65 | threads (a.k.a. <i class="dfn">light-weight processes</i> or <i class="dfn">tasks</i>) -- multiple |
| 66 | threads of control sharing their global data space. For |
| 67 | synchronization, simple locks (a.k.a. <i class="dfn">mutexes</i> or <i class="dfn">binary |
| 68 | semaphores</i>) are provided. |
| 69 | <a id='l2h-2700' xml:id='l2h-2700'></a> |
| 70 | |
| 71 | <P> |
| 72 | The module is optional. It is supported on Windows, Linux, SGI |
| 73 | IRIX, Solaris 2.x, as well as on systems that have a POSIX thread |
| 74 | (a.k.a. ``pthread'') implementation. For systems lacking the <tt class="module">thread</tt> |
| 75 | module, the <tt class="module"><a href="module-dummythread.html">dummy_thread</a></tt> module is available. |
| 76 | It duplicates this module's interface and can be |
| 77 | used as a drop-in replacement. |
| 78 | <a id='l2h-2701' xml:id='l2h-2701'></a> |
| 79 | <a id='l2h-2688' xml:id='l2h-2688'></a> |
| 80 | <P> |
| 81 | It defines the following constant and functions: |
| 82 | |
| 83 | <P> |
| 84 | <dl><dt><b><span class="typelabel">exception</span> <tt id='l2h-2689' xml:id='l2h-2689' class="exception">error</tt></b></dt> |
| 85 | <dd> |
| 86 | Raised on thread-specific errors. |
| 87 | </dd></dl> |
| 88 | |
| 89 | <P> |
| 90 | <dl><dt><b><tt id='l2h-2690' xml:id='l2h-2690'>LockType</tt></b></dt> |
| 91 | <dd> |
| 92 | This is the type of lock objects. |
| 93 | </dd></dl> |
| 94 | |
| 95 | <P> |
| 96 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 97 | <td><nobr><b><tt id='l2h-2691' xml:id='l2h-2691' class="function">start_new_thread</tt></b>(</nobr></td> |
| 98 | <td><var>function, args</var><big>[</big><var>, kwargs</var><big>]</big><var></var>)</td></tr></table></dt> |
| 99 | <dd> |
| 100 | Start a new thread and return its identifier. The thread executes the function |
| 101 | <var>function</var> with the argument list <var>args</var> (which must be a tuple). The |
| 102 | optional <var>kwargs</var> argument specifies a dictionary of keyword arguments. |
| 103 | When the function returns, the thread silently exits. When the function |
| 104 | terminates with an unhandled exception, a stack trace is printed and |
| 105 | then the thread exits (but other threads continue to run). |
| 106 | </dl> |
| 107 | |
| 108 | <P> |
| 109 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 110 | <td><nobr><b><tt id='l2h-2692' xml:id='l2h-2692' class="function">interrupt_main</tt></b>(</nobr></td> |
| 111 | <td><var></var>)</td></tr></table></dt> |
| 112 | <dd> |
| 113 | Raise a KeyboardInterrupt in the main thread. A subthread can use this |
| 114 | function to interrupt the main thread. |
| 115 | |
| 116 | <span class="versionnote">New in version 2.3.</span> |
| 117 | |
| 118 | </dl> |
| 119 | |
| 120 | <P> |
| 121 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 122 | <td><nobr><b><tt id='l2h-2693' xml:id='l2h-2693' class="function">exit</tt></b>(</nobr></td> |
| 123 | <td><var></var>)</td></tr></table></dt> |
| 124 | <dd> |
| 125 | Raise the <tt class="exception">SystemExit</tt> exception. When not caught, this |
| 126 | will cause the thread to exit silently. |
| 127 | </dl> |
| 128 | |
| 129 | <P> |
| 130 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 131 | <td><nobr><b><tt id='l2h-2694' xml:id='l2h-2694' class="function">allocate_lock</tt></b>(</nobr></td> |
| 132 | <td><var></var>)</td></tr></table></dt> |
| 133 | <dd> |
| 134 | Return a new lock object. Methods of locks are described below. The |
| 135 | lock is initially unlocked. |
| 136 | </dl> |
| 137 | |
| 138 | <P> |
| 139 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 140 | <td><nobr><b><tt id='l2h-2695' xml:id='l2h-2695' class="function">get_ident</tt></b>(</nobr></td> |
| 141 | <td><var></var>)</td></tr></table></dt> |
| 142 | <dd> |
| 143 | Return the `thread identifier' of the current thread. This is a |
| 144 | nonzero integer. Its value has no direct meaning; it is intended as a |
| 145 | magic cookie to be used e.g. to index a dictionary of thread-specific |
| 146 | data. Thread identifiers may be recycled when a thread exits and |
| 147 | another thread is created. |
| 148 | </dl> |
| 149 | |
| 150 | <P> |
| 151 | Lock objects have the following methods: |
| 152 | |
| 153 | <P> |
| 154 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 155 | <td><nobr><b><tt id='l2h-2696' xml:id='l2h-2696' class="method">acquire</tt></b>(</nobr></td> |
| 156 | <td><var></var><big>[</big><var>waitflag</var><big>]</big><var></var>)</td></tr></table></dt> |
| 157 | <dd> |
| 158 | Without the optional argument, this method acquires the lock |
| 159 | unconditionally, if necessary waiting until it is released by another |
| 160 | thread (only one thread at a time can acquire a lock -- that's their |
| 161 | reason for existence). If the integer |
| 162 | <var>waitflag</var> argument is present, the action depends on its |
| 163 | value: if it is zero, the lock is only acquired if it can be acquired |
| 164 | immediately without waiting, while if it is nonzero, the lock is |
| 165 | acquired unconditionally as before. The |
| 166 | return value is <code>True</code> if the lock is acquired successfully, |
| 167 | <code>False</code> if not. |
| 168 | </dl> |
| 169 | |
| 170 | <P> |
| 171 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 172 | <td><nobr><b><tt id='l2h-2697' xml:id='l2h-2697' class="method">release</tt></b>(</nobr></td> |
| 173 | <td><var></var>)</td></tr></table></dt> |
| 174 | <dd> |
| 175 | Releases the lock. The lock must have been acquired earlier, but not |
| 176 | necessarily by the same thread. |
| 177 | </dl> |
| 178 | |
| 179 | <P> |
| 180 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 181 | <td><nobr><b><tt id='l2h-2698' xml:id='l2h-2698' class="method">locked</tt></b>(</nobr></td> |
| 182 | <td><var></var>)</td></tr></table></dt> |
| 183 | <dd> |
| 184 | Return the status of the lock: <code>True</code> if it has been acquired by |
| 185 | some thread, <code>False</code> if not. |
| 186 | </dl> |
| 187 | |
| 188 | <P> |
| 189 | <strong>Caveats:</strong> |
| 190 | |
| 191 | <P> |
| 192 | |
| 193 | <UL> |
| 194 | <LI>Threads interact strangely with interrupts: the |
| 195 | <tt class="exception">KeyboardInterrupt</tt> exception will be received by an |
| 196 | arbitrary thread. (When the <tt class="module"><a href="module-signal.html">signal</a></tt><a id='l2h-2702' xml:id='l2h-2702'></a> |
| 197 | module is available, interrupts always go to the main thread.) |
| 198 | |
| 199 | <P> |
| 200 | </LI> |
| 201 | <LI>Calling <tt class="function">sys.exit()</tt> or raising the <tt class="exception">SystemExit</tt> |
| 202 | exception is equivalent to calling <tt class="function">exit()</tt>. |
| 203 | |
| 204 | <P> |
| 205 | </LI> |
| 206 | <LI>Not all built-in functions that may block waiting for I/O allow other |
| 207 | threads to run. (The most popular ones (<tt class="function">time.sleep()</tt>, |
| 208 | <tt class="method"><var>file</var>.read()</tt>, <tt class="function">select.select()</tt>) work as |
| 209 | expected.) |
| 210 | |
| 211 | <P> |
| 212 | </LI> |
| 213 | <LI>It is not possible to interrupt the <tt class="method">acquire()</tt> method on a lock |
| 214 | -- the <tt class="exception">KeyboardInterrupt</tt> exception will happen after the |
| 215 | lock has been acquired. |
| 216 | |
| 217 | <P> |
| 218 | </LI> |
| 219 | <LI>When the main thread exits, it is system defined whether the other |
| 220 | threads survive. On SGI IRIX using the native thread implementation, |
| 221 | they survive. On most other systems, they are killed without |
| 222 | executing <tt class="keyword">try</tt> ... <tt class="keyword">finally</tt> clauses or executing |
| 223 | object destructors. |
| 224 | <a id='l2h-2699' xml:id='l2h-2699'></a> |
| 225 | <P> |
| 226 | </LI> |
| 227 | <LI>When the main thread exits, it does not do any of its usual cleanup |
| 228 | (except that <tt class="keyword">try</tt> ... <tt class="keyword">finally</tt> clauses are honored), |
| 229 | and the standard I/O files are not flushed. |
| 230 | |
| 231 | <P> |
| 232 | </LI> |
| 233 | </UL> |
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