Mercurial > cpython-lru_cache-weakref
annotate Doc/library/threading.rst @ 54294:dba767bc3155 legacy-trunk
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| author | Benjamin Peterson <benjamin@python.org> |
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| date | Sat, 23 May 2009 20:59:09 +0000 |
| parents | f44d42a11b9b |
| children | 1bde8455222c |
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1 :mod:`threading` --- Higher-level threading interface |
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2 ===================================================== |
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3 |
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4 .. module:: threading |
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5 :synopsis: Higher-level threading interface. |
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6 |
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7 |
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8 This module constructs higher-level threading interfaces on top of the lower |
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9 level :mod:`thread` module. |
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10 See also the :mod:`mutex` and :mod:`Queue` modules. |
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11 |
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12 The :mod:`dummy_threading` module is provided for situations where |
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13 :mod:`threading` cannot be used because :mod:`thread` is missing. |
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14 |
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15 .. note:: |
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16 |
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17 Starting with Python 2.6, this module provides PEP 8 compliant aliases and |
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18 properties to replace the ``camelCase`` names that were inspired by Java's |
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19 threading API. This updated API is compatible with that of the |
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20 :mod:`multiprocessing` module. However, no schedule has been set for the |
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21 deprecation of the ``camelCase`` names and they remain fully supported in |
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22 both Python 2.x and 3.x. |
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23 |
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24 .. note:: |
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25 |
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26 Starting with Python 2.5, several Thread methods raise :exc:`RuntimeError` |
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27 instead of :exc:`AssertionError` if called erroneously. |
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29 |
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30 This module defines the following functions and objects: |
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31 |
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32 .. function:: active_count() |
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33 activeCount() |
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34 |
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35 Return the number of :class:`Thread` objects currently alive. The returned |
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36 count is equal to the length of the list returned by :func:`enumerate`. |
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37 |
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38 |
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39 .. function:: Condition() |
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40 :noindex: |
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41 |
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42 A factory function that returns a new condition variable object. A condition |
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43 variable allows one or more threads to wait until they are notified by another |
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44 thread. |
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45 |
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46 |
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47 .. function:: current_thread() |
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48 currentThread() |
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49 |
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50 Return the current :class:`Thread` object, corresponding to the caller's thread |
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51 of control. If the caller's thread of control was not created through the |
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52 :mod:`threading` module, a dummy thread object with limited functionality is |
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53 returned. |
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54 |
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55 |
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56 .. function:: enumerate() |
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57 |
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58 Return a list of all :class:`Thread` objects currently alive. The list |
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59 includes daemonic threads, dummy thread objects created by |
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60 :func:`current_thread`, and the main thread. It excludes terminated threads |
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61 and threads that have not yet been started. |
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62 |
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63 |
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64 .. function:: Event() |
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65 :noindex: |
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66 |
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67 A factory function that returns a new event object. An event manages a flag |
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68 that can be set to true with the :meth:`set` method and reset to false with the |
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69 :meth:`clear` method. The :meth:`wait` method blocks until the flag is true. |
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70 |
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71 |
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72 .. class:: local |
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73 |
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74 A class that represents thread-local data. Thread-local data are data whose |
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75 values are thread specific. To manage thread-local data, just create an |
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76 instance of :class:`local` (or a subclass) and store attributes on it:: |
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77 |
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78 mydata = threading.local() |
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79 mydata.x = 1 |
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80 |
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81 The instance's values will be different for separate threads. |
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82 |
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83 For more details and extensive examples, see the documentation string of the |
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84 :mod:`_threading_local` module. |
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85 |
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86 .. versionadded:: 2.4 |
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87 |
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88 |
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89 .. function:: Lock() |
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90 |
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91 A factory function that returns a new primitive lock object. Once a thread has |
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92 acquired it, subsequent attempts to acquire it block, until it is released; any |
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93 thread may release it. |
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94 |
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95 |
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96 .. function:: RLock() |
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97 |
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98 A factory function that returns a new reentrant lock object. A reentrant lock |
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99 must be released by the thread that acquired it. Once a thread has acquired a |
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100 reentrant lock, the same thread may acquire it again without blocking; the |
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101 thread must release it once for each time it has acquired it. |
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102 |
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103 |
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104 .. function:: Semaphore([value]) |
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105 :noindex: |
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106 |
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107 A factory function that returns a new semaphore object. A semaphore manages a |
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108 counter representing the number of :meth:`release` calls minus the number of |
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109 :meth:`acquire` calls, plus an initial value. The :meth:`acquire` method blocks |
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110 if necessary until it can return without making the counter negative. If not |
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111 given, *value* defaults to 1. |
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112 |
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113 |
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114 .. function:: BoundedSemaphore([value]) |
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115 |
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116 A factory function that returns a new bounded semaphore object. A bounded |
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117 semaphore checks to make sure its current value doesn't exceed its initial |
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118 value. If it does, :exc:`ValueError` is raised. In most situations semaphores |
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119 are used to guard resources with limited capacity. If the semaphore is released |
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120 too many times it's a sign of a bug. If not given, *value* defaults to 1. |
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121 |
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122 |
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123 .. class:: Thread |
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124 |
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125 A class that represents a thread of control. This class can be safely |
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126 subclassed in a limited fashion. |
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127 |
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128 |
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129 .. class:: Timer |
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130 |
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131 A thread that executes a function after a specified interval has passed. |
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132 |
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133 |
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134 .. function:: settrace(func) |
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135 |
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136 .. index:: single: trace function |
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137 |
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138 Set a trace function for all threads started from the :mod:`threading` module. |
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139 The *func* will be passed to :func:`sys.settrace` for each thread, before its |
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140 :meth:`run` method is called. |
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141 |
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142 .. versionadded:: 2.3 |
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143 |
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144 |
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145 .. function:: setprofile(func) |
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146 |
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147 .. index:: single: profile function |
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148 |
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149 Set a profile function for all threads started from the :mod:`threading` module. |
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150 The *func* will be passed to :func:`sys.setprofile` for each thread, before its |
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151 :meth:`run` method is called. |
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152 |
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153 .. versionadded:: 2.3 |
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154 |
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155 |
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156 .. function:: stack_size([size]) |
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157 |
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158 Return the thread stack size used when creating new threads. The optional |
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159 *size* argument specifies the stack size to be used for subsequently created |
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160 threads, and must be 0 (use platform or configured default) or a positive |
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161 integer value of at least 32,768 (32kB). If changing the thread stack size is |
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162 unsupported, a :exc:`ThreadError` is raised. If the specified stack size is |
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163 invalid, a :exc:`ValueError` is raised and the stack size is unmodified. 32kB |
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164 is currently the minimum supported stack size value to guarantee sufficient |
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165 stack space for the interpreter itself. Note that some platforms may have |
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166 particular restrictions on values for the stack size, such as requiring a |
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167 minimum stack size > 32kB or requiring allocation in multiples of the system |
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168 memory page size - platform documentation should be referred to for more |
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169 information (4kB pages are common; using multiples of 4096 for the stack size is |
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170 the suggested approach in the absence of more specific information). |
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171 Availability: Windows, systems with POSIX threads. |
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172 |
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173 .. versionadded:: 2.5 |
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174 |
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175 Detailed interfaces for the objects are documented below. |
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176 |
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177 The design of this module is loosely based on Java's threading model. However, |
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178 where Java makes locks and condition variables basic behavior of every object, |
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179 they are separate objects in Python. Python's :class:`Thread` class supports a |
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180 subset of the behavior of Java's Thread class; currently, there are no |
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181 priorities, no thread groups, and threads cannot be destroyed, stopped, |
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182 suspended, resumed, or interrupted. The static methods of Java's Thread class, |
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183 when implemented, are mapped to module-level functions. |
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184 |
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185 All of the methods described below are executed atomically. |
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186 |
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187 |
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188 .. _thread-objects: |
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189 |
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190 Thread Objects |
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191 -------------- |
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192 |
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193 This class represents an activity that is run in a separate thread of control. |
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194 There are two ways to specify the activity: by passing a callable object to the |
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195 constructor, or by overriding the :meth:`run` method in a subclass. No other |
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196 methods (except for the constructor) should be overridden in a subclass. In |
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197 other words, *only* override the :meth:`__init__` and :meth:`run` methods of |
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198 this class. |
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199 |
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200 Once a thread object is created, its activity must be started by calling the |
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201 thread's :meth:`start` method. This invokes the :meth:`run` method in a |
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202 separate thread of control. |
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203 |
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204 Once the thread's activity is started, the thread is considered 'alive'. It |
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205 stops being alive when its :meth:`run` method terminates -- either normally, or |
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206 by raising an unhandled exception. The :meth:`is_alive` method tests whether the |
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207 thread is alive. |
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208 |
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209 Other threads can call a thread's :meth:`join` method. This blocks the calling |
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210 thread until the thread whose :meth:`join` method is called is terminated. |
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211 |
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212 A thread has a name. The name can be passed to the constructor, and read or |
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213 changed through the :attr:`name` attribute. |
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214 |
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215 A thread can be flagged as a "daemon thread". The significance of this flag is |
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216 that the entire Python program exits when only daemon threads are left. The |
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217 initial value is inherited from the creating thread. The flag can be set |
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218 through the :attr:`daemon` property. |
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219 |
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220 There is a "main thread" object; this corresponds to the initial thread of |
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221 control in the Python program. It is not a daemon thread. |
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222 |
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223 There is the possibility that "dummy thread objects" are created. These are |
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224 thread objects corresponding to "alien threads", which are threads of control |
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225 started outside the threading module, such as directly from C code. Dummy |
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226 thread objects have limited functionality; they are always considered alive and |
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227 daemonic, and cannot be :meth:`join`\ ed. They are never deleted, since it is |
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228 impossible to detect the termination of alien threads. |
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229 |
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230 |
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231 .. class:: Thread(group=None, target=None, name=None, args=(), kwargs={}) |
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232 |
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233 This constructor should always be called with keyword arguments. Arguments are: |
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234 |
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235 *group* should be ``None``; reserved for future extension when a |
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236 :class:`ThreadGroup` class is implemented. |
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237 |
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238 *target* is the callable object to be invoked by the :meth:`run` method. |
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239 Defaults to ``None``, meaning nothing is called. |
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240 |
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241 *name* is the thread name. By default, a unique name is constructed of the form |
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242 "Thread-*N*" where *N* is a small decimal number. |
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243 |
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244 *args* is the argument tuple for the target invocation. Defaults to ``()``. |
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246 *kwargs* is a dictionary of keyword arguments for the target invocation. |
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247 Defaults to ``{}``. |
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249 If the subclass overrides the constructor, it must make sure to invoke the base |
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250 class constructor (``Thread.__init__()``) before doing anything else to the |
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251 thread. |
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254 .. method:: Thread.start() |
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256 Start the thread's activity. |
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258 It must be called at most once per thread object. It arranges for the object's |
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259 :meth:`run` method to be invoked in a separate thread of control. |
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260 |
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261 This method will raise a :exc:`RuntimeException` if called more than once on the |
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262 same thread object. |
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263 |
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264 |
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265 .. method:: Thread.run() |
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266 |
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267 Method representing the thread's activity. |
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268 |
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269 You may override this method in a subclass. The standard :meth:`run` method |
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270 invokes the callable object passed to the object's constructor as the *target* |
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271 argument, if any, with sequential and keyword arguments taken from the *args* |
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272 and *kwargs* arguments, respectively. |
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273 |
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274 |
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275 .. method:: Thread.join([timeout]) |
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276 |
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277 Wait until the thread terminates. This blocks the calling thread until the |
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278 thread whose :meth:`join` method is called terminates -- either normally or |
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279 through an unhandled exception -- or until the optional timeout occurs. |
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280 |
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281 When the *timeout* argument is present and not ``None``, it should be a floating |
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282 point number specifying a timeout for the operation in seconds (or fractions |
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283 thereof). As :meth:`join` always returns ``None``, you must call :meth:`isAlive` |
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284 after :meth:`join` to decide whether a timeout happened -- if the thread is |
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285 still alive, the :meth:`join` call timed out. |
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287 When the *timeout* argument is not present or ``None``, the operation will block |
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288 until the thread terminates. |
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290 A thread can be :meth:`join`\ ed many times. |
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292 :meth:`join` raises a :exc:`RuntimeError` if an attempt is made to join |
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293 the current thread as that would cause a deadlock. It is also an error to |
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294 :meth:`join` a thread before it has been started and attempts to do so |
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295 raises the same exception. |
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298 .. method:: Thread.getName() |
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299 Thread.setName() |
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300 |
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301 Old API for :attr:`~Thread.name`. |
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302 |
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304 .. attribute:: Thread.name |
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305 |
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306 A string used for identification purposes only. It has no semantics. |
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307 Multiple threads may be given the same name. The initial name is set by the |
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308 constructor. |
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309 |
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311 .. attribute:: Thread.ident |
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313 The 'thread identifier' of this thread or ``None`` if the thread has not been |
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314 started. This is a nonzero integer. See the :func:`thread.get_ident()` |
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315 function. Thread identifiers may be recycled when a thread exits and another |
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316 thread is created. The identifier is available even after the thread has |
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317 exited. |
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319 .. versionadded:: 2.6 |
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321 |
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322 .. method:: Thread.is_alive() |
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323 Thread.isAlive() |
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324 |
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325 Return whether the thread is alive. |
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326 |
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327 Roughly, a thread is alive from the moment the :meth:`start` method returns |
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328 until its :meth:`run` method terminates. The module function :func:`enumerate` |
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329 returns a list of all alive threads. |
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330 |
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331 |
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332 .. method:: Thread.isDaemon() |
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333 Thread.setDaemon() |
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334 |
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335 Old API for :attr:`~Thread.daemon`. |
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338 .. attribute:: Thread.daemon |
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339 |
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340 A boolean value indicating whether this thread is a daemon thread (True) or |
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341 not (False). This must be set before :meth:`start` is called, otherwise |
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342 :exc:`RuntimeError` is raised. Its initial value is inherited from the |
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343 creating thread; the main thread is not a daemon thread and therefore all |
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344 threads created in the main thread default to :attr:`daemon` = ``False``. |
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345 |
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346 The entire Python program exits when no alive non-daemon threads are left. |
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347 |
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348 |
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349 .. _lock-objects: |
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350 |
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351 Lock Objects |
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352 ------------ |
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353 |
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354 A primitive lock is a synchronization primitive that is not owned by a |
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355 particular thread when locked. In Python, it is currently the lowest level |
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356 synchronization primitive available, implemented directly by the :mod:`thread` |
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357 extension module. |
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358 |
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359 A primitive lock is in one of two states, "locked" or "unlocked". It is created |
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360 in the unlocked state. It has two basic methods, :meth:`acquire` and |
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361 :meth:`release`. When the state is unlocked, :meth:`acquire` changes the state |
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362 to locked and returns immediately. When the state is locked, :meth:`acquire` |
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363 blocks until a call to :meth:`release` in another thread changes it to unlocked, |
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364 then the :meth:`acquire` call resets it to locked and returns. The |
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365 :meth:`release` method should only be called in the locked state; it changes the |
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366 state to unlocked and returns immediately. If an attempt is made to release an |
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367 unlocked lock, a :exc:`RuntimeError` will be raised. |
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368 |
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369 When more than one thread is blocked in :meth:`acquire` waiting for the state to |
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370 turn to unlocked, only one thread proceeds when a :meth:`release` call resets |
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371 the state to unlocked; which one of the waiting threads proceeds is not defined, |
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372 and may vary across implementations. |
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373 |
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374 All methods are executed atomically. |
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375 |
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376 |
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377 .. method:: Lock.acquire([blocking=1]) |
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378 |
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379 Acquire a lock, blocking or non-blocking. |
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380 |
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381 When invoked without arguments, block until the lock is unlocked, then set it to |
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382 locked, and return true. |
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383 |
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384 When invoked with the *blocking* argument set to true, do the same thing as when |
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385 called without arguments, and return true. |
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386 |
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387 When invoked with the *blocking* argument set to false, do not block. If a call |
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388 without an argument would block, return false immediately; otherwise, do the |
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389 same thing as when called without arguments, and return true. |
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390 |
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391 |
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392 .. method:: Lock.release() |
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393 |
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394 Release a lock. |
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395 |
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396 When the lock is locked, reset it to unlocked, and return. If any other threads |
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397 are blocked waiting for the lock to become unlocked, allow exactly one of them |
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398 to proceed. |
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399 |
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400 Do not call this method when the lock is unlocked. |
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401 |
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402 There is no return value. |
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403 |
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404 |
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405 .. _rlock-objects: |
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406 |
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407 RLock Objects |
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408 ------------- |
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409 |
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410 A reentrant lock is a synchronization primitive that may be acquired multiple |
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411 times by the same thread. Internally, it uses the concepts of "owning thread" |
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412 and "recursion level" in addition to the locked/unlocked state used by primitive |
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413 locks. In the locked state, some thread owns the lock; in the unlocked state, |
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414 no thread owns it. |
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415 |
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416 To lock the lock, a thread calls its :meth:`acquire` method; this returns once |
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417 the thread owns the lock. To unlock the lock, a thread calls its |
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418 :meth:`release` method. :meth:`acquire`/:meth:`release` call pairs may be |
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419 nested; only the final :meth:`release` (the :meth:`release` of the outermost |
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420 pair) resets the lock to unlocked and allows another thread blocked in |
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421 :meth:`acquire` to proceed. |
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422 |
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423 |
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424 .. method:: RLock.acquire([blocking=1]) |
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425 |
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426 Acquire a lock, blocking or non-blocking. |
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427 |
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428 When invoked without arguments: if this thread already owns the lock, increment |
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429 the recursion level by one, and return immediately. Otherwise, if another |
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430 thread owns the lock, block until the lock is unlocked. Once the lock is |
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431 unlocked (not owned by any thread), then grab ownership, set the recursion level |
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432 to one, and return. If more than one thread is blocked waiting until the lock |
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433 is unlocked, only one at a time will be able to grab ownership of the lock. |
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434 There is no return value in this case. |
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435 |
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436 When invoked with the *blocking* argument set to true, do the same thing as when |
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437 called without arguments, and return true. |
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438 |
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439 When invoked with the *blocking* argument set to false, do not block. If a call |
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440 without an argument would block, return false immediately; otherwise, do the |
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441 same thing as when called without arguments, and return true. |
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442 |
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443 |
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444 .. method:: RLock.release() |
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445 |
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446 Release a lock, decrementing the recursion level. If after the decrement it is |
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447 zero, reset the lock to unlocked (not owned by any thread), and if any other |
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448 threads are blocked waiting for the lock to become unlocked, allow exactly one |
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449 of them to proceed. If after the decrement the recursion level is still |
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450 nonzero, the lock remains locked and owned by the calling thread. |
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451 |
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452 Only call this method when the calling thread owns the lock. A |
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453 :exc:`RuntimeError` is raised if this method is called when the lock is |
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454 unlocked. |
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455 |
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456 There is no return value. |
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457 |
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458 |
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459 .. _condition-objects: |
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460 |
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461 Condition Objects |
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462 ----------------- |
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463 |
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464 A condition variable is always associated with some kind of lock; this can be |
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465 passed in or one will be created by default. (Passing one in is useful when |
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466 several condition variables must share the same lock.) |
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467 |
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468 A condition variable has :meth:`acquire` and :meth:`release` methods that call |
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469 the corresponding methods of the associated lock. It also has a :meth:`wait` |
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470 method, and :meth:`notify` and :meth:`notifyAll` methods. These three must only |
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471 be called when the calling thread has acquired the lock, otherwise a |
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472 :exc:`RuntimeError` is raised. |
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473 |
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474 The :meth:`wait` method releases the lock, and then blocks until it is awakened |
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475 by a :meth:`notify` or :meth:`notifyAll` call for the same condition variable in |
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476 another thread. Once awakened, it re-acquires the lock and returns. It is also |
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477 possible to specify a timeout. |
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478 |
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479 The :meth:`notify` method wakes up one of the threads waiting for the condition |
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480 variable, if any are waiting. The :meth:`notifyAll` method wakes up all threads |
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481 waiting for the condition variable. |
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482 |
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483 Note: the :meth:`notify` and :meth:`notifyAll` methods don't release the lock; |
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484 this means that the thread or threads awakened will not return from their |
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485 :meth:`wait` call immediately, but only when the thread that called |
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486 :meth:`notify` or :meth:`notifyAll` finally relinquishes ownership of the lock. |
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487 |
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488 Tip: the typical programming style using condition variables uses the lock to |
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489 synchronize access to some shared state; threads that are interested in a |
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490 particular change of state call :meth:`wait` repeatedly until they see the |
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491 desired state, while threads that modify the state call :meth:`notify` or |
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492 :meth:`notifyAll` when they change the state in such a way that it could |
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493 possibly be a desired state for one of the waiters. For example, the following |
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494 code is a generic producer-consumer situation with unlimited buffer capacity:: |
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495 |
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496 # Consume one item |
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497 cv.acquire() |
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498 while not an_item_is_available(): |
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499 cv.wait() |
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500 get_an_available_item() |
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501 cv.release() |
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502 |
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503 # Produce one item |
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504 cv.acquire() |
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505 make_an_item_available() |
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506 cv.notify() |
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507 cv.release() |
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508 |
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509 To choose between :meth:`notify` and :meth:`notifyAll`, consider whether one |
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510 state change can be interesting for only one or several waiting threads. E.g. |
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511 in a typical producer-consumer situation, adding one item to the buffer only |
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512 needs to wake up one consumer thread. |
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513 |
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514 |
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515 .. class:: Condition([lock]) |
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516 |
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517 If the *lock* argument is given and not ``None``, it must be a :class:`Lock` or |
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518 :class:`RLock` object, and it is used as the underlying lock. Otherwise, a new |
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519 :class:`RLock` object is created and used as the underlying lock. |
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520 |
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521 |
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522 .. method:: Condition.acquire(*args) |
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523 |
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524 Acquire the underlying lock. This method calls the corresponding method on the |
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525 underlying lock; the return value is whatever that method returns. |
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526 |
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527 |
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528 .. method:: Condition.release() |
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529 |
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530 Release the underlying lock. This method calls the corresponding method on the |
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531 underlying lock; there is no return value. |
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532 |
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533 |
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534 .. method:: Condition.wait([timeout]) |
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535 |
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536 Wait until notified or until a timeout occurs. If the calling thread has not |
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537 acquired the lock when this method is called, a :exc:`RuntimeError` is raised. |
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538 |
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539 This method releases the underlying lock, and then blocks until it is awakened |
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540 by a :meth:`notify` or :meth:`notifyAll` call for the same condition variable in |
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541 another thread, or until the optional timeout occurs. Once awakened or timed |
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542 out, it re-acquires the lock and returns. |
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543 |
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544 When the *timeout* argument is present and not ``None``, it should be a floating |
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545 point number specifying a timeout for the operation in seconds (or fractions |
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546 thereof). |
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547 |
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548 When the underlying lock is an :class:`RLock`, it is not released using its |
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549 :meth:`release` method, since this may not actually unlock the lock when it was |
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550 acquired multiple times recursively. Instead, an internal interface of the |
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551 :class:`RLock` class is used, which really unlocks it even when it has been |
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552 recursively acquired several times. Another internal interface is then used to |
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553 restore the recursion level when the lock is reacquired. |
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554 |
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555 |
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556 .. method:: Condition.notify() |
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557 |
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558 Wake up a thread waiting on this condition, if any. If the calling thread |
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559 has not acquired the lock when this method is called, a :exc:`RuntimeError` |
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560 is raised. |
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561 |
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562 This method wakes up one of the threads waiting for the condition variable, |
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563 if any are waiting; it is a no-op if no threads are waiting. |
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564 |
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565 The current implementation wakes up exactly one thread, if any are waiting. |
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566 However, it's not safe to rely on this behavior. A future, optimized |
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567 implementation may occasionally wake up more than one thread. |
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568 |
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569 Note: the awakened thread does not actually return from its :meth:`wait` call |
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570 until it can reacquire the lock. Since :meth:`notify` does not release the |
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571 lock, its caller should. |
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572 |
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573 |
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574 .. method:: Condition.notify_all() |
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575 Condition.notifyAll() |
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576 |
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577 Wake up all threads waiting on this condition. This method acts like |
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578 :meth:`notify`, but wakes up all waiting threads instead of one. If the calling |
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579 thread has not acquired the lock when this method is called, a |
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580 :exc:`RuntimeError` is raised. |
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581 |
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582 |
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583 .. _semaphore-objects: |
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584 |
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585 Semaphore Objects |
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586 ----------------- |
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587 |
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588 This is one of the oldest synchronization primitives in the history of computer |
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589 science, invented by the early Dutch computer scientist Edsger W. Dijkstra (he |
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590 used :meth:`P` and :meth:`V` instead of :meth:`acquire` and :meth:`release`). |
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591 |
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592 A semaphore manages an internal counter which is decremented by each |
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593 :meth:`acquire` call and incremented by each :meth:`release` call. The counter |
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594 can never go below zero; when :meth:`acquire` finds that it is zero, it blocks, |
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595 waiting until some other thread calls :meth:`release`. |
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596 |
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597 |
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598 .. class:: Semaphore([value]) |
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599 |
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600 The optional argument gives the initial *value* for the internal counter; it |
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601 defaults to ``1``. If the *value* given is less than 0, :exc:`ValueError` is |
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602 raised. |
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603 |
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604 |
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605 .. method:: Semaphore.acquire([blocking]) |
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606 |
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607 Acquire a semaphore. |
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608 |
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609 When invoked without arguments: if the internal counter is larger than zero on |
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610 entry, decrement it by one and return immediately. If it is zero on entry, |
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611 block, waiting until some other thread has called :meth:`release` to make it |
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612 larger than zero. This is done with proper interlocking so that if multiple |
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613 :meth:`acquire` calls are blocked, :meth:`release` will wake exactly one of them |
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614 up. The implementation may pick one at random, so the order in which blocked |
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615 threads are awakened should not be relied on. There is no return value in this |
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616 case. |
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617 |
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618 When invoked with *blocking* set to true, do the same thing as when called |
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619 without arguments, and return true. |
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620 |
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621 When invoked with *blocking* set to false, do not block. If a call without an |
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622 argument would block, return false immediately; otherwise, do the same thing as |
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623 when called without arguments, and return true. |
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624 |
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625 |
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626 .. method:: Semaphore.release() |
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627 |
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628 Release a semaphore, incrementing the internal counter by one. When it was zero |
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629 on entry and another thread is waiting for it to become larger than zero again, |
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630 wake up that thread. |
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631 |
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632 |
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633 .. _semaphore-examples: |
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634 |
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635 :class:`Semaphore` Example |
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636 ^^^^^^^^^^^^^^^^^^^^^^^^^^ |
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637 |
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638 Semaphores are often used to guard resources with limited capacity, for example, |
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639 a database server. In any situation where the size of the resource size is |
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640 fixed, you should use a bounded semaphore. Before spawning any worker threads, |
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641 your main thread would initialize the semaphore:: |
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642 |
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643 maxconnections = 5 |
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644 ... |
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645 pool_sema = BoundedSemaphore(value=maxconnections) |
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646 |
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647 Once spawned, worker threads call the semaphore's acquire and release methods |
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648 when they need to connect to the server:: |
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649 |
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650 pool_sema.acquire() |
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651 conn = connectdb() |
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652 ... use connection ... |
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653 conn.close() |
|
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654 pool_sema.release() |
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655 |
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656 The use of a bounded semaphore reduces the chance that a programming error which |
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657 causes the semaphore to be released more than it's acquired will go undetected. |
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658 |
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659 |
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660 .. _event-objects: |
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661 |
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662 Event Objects |
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663 ------------- |
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664 |
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665 This is one of the simplest mechanisms for communication between threads: one |
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666 thread signals an event and other threads wait for it. |
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667 |
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668 An event object manages an internal flag that can be set to true with the |
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669 :meth:`set` method and reset to false with the :meth:`clear` method. The |
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670 :meth:`wait` method blocks until the flag is true. |
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671 |
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672 |
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673 .. class:: Event() |
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674 |
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675 The internal flag is initially false. |
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676 |
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677 |
|
47601
8dc4312a2f75
add aliases to threading module
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678 .. method:: Event.is_set() |
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679 Event.isSet() |
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680 |
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681 Return true if and only if the internal flag is true. |
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682 |
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683 |
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684 .. method:: Event.set() |
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685 |
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686 Set the internal flag to true. All threads waiting for it to become true are |
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687 awakened. Threads that call :meth:`wait` once the flag is true will not block at |
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688 all. |
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689 |
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690 |
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691 .. method:: Event.clear() |
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692 |
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693 Reset the internal flag to false. Subsequently, threads calling :meth:`wait` |
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694 will block until :meth:`set` is called to set the internal flag to true again. |
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695 |
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696 |
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697 .. method:: Event.wait([timeout]) |
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698 |
|
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b5aa8aa78c0f
#1674032: return value of flag from Event.wait(). OKed by Guido.
Georg Brandl <georg@python.org>
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|
699 Block until the internal flag is true. If the internal flag is true on entry, |
|
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#1674032: return value of flag from Event.wait(). OKed by Guido.
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|
700 return immediately. Otherwise, block until another thread calls :meth:`set` |
|
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|
701 to set the flag to true, or until the optional timeout occurs. |
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702 |
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|
703 When the timeout argument is present and not ``None``, it should be a floating |
|
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704 point number specifying a timeout for the operation in seconds (or fractions |
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705 thereof). |
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706 |
|
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b5aa8aa78c0f
#1674032: return value of flag from Event.wait(). OKed by Guido.
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parents:
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|
707 This method returns the internal flag on exit, so it will always return |
|
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|
708 ``True`` except if a timeout is given and the operation times out. |
|
b5aa8aa78c0f
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|
709 |
|
b5aa8aa78c0f
#1674032: return value of flag from Event.wait(). OKed by Guido.
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|
710 .. versionchanged:: 2.7 |
|
b5aa8aa78c0f
#1674032: return value of flag from Event.wait(). OKed by Guido.
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|
711 Previously, the method always returned ``None``. |
|
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#1674032: return value of flag from Event.wait(). OKed by Guido.
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|
712 |
|
42604
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|
713 |
|
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|
714 .. _timer-objects: |
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|
715 |
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|
716 Timer Objects |
|
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|
717 ------------- |
|
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718 |
|
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|
719 This class represents an action that should be run only after a certain amount |
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|
720 of time has passed --- a timer. :class:`Timer` is a subclass of :class:`Thread` |
|
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|
721 and as such also functions as an example of creating custom threads. |
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|
722 |
|
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723 Timers are started, as with threads, by calling their :meth:`start` method. The |
|
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|
724 timer can be stopped (before its action has begun) by calling the :meth:`cancel` |
|
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|
725 method. The interval the timer will wait before executing its action may not be |
|
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|
726 exactly the same as the interval specified by the user. |
|
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|
727 |
|
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|
728 For example:: |
|
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|
729 |
|
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|
730 def hello(): |
|
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|
731 print "hello, world" |
|
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|
732 |
|
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|
733 t = Timer(30.0, hello) |
|
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|
734 t.start() # after 30 seconds, "hello, world" will be printed |
|
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735 |
|
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|
736 |
|
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737 .. class:: Timer(interval, function, args=[], kwargs={}) |
|
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738 |
|
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739 Create a timer that will run *function* with arguments *args* and keyword |
|
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740 arguments *kwargs*, after *interval* seconds have passed. |
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741 |
|
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|
742 |
|
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|
743 .. method:: Timer.cancel() |
|
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|
744 |
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745 Stop the timer, and cancel the execution of the timer's action. This will only |
|
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746 work if the timer is still in its waiting stage. |
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|
747 |
|
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|
748 |
|
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|
749 .. _with-locks: |
|
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|
750 |
|
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751 Using locks, conditions, and semaphores in the :keyword:`with` statement |
|
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|
752 ------------------------------------------------------------------------ |
|
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753 |
|
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754 All of the objects provided by this module that have :meth:`acquire` and |
|
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755 :meth:`release` methods can be used as context managers for a :keyword:`with` |
|
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756 statement. The :meth:`acquire` method will be called when the block is entered, |
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757 and :meth:`release` will be called when the block is exited. |
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758 |
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759 Currently, :class:`Lock`, :class:`RLock`, :class:`Condition`, |
|
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760 :class:`Semaphore`, and :class:`BoundedSemaphore` objects may be used as |
|
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761 :keyword:`with` statement context managers. For example:: |
|
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|
762 |
|
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763 import threading |
|
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|
764 |
|
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765 some_rlock = threading.RLock() |
|
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|
766 |
|
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767 with some_rlock: |
|
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|
768 print "some_rlock is locked while this executes" |
|
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|
769 |
|
45743
9395a22a966a
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Georg Brandl <georg@python.org>
parents:
45522
diff
changeset
|
770 |
|
9395a22a966a
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|
771 .. _threaded-imports: |
|
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changeset
|
772 |
|
9395a22a966a
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changeset
|
773 Importing in threaded code |
|
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|
774 -------------------------- |
|
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changeset
|
775 |
|
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changeset
|
776 While the import machinery is thread safe, there are two key |
|
9395a22a966a
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changeset
|
777 restrictions on threaded imports due to inherent limitations in the way |
|
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45522
diff
changeset
|
778 that thread safety is provided: |
|
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changeset
|
779 |
|
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#1720705: add docs about import/threading interaction, wording by Nick.
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|
780 * Firstly, other than in the main module, an import should not have the |
|
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changeset
|
781 side effect of spawning a new thread and then waiting for that thread in |
|
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782 any way. Failing to abide by this restriction can lead to a deadlock if |
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9395a22a966a
#1720705: add docs about import/threading interaction, wording by Nick.
Georg Brandl <georg@python.org>
parents:
45522
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783 the spawned thread directly or indirectly attempts to import a module. |
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9395a22a966a
#1720705: add docs about import/threading interaction, wording by Nick.
Georg Brandl <georg@python.org>
parents:
45522
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784 * Secondly, all import attempts must be completed before the interpreter |
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9395a22a966a
#1720705: add docs about import/threading interaction, wording by Nick.
Georg Brandl <georg@python.org>
parents:
45522
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changeset
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785 starts shutting itself down. This can be most easily achieved by only |
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9395a22a966a
#1720705: add docs about import/threading interaction, wording by Nick.
Georg Brandl <georg@python.org>
parents:
45522
diff
changeset
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786 performing imports from non-daemon threads created through the threading |
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9395a22a966a
#1720705: add docs about import/threading interaction, wording by Nick.
Georg Brandl <georg@python.org>
parents:
45522
diff
changeset
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787 module. Daemon threads and threads created directly with the thread |
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9395a22a966a
#1720705: add docs about import/threading interaction, wording by Nick.
Georg Brandl <georg@python.org>
parents:
45522
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changeset
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788 module will require some other form of synchronization to ensure they do |
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9395a22a966a
#1720705: add docs about import/threading interaction, wording by Nick.
Georg Brandl <georg@python.org>
parents:
45522
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changeset
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789 not attempt imports after system shutdown has commenced. Failure to |
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9395a22a966a
#1720705: add docs about import/threading interaction, wording by Nick.
Georg Brandl <georg@python.org>
parents:
45522
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changeset
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790 abide by this restriction will lead to intermittent exceptions and |
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9395a22a966a
#1720705: add docs about import/threading interaction, wording by Nick.
Georg Brandl <georg@python.org>
parents:
45522
diff
changeset
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791 crashes during interpreter shutdown (as the late imports attempt to |
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9395a22a966a
#1720705: add docs about import/threading interaction, wording by Nick.
Georg Brandl <georg@python.org>
parents:
45522
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changeset
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792 access machinery which is no longer in a valid state). |