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testCopysign was defined twice in test_math; combine the definitions
author Mark Dickinson <dickinsm@gmail.com>
date Sat, 06 Feb 2010 23:11:25 +0000
parents bdc390721f97
children ad2f5eacd2a0
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1 :mod:`queue` --- A synchronized queue class
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2 ===========================================
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3
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4 .. module:: Queue
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5 :synopsis: A synchronized queue class.
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6
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7 .. note::
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8 The :mod:`Queue` module has been renamed to :mod:`queue` in Python 3.0. The
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9 :term:`2to3` tool will automatically adapt imports when converting your
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10 sources to 3.0.
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12
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13 The :mod:`Queue` module implements multi-producer, multi-consumer queues.
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14 It is especially useful in threaded programming when information must be
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15 exchanged safely between multiple threads. The :class:`Queue` class in this
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16 module implements all the required locking semantics. It depends on the
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17 availability of thread support in Python; see the :mod:`threading`
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18 module.
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19
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20 Implements three types of queue whose only difference is the order that
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21 the entries are retrieved. In a FIFO queue, the first tasks added are
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22 the first retrieved. In a LIFO queue, the most recently added entry is
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23 the first retrieved (operating like a stack). With a priority queue,
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24 the entries are kept sorted (using the :mod:`heapq` module) and the
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25 lowest valued entry is retrieved first.
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26
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27 The :mod:`Queue` module defines the following classes and exceptions:
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28
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29 .. class:: Queue(maxsize)
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30
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31 Constructor for a FIFO queue. *maxsize* is an integer that sets the upperbound
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32 limit on the number of items that can be placed in the queue. Insertion will
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33 block once this size has been reached, until queue items are consumed. If
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34 *maxsize* is less than or equal to zero, the queue size is infinite.
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35
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36 .. class:: LifoQueue(maxsize)
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37
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38 Constructor for a LIFO queue. *maxsize* is an integer that sets the upperbound
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39 limit on the number of items that can be placed in the queue. Insertion will
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40 block once this size has been reached, until queue items are consumed. If
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41 *maxsize* is less than or equal to zero, the queue size is infinite.
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42
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43 .. versionadded:: 2.6
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44
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45 .. class:: PriorityQueue(maxsize)
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46
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47 Constructor for a priority queue. *maxsize* is an integer that sets the upperbound
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48 limit on the number of items that can be placed in the queue. Insertion will
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49 block once this size has been reached, until queue items are consumed. If
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50 *maxsize* is less than or equal to zero, the queue size is infinite.
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51
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52 The lowest valued entries are retrieved first (the lowest valued entry is the
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53 one returned by ``sorted(list(entries))[0]``). A typical pattern for entries
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54 is a tuple in the form: ``(priority_number, data)``.
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56 .. versionadded:: 2.6
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57
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58 .. exception:: Empty
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59
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60 Exception raised when non-blocking :meth:`get` (or :meth:`get_nowait`) is called
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61 on a :class:`Queue` object which is empty.
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62
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63
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64 .. exception:: Full
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65
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66 Exception raised when non-blocking :meth:`put` (or :meth:`put_nowait`) is called
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67 on a :class:`Queue` object which is full.
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68
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69 .. seealso::
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70
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71 :class:`collections.deque` is an alternative implementation of unbounded
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72 queues with fast atomic :func:`append` and :func:`popleft` operations that
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73 do not require locking.
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74
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75
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76 .. _queueobjects:
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77
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78 Queue Objects
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79 -------------
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80
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81 Queue objects (:class:`Queue`, :class:`LifoQueue`, or :class:`PriorityQueue`)
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82 provide the public methods described below.
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84
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85 .. method:: Queue.qsize()
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86
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87 Return the approximate size of the queue. Note, qsize() > 0 doesn't
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88 guarantee that a subsequent get() will not block, nor will qsize() < maxsize
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89 guarantee that put() will not block.
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90
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91
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92 .. method:: Queue.empty()
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93
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94 Return ``True`` if the queue is empty, ``False`` otherwise. If empty()
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95 returns ``True`` it doesn't guarantee that a subsequent call to put()
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96 will not block. Similarly, if empty() returns ``False`` it doesn't
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97 guarantee that a subsequent call to get() will not block.
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98
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99
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100 .. method:: Queue.full()
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101
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102 Return ``True`` if the queue is full, ``False`` otherwise. If full()
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103 returns ``True`` it doesn't guarantee that a subsequent call to get()
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104 will not block. Similarly, if full() returns ``False`` it doesn't
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105 guarantee that a subsequent call to put() will not block.
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106
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107
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108 .. method:: Queue.put(item[, block[, timeout]])
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109
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110 Put *item* into the queue. If optional args *block* is true and *timeout* is
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111 None (the default), block if necessary until a free slot is available. If
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112 *timeout* is a positive number, it blocks at most *timeout* seconds and raises
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113 the :exc:`Full` exception if no free slot was available within that time.
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114 Otherwise (*block* is false), put an item on the queue if a free slot is
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115 immediately available, else raise the :exc:`Full` exception (*timeout* is
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116 ignored in that case).
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117
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118 .. versionadded:: 2.3
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119 The *timeout* parameter.
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120
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121
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122 .. method:: Queue.put_nowait(item)
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123
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124 Equivalent to ``put(item, False)``.
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125
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126
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127 .. method:: Queue.get([block[, timeout]])
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128
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129 Remove and return an item from the queue. If optional args *block* is true and
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130 *timeout* is None (the default), block if necessary until an item is available.
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131 If *timeout* is a positive number, it blocks at most *timeout* seconds and
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132 raises the :exc:`Empty` exception if no item was available within that time.
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133 Otherwise (*block* is false), return an item if one is immediately available,
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134 else raise the :exc:`Empty` exception (*timeout* is ignored in that case).
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135
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136 .. versionadded:: 2.3
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137 The *timeout* parameter.
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138
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139
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140 .. method:: Queue.get_nowait()
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141
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142 Equivalent to ``get(False)``.
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143
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144 Two methods are offered to support tracking whether enqueued tasks have been
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145 fully processed by daemon consumer threads.
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148 .. method:: Queue.task_done()
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149
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150 Indicate that a formerly enqueued task is complete. Used by queue consumer
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151 threads. For each :meth:`get` used to fetch a task, a subsequent call to
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152 :meth:`task_done` tells the queue that the processing on the task is complete.
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153
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154 If a :meth:`join` is currently blocking, it will resume when all items have been
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155 processed (meaning that a :meth:`task_done` call was received for every item
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156 that had been :meth:`put` into the queue).
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157
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158 Raises a :exc:`ValueError` if called more times than there were items placed in
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159 the queue.
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160
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161 .. versionadded:: 2.5
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162
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163
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164 .. method:: Queue.join()
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165
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166 Blocks until all items in the queue have been gotten and processed.
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167
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168 The count of unfinished tasks goes up whenever an item is added to the queue.
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169 The count goes down whenever a consumer thread calls :meth:`task_done` to
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170 indicate that the item was retrieved and all work on it is complete. When the
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171 count of unfinished tasks drops to zero, :meth:`join` unblocks.
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172
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173 .. versionadded:: 2.5
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174
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175 Example of how to wait for enqueued tasks to be completed::
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176
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177 def worker():
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178 while True:
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179 item = q.get()
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180 do_work(item)
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181 q.task_done()
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182
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183 q = Queue()
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184 for i in range(num_worker_threads):
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185 t = Thread(target=worker)
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186 t.daemon = True
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187 t.start()
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188
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189 for item in source():
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190 q.put(item)
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191
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192 q.join() # block until all tasks are done
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193