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annotate Doc/lib/libqueue.tex @ 39940:dfd809dcf680 2.5

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Bug #1446043: correctly raise a LookupError if an encoding name given to encodings.search_function() contains a dot. (backport from rev. 52075)
author Georg Brandl <georg@python.org>
date Sat, 30 Sep 2006 11:22:35 +0000
parents bed3f52f0dc6
children 06145fbc7ab9
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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1
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2 \section{\module{Queue} ---
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3 A synchronized queue class}
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4
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5 \declaremodule{standard}{Queue}
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6 \modulesynopsis{A synchronized queue class.}
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7
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9 The \module{Queue} module implements a multi-producer, multi-consumer
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10 FIFO queue. It is especially useful in threads programming when
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11 information must be exchanged safely between multiple threads. The
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12 \class{Queue} class in this module implements all the required locking
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13 semantics. It depends on the availability of thread support in
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14 Python.
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15
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16 The \module{Queue} module defines the following class and exception:
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18
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19 \begin{classdesc}{Queue}{maxsize}
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20 Constructor for the class. \var{maxsize} is an integer that sets the
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21 upperbound limit on the number of items that can be placed in the
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22 queue. Insertion will block once this size has been reached, until
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23 queue items are consumed. If \var{maxsize} is less than or equal to
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24 zero, the queue size is infinite.
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25 \end{classdesc}
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26
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27 \begin{excdesc}{Empty}
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28 Exception raised when non-blocking \method{get()} (or
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29 \method{get_nowait()}) is called on a \class{Queue} object which is
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30 empty.
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31 \end{excdesc}
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32
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33 \begin{excdesc}{Full}
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34 Exception raised when non-blocking \method{put()} (or
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35 \method{put_nowait()}) is called on a \class{Queue} object which is
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36 full.
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37 \end{excdesc}
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38
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39 \subsection{Queue Objects}
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40 \label{QueueObjects}
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41
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42 Class \class{Queue} implements queue objects and has the methods
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43 described below. This class can be derived from in order to implement
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44 other queue organizations (e.g. stack) but the inheritable interface
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45 is not described here. See the source code for details. The public
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46 methods are:
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47
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48 \begin{methoddesc}{qsize}{}
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49 Return the approximate size of the queue. Because of multithreading
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50 semantics, this number is not reliable.
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51 \end{methoddesc}
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52
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53 \begin{methoddesc}{empty}{}
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54 Return \code{True} if the queue is empty, \code{False} otherwise.
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55 Because of multithreading semantics, this is not reliable.
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56 \end{methoddesc}
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57
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58 \begin{methoddesc}{full}{}
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59 Return \code{True} if the queue is full, \code{False} otherwise.
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60 Because of multithreading semantics, this is not reliable.
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61 \end{methoddesc}
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62
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63 \begin{methoddesc}{put}{item\optional{, block\optional{, timeout}}}
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64 Put \var{item} into the queue. If optional args \var{block} is true
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65 and \var{timeout} is None (the default), block if necessary until a
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66 free slot is available. If \var{timeout} is a positive number, it
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67 blocks at most \var{timeout} seconds and raises the \exception{Full}
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68 exception if no free slot was available within that time.
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69 Otherwise (\var{block} is false), put an item on the queue if a free
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70 slot is immediately available, else raise the \exception{Full}
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71 exception (\var{timeout} is ignored in that case).
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72
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73 \versionadded[the timeout parameter]{2.3}
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74
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75 \end{methoddesc}
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76
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77 \begin{methoddesc}{put_nowait}{item}
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78 Equivalent to \code{put(\var{item}, False)}.
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79 \end{methoddesc}
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80
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81 \begin{methoddesc}{get}{\optional{block\optional{, timeout}}}
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82 Remove and return an item from the queue. If optional args
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83 \var{block} is true and \var{timeout} is None (the default),
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84 block if necessary until an item is available. If \var{timeout} is
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85 a positive number, it blocks at most \var{timeout} seconds and raises
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86 the \exception{Empty} exception if no item was available within that
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87 time. Otherwise (\var{block} is false), return an item if one is
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88 immediately available, else raise the \exception{Empty} exception
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89 (\var{timeout} is ignored in that case).
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90
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91 \versionadded[the timeout parameter]{2.3}
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92
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93 \end{methoddesc}
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94
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95 \begin{methoddesc}{get_nowait}{}
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96 Equivalent to \code{get(False)}.
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97 \end{methoddesc}
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98
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99 Two methods are offered to support tracking whether enqueued tasks have
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100 been fully processed by daemon consumer threads.
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101
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102 \begin{methoddesc}{task_done}{}
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103 Indicate that a formerly enqueued task is complete. Used by queue consumer
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104 threads. For each \method{get()} used to fetch a task, a subsequent call to
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105 \method{task_done()} tells the queue that the processing on the task is complete.
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106
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107 If a \method{join()} is currently blocking, it will resume when all items
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108 have been processed (meaning that a \method{task_done()} call was received
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109 for every item that had been \method{put()} into the queue).
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110
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111 Raises a \exception{ValueError} if called more times than there were items
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112 placed in the queue.
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113 \versionadded{2.5}
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114 \end{methoddesc}
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115
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116 \begin{methoddesc}{join}{}
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117 Blocks until all items in the queue have been gotten and processed.
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118
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119 The count of unfinished tasks goes up whenever an item is added to the
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120 queue. The count goes down whenever a consumer thread calls \method{task_done()}
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121 to indicate that the item was retrieved and all work on it is complete.
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122 When the count of unfinished tasks drops to zero, join() unblocks.
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123 \versionadded{2.5}
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124 \end{methoddesc}
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125
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126 Example of how to wait for enqueued tasks to be completed:
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127
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128 \begin{verbatim}
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129 def worker():
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130 while True:
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131 item = q.get()
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132 do_work(item)
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133 q.task_done()
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134
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135 q = Queue()
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136 for i in range(num_worker_threads):
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137 t = Thread(target=worker)
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138 t.setDaemon(True)
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139 t.start()
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140
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141 for item in source():
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142 q.put(item)
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143
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144 q.join() # block until all tasks are done
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145 \end{verbatim}