1 :mod:`queue` --- A synchronized queue class 2 =========================================== 3 4 .. module:: queue 5 :synopsis: A synchronized queue class. 6 7 **Source code:** :source:`Lib/queue.py` 8 9 -------------- 10 11 The :mod:`queue` module implements multi-producer, multi-consumer queues. 12 It is especially useful in threaded programming when information must be 13 exchanged safely between multiple threads. The :class:`Queue` class in this 14 module implements all the required locking semantics. It depends on the 15 availability of thread support in Python; see the :mod:`threading` 16 module. 17 18 The module implements three types of queue, which differ only in the order in 19 which the entries are retrieved. In a :abbr:`FIFO (first-in, first-out)` 20 queue, the first tasks added are the first retrieved. In a 21 :abbr:`LIFO (last-in, first-out)` queue, the most recently added entry is 22 the first retrieved (operating like a stack). With a priority queue, 23 the entries are kept sorted (using the :mod:`heapq` module) and the 24 lowest valued entry is retrieved first. 25 26 Internally, the module uses locks to temporarily block competing threads; 27 however, it is not designed to handle reentrancy within a thread. 28 29 The :mod:`queue` module defines the following classes and exceptions: 30 31 .. class:: Queue(maxsize=0) 32 33 Constructor for a :abbr:`FIFO (first-in, first-out)` queue. *maxsize* is 34 an integer that sets the upperbound 35 limit on the number of items that can be placed in the queue. Insertion will 36 block once this size has been reached, until queue items are consumed. If 37 *maxsize* is less than or equal to zero, the queue size is infinite. 38 39 .. class:: LifoQueue(maxsize=0) 40 41 Constructor for a :abbr:`LIFO (last-in, first-out)` queue. *maxsize* is 42 an integer that sets the upperbound 43 limit on the number of items that can be placed in the queue. Insertion will 44 block once this size has been reached, until queue items are consumed. If 45 *maxsize* is less than or equal to zero, the queue size is infinite. 46 47 48 .. class:: PriorityQueue(maxsize=0) 49 50 Constructor for a priority queue. *maxsize* is an integer that sets the upperbound 51 limit on the number of items that can be placed in the queue. Insertion will 52 block once this size has been reached, until queue items are consumed. If 53 *maxsize* is less than or equal to zero, the queue size is infinite. 54 55 The lowest valued entries are retrieved first (the lowest valued entry is the 56 one returned by ``sorted(list(entries))[0]``). A typical pattern for entries 57 is a tuple in the form: ``(priority_number, data)``. 58 59 60 .. exception:: Empty 61 62 Exception raised when non-blocking :meth:`~Queue.get` (or 63 :meth:`~Queue.get_nowait`) is called 64 on a :class:`Queue` object which is empty. 65 66 67 .. exception:: Full 68 69 Exception raised when non-blocking :meth:`~Queue.put` (or 70 :meth:`~Queue.put_nowait`) is called 71 on a :class:`Queue` object which is full. 72 73 74 .. _queueobjects: 75 76 Queue Objects 77 ------------- 78 79 Queue objects (:class:`Queue`, :class:`LifoQueue`, or :class:`PriorityQueue`) 80 provide the public methods described below. 81 82 83 .. method:: Queue.qsize() 84 85 Return the approximate size of the queue. Note, qsize() > 0 doesn't 86 guarantee that a subsequent get() will not block, nor will qsize() < maxsize 87 guarantee that put() will not block. 88 89 90 .. method:: Queue.empty() 91 92 Return ``True`` if the queue is empty, ``False`` otherwise. If empty() 93 returns ``True`` it doesn't guarantee that a subsequent call to put() 94 will not block. Similarly, if empty() returns ``False`` it doesn't 95 guarantee that a subsequent call to get() will not block. 96 97 98 .. method:: Queue.full() 99 100 Return ``True`` if the queue is full, ``False`` otherwise. If full() 101 returns ``True`` it doesn't guarantee that a subsequent call to get() 102 will not block. Similarly, if full() returns ``False`` it doesn't 103 guarantee that a subsequent call to put() will not block. 104 105 106 .. method:: Queue.put(item, block=True, timeout=None) 107 108 Put *item* into the queue. If optional args *block* is true and *timeout* is 109 ``None`` (the default), block if necessary until a free slot is available. If 110 *timeout* is a positive number, it blocks at most *timeout* seconds and raises 111 the :exc:`Full` exception if no free slot was available within that time. 112 Otherwise (*block* is false), put an item on the queue if a free slot is 113 immediately available, else raise the :exc:`Full` exception (*timeout* is 114 ignored in that case). 115 116 117 .. method:: Queue.put_nowait(item) 118 119 Equivalent to ``put(item, False)``. 120 121 122 .. method:: Queue.get(block=True, timeout=None) 123 124 Remove and return an item from the queue. If optional args *block* is true and 125 *timeout* is ``None`` (the default), block if necessary until an item is available. 126 If *timeout* is a positive number, it blocks at most *timeout* seconds and 127 raises the :exc:`Empty` exception if no item was available within that time. 128 Otherwise (*block* is false), return an item if one is immediately available, 129 else raise the :exc:`Empty` exception (*timeout* is ignored in that case). 130 131 132 .. method:: Queue.get_nowait() 133 134 Equivalent to ``get(False)``. 135 136 Two methods are offered to support tracking whether enqueued tasks have been 137 fully processed by daemon consumer threads. 138 139 140 .. method:: Queue.task_done() 141 142 Indicate that a formerly enqueued task is complete. Used by queue consumer 143 threads. For each :meth:`get` used to fetch a task, a subsequent call to 144 :meth:`task_done` tells the queue that the processing on the task is complete. 145 146 If a :meth:`join` is currently blocking, it will resume when all items have been 147 processed (meaning that a :meth:`task_done` call was received for every item 148 that had been :meth:`put` into the queue). 149 150 Raises a :exc:`ValueError` if called more times than there were items placed in 151 the queue. 152 153 154 .. method:: Queue.join() 155 156 Blocks until all items in the queue have been gotten and processed. 157 158 The count of unfinished tasks goes up whenever an item is added to the queue. 159 The count goes down whenever a consumer thread calls :meth:`task_done` to 160 indicate that the item was retrieved and all work on it is complete. When the 161 count of unfinished tasks drops to zero, :meth:`join` unblocks. 162 163 164 Example of how to wait for enqueued tasks to be completed:: 165 166 def worker(): 167 while True: 168 item = q.get() 169 if item is None: 170 break 171 do_work(item) 172 q.task_done() 173 174 q = queue.Queue() 175 threads = [] 176 for i in range(num_worker_threads): 177 t = threading.Thread(target=worker) 178 t.start() 179 threads.append(t) 180 181 for item in source(): 182 q.put(item) 183 184 # block until all tasks are done 185 q.join() 186 187 # stop workers 188 for i in range(num_worker_threads): 189 q.put(None) 190 for t in threads: 191 t.join() 192 193 194 .. seealso:: 195 196 Class :class:`multiprocessing.Queue` 197 A queue class for use in a multi-processing (rather than multi-threading) 198 context. 199 200 :class:`collections.deque` is an alternative implementation of unbounded 201 queues with fast atomic :meth:`~collections.deque.append` and 202 :meth:`~collections.deque.popleft` operations that do not require locking. 203 204
