1 :mod:`Queue` --- A synchronized queue class 2 =========================================== 3 4 .. module:: Queue 5 :synopsis: A synchronized queue class. 6 7 .. note:: 8 The :mod:`Queue` module has been renamed to :mod:`queue` in Python 3. The 9 :term:`2to3` tool will automatically adapt imports when converting your 10 sources to Python 3. 11 12 **Source code:** :source:`Lib/Queue.py` 13 14 -------------- 15 16 The :mod:`Queue` module implements multi-producer, multi-consumer queues. 17 It is especially useful in threaded programming when information must be 18 exchanged safely between multiple threads. The :class:`~Queue.Queue` class in this 19 module implements all the required locking semantics. It depends on the 20 availability of thread support in Python; see the :mod:`threading` 21 module. 22 23 The module implements three types of queue, which differ only in the order in 24 which the entries are retrieved. In a FIFO queue, the first tasks added are 25 the first retrieved. In a LIFO queue, the most recently added entry is 26 the first retrieved (operating like a stack). With a priority queue, 27 the entries are kept sorted (using the :mod:`heapq` module) and the 28 lowest valued entry is retrieved first. 29 30 The :mod:`Queue` module defines the following classes and exceptions: 31 32 .. class:: Queue(maxsize=0) 33 34 Constructor for a FIFO queue. *maxsize* is 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 LIFO queue. *maxsize* is an integer that sets the upperbound 42 limit on the number of items that can be placed in the queue. Insertion will 43 block once this size has been reached, until queue items are consumed. If 44 *maxsize* is less than or equal to zero, the queue size is infinite. 45 46 .. versionadded:: 2.6 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 .. versionadded:: 2.6 60 61 .. exception:: Empty 62 63 Exception raised when non-blocking :meth:`~Queue.get` (or 64 :meth:`~Queue.get_nowait`) is called 65 on a :class:`~Queue.Queue` object which is empty. 66 67 68 .. exception:: Full 69 70 Exception raised when non-blocking :meth:`~Queue.put` (or 71 :meth:`~Queue.put_nowait`) is called 72 on a :class:`~Queue.Queue` object which is full. 73 74 .. seealso:: 75 76 :class:`collections.deque` is an alternative implementation of unbounded 77 queues with fast atomic :func:`append` and :func:`popleft` operations that 78 do not require locking. 79 80 81 .. _queueobjects: 82 83 Queue Objects 84 ------------- 85 86 Queue objects (:class:`~Queue.Queue`, :class:`LifoQueue`, or :class:`PriorityQueue`) 87 provide the public methods described below. 88 89 90 .. method:: Queue.qsize() 91 92 Return the approximate size of the queue. Note, qsize() > 0 doesn't 93 guarantee that a subsequent get() will not block, nor will qsize() < maxsize 94 guarantee that put() will not block. 95 96 97 .. method:: Queue.empty() 98 99 Return ``True`` if the queue is empty, ``False`` otherwise. If empty() 100 returns ``True`` it doesn't guarantee that a subsequent call to put() 101 will not block. Similarly, if empty() returns ``False`` it doesn't 102 guarantee that a subsequent call to get() will not block. 103 104 105 .. method:: Queue.full() 106 107 Return ``True`` if the queue is full, ``False`` otherwise. If full() 108 returns ``True`` it doesn't guarantee that a subsequent call to get() 109 will not block. Similarly, if full() returns ``False`` it doesn't 110 guarantee that a subsequent call to put() will not block. 111 112 113 .. method:: Queue.put(item[, block[, timeout]]) 114 115 Put *item* into the queue. If optional args *block* is true and *timeout* is 116 ``None`` (the default), block if necessary until a free slot is available. If 117 *timeout* is a positive number, it blocks at most *timeout* seconds and raises 118 the :exc:`Full` exception if no free slot was available within that time. 119 Otherwise (*block* is false), put an item on the queue if a free slot is 120 immediately available, else raise the :exc:`Full` exception (*timeout* is 121 ignored in that case). 122 123 .. versionadded:: 2.3 124 The *timeout* parameter. 125 126 127 .. method:: Queue.put_nowait(item) 128 129 Equivalent to ``put(item, False)``. 130 131 132 .. method:: Queue.get([block[, timeout]]) 133 134 Remove and return an item from the queue. If optional args *block* is true and 135 *timeout* is ``None`` (the default), block if necessary until an item is available. 136 If *timeout* is a positive number, it blocks at most *timeout* seconds and 137 raises the :exc:`Empty` exception if no item was available within that time. 138 Otherwise (*block* is false), return an item if one is immediately available, 139 else raise the :exc:`Empty` exception (*timeout* is ignored in that case). 140 141 .. versionadded:: 2.3 142 The *timeout* parameter. 143 144 145 .. method:: Queue.get_nowait() 146 147 Equivalent to ``get(False)``. 148 149 Two methods are offered to support tracking whether enqueued tasks have been 150 fully processed by daemon consumer threads. 151 152 153 .. method:: Queue.task_done() 154 155 Indicate that a formerly enqueued task is complete. Used by queue consumer 156 threads. For each :meth:`get` used to fetch a task, a subsequent call to 157 :meth:`task_done` tells the queue that the processing on the task is complete. 158 159 If a :meth:`join` is currently blocking, it will resume when all items have been 160 processed (meaning that a :meth:`task_done` call was received for every item 161 that had been :meth:`put` into the queue). 162 163 Raises a :exc:`ValueError` if called more times than there were items placed in 164 the queue. 165 166 .. versionadded:: 2.5 167 168 169 .. method:: Queue.join() 170 171 Blocks until all items in the queue have been gotten and processed. 172 173 The count of unfinished tasks goes up whenever an item is added to the queue. 174 The count goes down whenever a consumer thread calls :meth:`task_done` to 175 indicate that the item was retrieved and all work on it is complete. When the 176 count of unfinished tasks drops to zero, :meth:`join` unblocks. 177 178 .. versionadded:: 2.5 179 180 Example of how to wait for enqueued tasks to be completed:: 181 182 def worker(): 183 while True: 184 item = q.get() 185 do_work(item) 186 q.task_done() 187 188 q = Queue() 189 for i in range(num_worker_threads): 190 t = Thread(target=worker) 191 t.daemon = True 192 t.start() 193 194 for item in source(): 195 q.put(item) 196 197 q.join() # block until all tasks are done 198 199
