\section{\module{Queue} ---         A synchronized queue class} \declaremodule{standard}{Queue}\modulesynopsis{A synchronized queue class.}  The \module{Queue} module implements a multi-producer, multi-consumerFIFO queue.  It is especially useful in threads programming wheninformation must be exchanged safely between multiple threads.  The\class{Queue} class in this module implements all the required lockingsemantics.  It depends on the availability of thread support inPython. The \module{Queue} module defines the following class and exception:  \begin{classdesc}{Queue}{maxsize}Constructor for the class.  \var{maxsize} is an integer that sets theupperbound limit on the number of items that can be placed in thequeue.  Insertion will block once this size has been reached, untilqueue items are consumed.  If \var{maxsize} is less than or equal tozero, the queue size is infinite.\end{classdesc} \begin{excdesc}{Empty}Exception raised when non-blocking \method{get()} (or\method{get_nowait()}) is called on a \class{Queue} object which isempty.\end{excdesc} \begin{excdesc}{Full}Exception raised when non-blocking \method{put()} (or\method{put_nowait()}) is called on a \class{Queue} object which isfull.\end{excdesc} \subsection{Queue Objects}\label{QueueObjects} Class \class{Queue} implements queue objects and has the methodsdescribed below.  This class can be derived from in order to implementother queue organizations (e.g. stack) but the inheritable interfaceis not described here.  See the source code for details.  The publicmethods are: \begin{methoddesc}{qsize}{}Return the approximate size of the queue.  Because of multithreadingsemantics, this number is not reliable.\end{methoddesc} \begin{methoddesc}{empty}{}Return \code{True} if the queue is empty, \code{False} otherwise.Because of multithreading semantics, this is not reliable.\end{methoddesc} \begin{methoddesc}{full}{}Return \code{True} if the queue is full, \code{False} otherwise.Because of multithreading semantics, this is not reliable.\end{methoddesc} \begin{methoddesc}{put}{item\optional{, block\optional{, timeout}}}Put \var{item} into the queue. If optional args \var{block} is trueand \var{timeout} is None (the default), block if necessary until afree slot is available. If \var{timeout} is a positive number, itblocks at most \var{timeout} seconds and raises the \exception{Full}exception if no free slot was available within that time.Otherwise (\var{block} is false), put an item on the queue if a freeslot is immediately available, else raise the \exception{Full}exception (\var{timeout} is ignored in that case). \versionadded[the timeout parameter]{2.3} \end{methoddesc} \begin{methoddesc}{put_nowait}{item}Equivalent to \code{put(\var{item}, False)}.\end{methoddesc} \begin{methoddesc}{get}{\optional{block\optional{, timeout}}}Remove and return an item from the queue. If optional args\var{block} is true and \var{timeout} is None (the default),block if necessary until an item is available. If \var{timeout} isa positive number, it blocks at most \var{timeout} seconds and raisesthe \exception{Empty} exception if no item was available within thattime. Otherwise (\var{block} is false), return an item if one isimmediately available, else raise the \exception{Empty} exception(\var{timeout} is ignored in that case). \versionadded[the timeout parameter]{2.3} \end{methoddesc} \begin{methoddesc}{get_nowait}{}Equivalent to \code{get(False)}.\end{methoddesc} Two methods are offered to support tracking whether enqueued tasks havebeen fully processed by daemon consumer threads. \begin{methoddesc}{task_done}{}Indicate that a formerly enqueued task is complete.  Used by queue consumerthreads.  For each \method{get()} used to fetch a task, a subsequent call to\method{task_done()} tells the queue that the processing on the task is complete. If a \method{join()} is currently blocking, it will resume when all itemshave been processed (meaning that a \method{task_done()} call was receivedfor every item that had been \method{put()} into the queue). Raises a \exception{ValueError} if called more times than there were itemsplaced in the queue.\versionadded{2.5}\end{methoddesc} \begin{methoddesc}{join}{}Blocks until all items in the queue have been gotten and processed. The count of unfinished tasks goes up whenever an item is added to thequeue. The count goes down whenever a consumer thread calls \method{task_done()}to indicate that the item was retrieved and all work on it is complete.When the count of unfinished tasks drops to zero, join() unblocks.\versionadded{2.5}\end{methoddesc} Example of how to wait for enqueued tasks to be completed: \begin{verbatim}    def worker():         while True:             item = q.get()             do_work(item)             q.task_done()      q = Queue()     for i in range(num_worker_threads):          t = Thread(target=worker)         t.setDaemon(True)         t.start()      for item in source():        q.put(item)      q.join()       # block until all tasks are done\end{verbatim}