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# Python - Multithreaded Programming

Running several threads is similar to running several different programs concurrently, but with the following benefits:

- Multiple threads within a process share the same data space with the main thread and can therefore share information or communicate with each other more easily than if they were separate processes.

- Threads sometimes called light-weight processes and they do not require much memory overhead; theycare cheaper than processes.

A thread has a beginning, an execution sequence, and a conclusion. It has an instruction pointer that keeps track of where within its context it is currently running.

- It can be pre-empted (interrupted)

- It can temporarily be put on hold (also known as sleeping) while other threads are running - this is called yielding.

# Starting a New Thread:

To spawn another thread, you need to call following method available in thread module:

    thread.start_new_thread ( function, args[, kwargs] )

This method call enables a fast and efficient way to create new threads in both Linux and Windows.

The method call returns immediately and the child thread starts and calls function with the passed list of agrs. When function returns, the thread terminates.

Here args is a tuple of arguments; use an empty tuple to call function without passing any arguments. kwargs is an optional dictionary of keyword arguments.

## Example:

    #!/usr/bin/python

    import thread
    import time

    # Define a function for the thread
    def print_time( threadName, delay):
       count = 0
       while count < 5:
          time.sleep(delay)
          count += 1
          print "%s: %s" % ( threadName, time.ctime(time.time()) )

    # Create two threads as follows
    try:
       thread.start_new_thread( print_time, ("Thread-1", 2, ) )
       thread.start_new_thread( print_time, ("Thread-2", 4, ) )
    except:
       print "Error: unable to start thread"

    while 1:
       pass

This would produce following result:

    Thread-1: Thu Jan 22 15:42:17 2009
    Thread-1: Thu Jan 22 15:42:19 2009
    Thread-2: Thu Jan 22 15:42:19 2009
    Thread-1: Thu Jan 22 15:42:21 2009
    Thread-2: Thu Jan 22 15:42:23 2009
    Thread-1: Thu Jan 22 15:42:23 2009
    Thread-1: Thu Jan 22 15:42:25 2009
    Thread-2: Thu Jan 22 15:42:27 2009
    Thread-2: Thu Jan 22 15:42:31 2009
    Thread-2: Thu Jan 22 15:42:35 2009

Although it is very effective for low-level threading, but the thread module is very limited compared to the newer threading module.

# The Threading Module:

The newer threading module included with Python 2.4 provides much more powerful, high-level support for threads than the thread module discussed in the previous section.

The threading module exposes all the methods of the thread module and provides some additional methods:

- threading.activeCount(): Returns the number of thread objects that are active.

- threading.currentThread(): Returns the number of thread objects in the caller's thread control.

- threading.enumerate(): Returns a list of all thread objects that are currently active.

In addition to the methods, the threading module has the Thread class that implements threading. The methods provided by the Thread class are as follows:

- run(): The run() method is the entry point for a thread.

- start(): The start() method starts a thread by calling the run method.

- join([time]): The join() waits for threads to terminate.

- isAlive(): The isAlive() method checks whether a thread is still executing.

- getName(): The getName() method returns the name of a thread.

- setName(): The setName() method sets the name of a thread.

## Creating Thread using Threading Module:

To implement a new thread using the threading module, you have to do the following:

- Define a new subclass of the Thread class.

- Override the __init__(self [,args]) method to add additional arguments.

- Then override the run(self [,args]) method to implement what the thread should do when started.

Once you have created the new Thread subclass, you can create an instance of it and then start a new thread by invoking the start() or run() methods.

## Example:

    #!/usr/bin/python

    import threading
    import time

    exitFlag = 0

    class myThread (threading.Thread):
        def __init__(self, threadID, name, counter):
            self.threadID = threadID
            self.name = name
            self.counter = counter
            threading.Thread.__init__(self)
        def run(self):
            print "Starting " + self.name
            print_time(self.name, self.counter, 5)
            print "Exiting " + self.name

    def print_time(threadName, delay, counter):
        while counter:
            if exitFlag:
                thread.exit()
            time.sleep(delay)
            print "%s: %s" % (threadName, time.ctime(time.time()))
            counter -= 1

    # Create new threads
    thread1 = myThread(1, "Thread-1", 1)
    thread2 = myThread(2, "Thread-2", 2)

    # Start new Threads
    thread1.start()
    thread2.run()

    while thread2.isAlive():
        if not thread1.isAlive():
            exitFlag = 1
        pass
    print "Exiting Main Thread"

This would produce following result:

    Starting Thread-2
    Starting Thread-1
    Thread-1: Thu Jan 22 15:53:05 2009
    Thread-2: Thu Jan 22 15:53:06 2009
    Thread-1: Thu Jan 22 15:53:06 2009
    Thread-1: Thu Jan 22 15:53:07 2009
    Thread-2: Thu Jan 22 15:53:08 2009
    Thread-1: Thu Jan 22 15:53:08 2009
    Thread-1: Thu Jan 22 15:53:09 2009
    Exiting Thread-1
    Thread-2: Thu Jan 22 15:53:10 2009
    Thread-2: Thu Jan 22 15:53:12 2009
    Thread-2: Thu Jan 22 15:53:14 2009
    Exiting Thread-2
    Exiting Main Thread

# Synchronizing Threads:

The threading module provided with Python includes a simple-to-implement locking mechanism that will allow you to synchronize threads. A new lock is created by calling the Lock() method, which returns the new lock.

The acquire(blocking) method the new lock object would be used to force threads to run synchronously. The optional blocking parameter enables you to control whether the thread will wait to acquire the lock.

If blocking is set to 0, the thread will return immediately with a 0 value if the lock cannot be acquired and with a 1 if the lock was acquired. If blocking is set to 1, the thread will block and wait for the lock to be released.

The release() method of the the new lock object would be used to release the lock when it is no longer required.

## Example:

    #!/usr/bin/python

    import threading
    import time

    class myThread (threading.Thread):
        def __init__(self, threadID, name, counter):
            self.threadID = threadID
            self.name = name
            self.counter = counter
            threading.Thread.__init__(self)
        def run(self):
            print "Starting " + self.name
            # Get lock to synchronize threads
            threadLock.acquire()
            print_time(self.name, self.counter, 3)
            # Free lock to release next thread
            threadLock.release()

    def print_time(threadName, delay, counter):
        while counter:
            time.sleep(delay)
            print "%s: %s" % (threadName, time.ctime(time.time()))
            counter -= 1

    threadLock = threading.Lock()
    threads = []

    # Create new threads
    thread1 = myThread(1, "Thread-1", 1)
    thread2 = myThread(2, "Thread-2", 2)

    # Start new Threads
    thread1.start()
    thread2.start()

    # Add threads to thread list
    threads.append(thread1)
    threads.append(thread2)

    # Wait for all threads to complete
    for t in threads:
        t.join()
    print "Exiting Main Thread"

This would produce following result:

    Starting Thread-1
    Starting Thread-2
    Thread01: Thu Jan 22 16:04:38 2009
    Thread01: Thu Jan 22 16:04:39 2009
    Thread01: Thu Jan 22 16:04:40 2009
    Thread02: Thu Jan 22 16:04:42 2009
    Thread02: Thu Jan 22 16:04:44 2009
    Thread02: Thu Jan 22 16:04:46 2009
    Exiting Main Thread

# Multithreaded Priority Queue:

The Queue module allows you to create a new queue object that can hold a specific number of items. There are following methods to control the Queue:

- get(): The get() removes and returns an item from the queue.

- put(): The put adds item to a queue.

- qsize() : The qsize() returns the number of items that are currently in the queue.

- empty(): The empty( ) returns True if queue is empty; otherwise, False.

- full(): the full() returns True if queue is full; otherwise, False.

## Example:

    #!/usr/bin/python

    import Queue
    import threading
    import time

    exitFlag = 0

    class myThread (threading.Thread):
        def __init__(self, threadID, name, q):
            self.threadID = threadID
            self.name = name
            self.q = q
            threading.Thread.__init__(self)
        def run(self):
            print "Starting " + self.name
            process_data(self.name, self.q)
            print "Exiting " + self.name

    def process_data(threadName, q):
        while not exitFlag:
            queueLock.acquire()
            if not workQueue.empty():
                data = q.get()
                queueLock.release()
                print "%s processing %s" % (threadName, data)
            else:
                queueLock.release()
            time.sleep(1)

    threadList = ["Thread-1", "Thread-2", "Thread-3"]
    nameList = ["One", "Two", "Three", "Four", "Five"]
    queueLock = threading.Lock()
    workQueue = Queue.Queue(10)
    threads = []
    threadID = 1

    # Create new threads
    for tName in threadList:
        thread = myThread(threadID, tName, workQueue)
        thread.start()
        threads.append(thread)
        threadID += 1

    # Fill the queue
    queueLock.acquire()
    for word in nameList:
        workQueue.put(word)
    queueLock.release()

    # Wait for queue to empty
    while not workQueue.empty():
        pass

    # Notify threads it's time to exit
    exitFlag = 1

    # Wait for all threads to complete
    for t in threads:
        t.join()
    print "Exiting Main Thread"

This would produce following result:

    Starting Thread-2
    Starting Thread-1
    Starting Thread-3
    Thread-2 processing One
    Thread-1 processing Two
    Thread-3 processing Three
    Thread-2 processing Four
    Thread-1 processing Five
    Exiting Thread-3
    Exiting Thread-2
    Exiting Thread-1
    Exiting Main Thread

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