Friday, 18 March 2016

Sprint 1: Python Multithreaded Programming by Siang Wei Lee

Sprint 1: Python Multithreaded Programming by Siang Wei Lee

 

Team: PI over Four

Author: Siang Wei Lee

Date: 1/3/2016


Introduction:
During week 2 of sprint 1, I was making analysis on the Python multithreaded programming. Multithreaded has a benefit that it can share the same data space with the main thread within a process. Besides, it also share information or communicate with each other more easily than if they were separate processes. The information below is showing some of the basic of python multithreaded programming.

 


Starting a New Thread

To spawn another thread, you need to call following method available in threadmodule:
thread.start_new_thread ( function, args[, kwargs] )
This method call enables a fast and efficient way to create new threads and also 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
When the above code is executed, it produces the following result.
Thread-1: Fri Jan 22 10:23:16 2016
Thread-1: Fri Jan 22 10:23:18 2016
Thread-2: Fri Jan 22 10:23:19 2016
Thread-1: Fri Jan 22 10:23:21 2016
Thread-2: Fri Jan 22 10:23:24 2016
Thread-1: Fri Jan 22 10:23:26 2016
Thread-1: Fri Jan 22 10:23:29 2016
Thread-2: Fri Jan 22 10:23:31 2016
Thread-2: Fri Jan 22 10:23:35 2016
Thread-2: Fri Jan 22 10:23:39 2016
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 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(), which in turn calls run()method.

Example

#!/usr/bin/python
import threading
import time
exitFlag = 0
 
class myThread (threading.Thread):
    def __init__(self, threadID, name, counter):
        threading.Thread.__init__(self)
        self.threadID = threadID
        self.name = name
        self.counter = counter
    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:
            threadName.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.start()
 
print "Exiting Main Thread"
When the above code is executed, it produces the following result.
Starting Thread-1
Starting Thread-2
Exiting Main Thread
Thread-1: Fri Jan 22 11:10:05 2016
Thread-1: Fri Jan 22 11:10:06 2016
Thread-2: Fri Jan 22 11:10:06 2016
Thread-1: Fri Jan 22 11:10:07 2016
Thread-1: Fri Jan 22 11:10:09 2016
Thread-2: Fri Jan 22 11:10:10 2016
Thread-1: Fri Jan 22 11:10:10 2016
Exiting Thread-1
Thread-2: Fri Jan 22 11:10:11 2016
Thread-2: Fri Jan 22 11:10:12 2016
Thread-2: Fri Jan 22 11:10:12 2016
Exiting Thread-2



Synchronizing Threads

The threading module provided with Python includes a simple-to-implement locking mechanism that allows you to synchronize threads. A new lock is created by calling the Lock() method, which returns the new lock.
The acquire(blocking) method of the new lock object is used to force threads to run synchronously. The optional blocking parameter enables you to control whether the thread waits to acquire the lock.
If blocking is set to 0, the thread returns 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 blocks and wait for the lock to be released.
The release() method of the new lock object is 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):
        threading.Thread.__init__(self)
        self.threadID = threadID
        self.name = name
        self.counter = counter
    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"
When the above code is executed, it produces the following result.
Starting Thread-1
Starting Thread-2
Thread-1: Fri Jan 22 11:50:28 2016
Thread-1: Fri Jan 22 11:50:29 2013
Thread-1: Fri Jan 22 11:50:31 2013
Thread-2: Fri Jan 22 11:50:33 2013
Thread-2: Fri Jan 22 11:50:34 2013
Thread-2: Fri Jan 22 11:50:36 2013
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):
        threading.Thread.__init__(self)
        self.threadID = threadID
        self.name = name
        self.q = q
    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"
When the above code is executed, it produces the following result −
Starting Thread-1
Starting Thread-2
Starting Thread-3
Thread-1 processing One
Thread-2 processing Two
Thread-3 processing Three
Thread-1 processing Four
Thread-2 processing Five
Exiting Thread-3
Exiting Thread-1
Exiting Thread-2
Exiting Main Thread


By Siang Wei Lee       # 2

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