Threads in python
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This document provides a summary of threads in Python. It begins by defining what a thread is and how it allows for multitasking by time-division multiplexing the processor between threads. It then discusses how to start new threads in Python using the thread and threading modules, including examples. It also covers how to create threads that subclass the Thread class and how to synchronize threads using locks.
![Python Threading:
●
●
●
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] )
●
●
The method call returns immediately and the child thread starts and calls function with the
passed list of args. 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.](https://image.slidesharecdn.com/threadsinpython-131116024153-phpapp01/85/Threads-in-python-9-320.jpg)
![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.](https://image.slidesharecdn.com/threadsinpython-131116024153-phpapp01/85/Threads-in-python-13-320.jpg)
![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 will in turn call run() method.](https://image.slidesharecdn.com/threadsinpython-131116024153-phpapp01/85/Threads-in-python-14-320.jpg)
![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 = []](https://image.slidesharecdn.com/threadsinpython-131116024153-phpapp01/85/Threads-in-python-18-320.jpg)
Threads in python
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- 6. Introduction What are threads? ● ● A thread is a light-weight process A thread of execution is the smallest sequence of programmed instructions that can be managed independently by an operating system scheduler process Time A process with two threads of execution on a single processor. ● for example copying a file and showing the progress, or playing a music while showing some pictures as a slideshow, or a listener handles cancel event for aborting a file copy operation
- 9. Python Threading: ● ● ● 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] ) ● ● The method call returns immediately and the child thread starts and calls function with the passed list of args. 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.
- 10. 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
- 11. When the above code is executed, it produces the following result: Thread-1: Tue Sep 3 10:10:38 2013 Thread-2: Tue Sep 3 10:10:40 2013 Thread-1: Tue Sep 3 10:10:40 2013 Thread-1: Tue Sep 3 10:10:42 2013 Thread-1: Tue Sep 3 10:10:44 2013 Thread-2: Tue Sep 3 10:10:44 2013 Thread-1: Tue Sep 3 10:10:46 2013 Thread-2: Tue Sep 3 10:10:48 2013 Thread-2: Tue Sep 3 10:10:52 2013 Thread-2: Tue Sep 3 10:10:56 2013
- 13. 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.
- 14. 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 will in turn call run() method.
- 15. 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: thread.exit() time.sleep(delay) print "%s: %s" % (threadName, time.ctime(time.time())) counter -= 1
- 16. # 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: Thu Mar 21 Thread-1: Thu Mar 21 Thread-2: Thu Mar 21 Thread-1: Thu Mar 21 Thread-1: Thu Mar 21 Thread-2: Thu Mar 21 Thread-1: Thu Mar 21 Exiting Thread-1 Thread-2: Thu Mar 21 Thread-2: Thu Mar 21 Thread-2: Thu Mar 21 Exiting Thread-2 09:10:03 09:10:04 09:10:04 09:10:05 09:10:06 09:10:06 09:10:07 2013 2013 2013 2013 2013 2013 2013 09:10:08 2013 09:10:10 2013 09:10:12 2013
- 17. Synchronizing Threads: ● ● ● ● ● ● The threading module provided with Python includes a simple-toimplement 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 of 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.
- 18. 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 = []
- 19. # 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: Thu Mar 21 Thread-1: Thu Mar 21 Thread-1: Thu Mar 21 Thread-2: Thu Mar 21 Thread-2: Thu Mar 21 Thread-2: Thu Mar 21 Exiting Main Thread 09:11:28 09:11:29 09:11:30 09:11:32 09:11:34 09:11:36 2013 2013 2013 2013 2013 2013
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