Multithreading¶
Running several threads is similar to running several different programs concurrently
Advantages :
Multiple threads within a process share the same data space with the main thread
Share information
communicate with each other
A Thread has :
Beginning,
Execution sequence
Conclusion
Salient Features :
IT has an instruction pointer ( keep track of context)
It can be pre-empted (interrupted)
It can temporarily be put on hold (yielding)
Kinds of Threads :
Kernel thread (part of operating system)
User thread
Modules supported in Python3
_thread (backward compatible)
threading
Starting a New Thread¶
To spawn another thread, you need to call the following method
_thread.start_new_thread ( function, args[, kwargs] )
Example
#!/usr/bin/python3
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
Output:
Thread-1: Wed Sep 15 11:23:04 2021
Thread-1: Wed Sep 15 11:23:06 2021
Thread-2: Wed Sep 15 11:23:06 2021
Thread-1: Wed Sep 15 11:23:08 2021
Thread-1: Wed Sep 15 11:23:10 2021
Thread-2: Wed Sep 15 11:23:10 2021
Thread-1: Wed Sep 15 11:23:12 2021
Thread-2: Wed Sep 15 11:23:14 2021
Thread-2: Wed Sep 15 11:23:18 2021
Thread-2: Wed Sep 15 11:23:22 2021
Program goes in an infinite loop. You will have to press ctrl-c to stop
Threading Module¶
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.
The threading module has the Thread class that implements threading methods provided by the Thread class
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¶
Steps to implement a new thread using the threading module
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
Then start a new thread by invoking the start(), which in turn calls the run() method
Example
#!/usr/bin/python3
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()
thread1.join()
thread2.join()
print ("Exiting Main Thread")
Output:
Starting Thread-1
Starting Thread-2
Thread-1: Wed Sep 15 11:34:21 2021
Thread-2: Wed Sep 15 11:34:22 2021
Thread-1: Wed Sep 15 11:34:22 2021
Thread-1: Wed Sep 15 11:34:23 2021
Thread-2: Wed Sep 15 11:34:24 2021
Thread-1: Wed Sep 15 11:34:24 2021
Thread-1: Wed Sep 15 11:34:25 2021
Exiting Thread-1
Thread-2: Wed Sep 15 11:34:26 2021
Thread-2: Wed Sep 15 11:34:28 2021
Thread-2: Wed Sep 15 11:34:30 2021
Exiting Thread-2
Exiting Main Thread
Synchronizing Threads¶
The threading module has locking mechanism that allows you to synchronize threads.
Steps to implement synchronization using the threading module
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 the 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/python3
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")
Output:
Starting Thread-1
Starting Thread-2
Thread-1: Wed Sep 15 11:42:28 2021
Thread-1: Wed Sep 15 11:42:29 2021
Thread-1: Wed Sep 15 11:42:30 2021
Thread-2: Wed Sep 15 11:42:32 2021
Thread-2: Wed Sep 15 11:42:34 2021
Thread-2: Wed Sep 15 11:42:36 2021
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
Methods to control 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/python3
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")
Output:
Starting Thread-1
Starting Thread-2
Starting Thread-3
Thread-3 processing One
Thread-3 processing Two
Thread-3 processing Three
Thread-3 processing Four
Thread-3 processing Five
Exiting Thread-1
Exiting Thread-3
Exiting Thread-2
Exiting Main Thread