Multi Tasking:
Executing several tasks simultaneously is the concept of multitasking.
There are 2 types of Multi-Tasking
- Process-based Multi-Tasking
- Thread based Multi-Tasking
1. Process-based Multi-Tasking:
Executing several tasks simultaneously where each task is a separate independent process is called process-based multi-tasking.
Eg: while typing a python program in the editor we can listen mp3 audio songs from the same system. At the same time we can download a file from the internet. All these taks are executing simultaneously and independent of each other. Hence it is process based multi tasking.
This type of multi tasking is best suitable at operating system level.
2. Thread based MultiTasking:
Executing several tasks simultaneously where each task is a seperate independent part of the same program, is called Thread based multi tasking, and each independent part is called a Thread.
This type of multi tasking is best suitable at programmatic level.
The main important application areas of multi threading are:
- To implement Multimedia graphics
- To develop animations
- To develop video games
- To develop web and application servers etc..
Python provides one inbuilt module "threading" to provide support for developing threads. Hence developing multi threaded Programs is very easy in python.
Every Python Program by default contains one thread which is nothing but MainThread.
Q.Program to print name of current executing thread:
1) import threading
2) print("Current Executing Thread:",threading.current_thread().getName())
o/p: Current Executing Thread: MainThread
The ways of Creating Thread in Python:
We can create a thread in Python by using 3 ways
- Creating a Thread without using any class
- Creating a Thread by extending Thread class
- Creating a Thread without extending Thread class
1. Creating a Thread without using any class:
1) from threading import *
2) def display():
3) for i in range(1,11):
4) print("Child Thread")
5) t=Thread(target=display) #creating Thread object
6) t.start() #starting of Thread
7) for i in range(1,11):
8) print("Main Thread") If multiple threads present in our program, then we cannot expect execution order and hence we cannot expect exact output for the multi threaded programs. B'z of this we cannot provide exact output for the above program.It is varied from machine to machine and run to run.
2. Creating a Thread by extending Thread class
We have to create child class for Thread class. In that child class we have to override run() method with our required job. Whenever we call start() method then automatically run() method will be executed and performs our job.
1) from threading import *
2) class MyThread(Thread):
3) def run(self):
4) for i in range(10):
5) print("Child Thread-1")
6) t=MyThread()
7) t.start()
8) for i in range(10):
9) print("Main Thread-1") 3. Creating a Thread without extending Thread class:
1) from threading import *
2) class Test:
3) def display(self):
4) for i in range(10):
5) print("Child Thread-2")
6) obj=Test()
7) t=Thread(target=obj.display)
8) t.start()
9) for i in range(10):
10) print("Main Thread-2") Setting and Getting Name of a Thread:
Every thread in python has name. It may be default name generated by Python or Customized Name provided by programmer.
We can get and set name of thread by using the following Thread class methods.
t.getName() Returns Name of Thread
t.setName(newName) To set our own name
Note: Every Thread has implicit variable "name" to represent name of Thread.
Eg:
1) from threading import *
2) print(current_thread().getName())
3) current_thread().setName("Pawan Kalyan")
4) print(current_thread().getName())
5) print(current_thread().name) Output: MainThread Pawan Kalyan Pawan Kalyan
Thread Identification Number (ident):
For every thread internally a unique identification number is available. We can access this id by using implicit variable "ident"
1) from threading import *
2) def test():
3) print("Child Thread")
4) t=Thread(target=test)
5) t.start()
6) print("Main Thread Identification Number:",current_thread().ident)
7) print("Child Thread Identification Number:",t.ident) Output:
Child Thread Main Thread Identification Number: 2492 Child Thread Identification Number: 2768
enumerate() function:
This function returns a list of all active threads currently running.
Eg:
1) from threading import *
2) import time
3) def display():
4) print(current_thread().getName(),"...started")
5) time.sleep(3)
6) print(current_thread().getName(),"...ended")
7) t1=Thread(target=display,name="ChildThread1")
8) t2=Thread(target=display,name="ChildThread2")
9) t3=Thread(target=display,name="ChildThread3")
10) t1.start()
11) t2.start()
12) t3.start()
13) l=enumerate()
14) for t in l:
15) print("Thread Name:",t.name)
16) time.sleep(5)
17) l=enumerate()
18) for t in l:
19) print("Thread Name:",t.name) Output:
D:\python_classes>py test.py ChildThread1 ...started ChildThread2 ...started ChildThread3 ...started Thread Name: MainThread Thread Name: ChildThread1 Thread Name: ChildThread2 Thread Name: ChildThread3 ChildThread1 ...ended ChildThread2 ...ended ChildThread3 ...ended Thread Name: MainThread
isAlive():
isAlive() method checks whether a thread is still executing or not.
Eg:
1) from threading import * 2) import time 3) def display(): 4) print(current_thread().getName(),"...started") 5) time.sleep(3) 6) print(current_thread().getName(),"...ended") 7) t1=Thread(target=display,name="ChildThread1") 8) t2=Thread(target=display,name="ChildThread2") 9) t1.start() 10) t2.start() 11) 12) print(t1.name,"is Alive :",t1.isAlive()) 13) print(t2.name,"is Alive :",t2.isAlive()) 14) time.sleep(5) 15) print(t1.name,"is Alive :",t1.isAlive()) 16) print(t2.name,"is Alive :",t2.isAlive())
Output:
D:\python_classes>py test.py ChildThread1 ...started ChildThread2 ...started ChildThread1 is Alive : True ChildThread2 is Alive : True ChildThread1 ...ended ChildThread2 ...ended ChildThread1 is Alive : False ChildThread2 is Alive : False
join() method:
If a thread wants to wait until completing some other thread then we should go for join() method.
Eg:
1) from threading import *
2) import time
3) def display():
4) for i in range(10):
5) print("Seetha Thread")
6) time.sleep(2)
7)
8) t=Thread(target=display)
9) t.start()
10) t.join()#This Line executed by Main Thread
11) for i in range(10):
12) print("Rama Thread") In the above example Main Thread waited until completing child thread. In this case output is:
Seetha Thread Seetha Thread Seetha Thread Seetha Thread Seetha Thread Seetha Thread Seetha Thread Seetha Thread Seetha Thread Seetha Thread Rama Thread Rama Thread Rama Thread Rama Thread Rama Thread Rama Thread Rama Thread Rama Thread Rama Thread Rama Thread
Python: Daemon Threads
The threads which are running in the background are called Daemon Threads.
The main objective of Daemon Threads is to provide support for Non Daemon Threads( like main thread)
Eg: Garbage Collector
Whenever Main Thread runs with low memory, immediately PVM runs Garbage Collector to destroy useless objects and to provide free memory,so that Main Thread can continue its execution without having any memory problems.
We can check whether thread is Daemon or not by using t.isDaemon() method of Thread class or by using daemon property.
Eg:
1) from threading import * 2) print(current_thread().isDaemon()) #False 3) print(current_thread().daemon) #False
We can change Daemon nature by using setDaemon() method of Thread class.
t.setDaemon(True)
But we can use this method before starting of Thread.i.e once thread started,we cannot change its Daemon nature,otherwise we will get
RuntimeException:cannot set daemon status of active thread
Eg:
1) from threading import * 2) print(current_thread().isDaemon()) 3) current_thread().setDaemon(True)
RuntimeError: cannot set daemon status of active thread
Default Nature:
By default Main Thread is always non-daemon.But for the remaining threads Daemon nature will be inherited from parent to child.i.e if the Parent Thread is Daemon then child thread is also Daemon and if the Parent Thread is Non Daemon then ChildThread is also Non Daemon.
Eg:
1) from threading import *
2) def job():
3) print("Child Thread")
4) t=Thread(target=job)
5) print(t.isDaemon())#False
6) t.setDaemon(True)
7) print(t.isDaemon()) #True
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