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Python Thread Class | start() Method with Example

Python Thread.start() Method: Here, we are going to learn about the start() method of Thread class in Python with its definition, syntax, and examples.
Submitted by Hritika Rajput, on May 07, 2020

Python Thread.start() Method

Thread.start() method is an inbuilt method of the Thread class of the threading module in Python. It is used to start a thread's activity. This method calls the run() method internally which then executes the target method. This method must be called at most one time for one thread. If it is called more than once, it raises a RuntimeError.

Module:

    from threading import Thread

Syntax:

    start()

Parameter(s):

  • None

Return value:

The return type of this method is <class 'NoneType'>, it does not return anything.

Example:

# Python program to explain the
# use of start() method in Thread class

import time
import threading

def thread_1(i):
    time.sleep(5)
    print('Value by Thread 1:', i)

def thread_2(i):
    print('Value by Thread 2:', i)
    
# Creating two sample threads 
thread1 = threading.Thread(target=thread_1, args=(1,))
thread2 = threading.Thread(target=thread_2, args=(2,))

# Starting two threads
thread1.start()
thread2.start()

Output

Value by Thread 2: 2
Value by Thread 1: 1

Example:

# Python program to explain the
# use of start() method in Thread class

import threading

def thread_1(i):
    print('Value by Thread 1:', i)

def thread_2(i):
    print('Value by Thread 2:', i)

def thread_3(i):
    print('Value by Thread 3:', i)    

    
# Creating three sample threads 
thread1 = threading.Thread(target=thread_1, args=(1,))
thread2 = threading.Thread(target=thread_2, args=(2,))
thread3 = threading.Thread(target=thread_3, args=(3,))

# Starting three threads
thread1.start()
thread2.start()
thread3.start()
    
thread1.start()

Output

Value by Thread 1: 1
Value by Thread 2: 2
Value by Thread 3: 3
Traceback (most recent call last):
  File "main.py", line 26, in <module>
    thread1.start()
  File "/usr/lib/python3.8/threading.py", line 848, in start
    raise RuntimeError("threads can only be started once")
RuntimeError: threads can only be started once
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