Python - Starting a Thread

In Python, starting a thread involves using the start() method provided by the Thread class in the threading module. This method initiates the thread's activity and automatically calls its run() method in a separate thread of execution. Meaning that, when you call start() on each thread object (for example., thread1, thread2, thread3) to initiate their execution.

Python to launch separate threads that concurrently execute the run() method defined in each Thread instance. And the main thread continues its execution after starting the child threads.

In this tutorial, you will see a detailed explanation and example of how to use the start() method effectively in multi-threaded programming to understand its behavior in multi-thread applications.

Starting a Thread in Python

The start() method is fundamental for beginning the execution of a thread. It sets up the thread's environment and schedules it to run. Importantly, it should only be called once per Thread object. If this method is called more than once on the same Thread object, it will raise a RuntimeError.

Here is the syntax for using the start() method on a Thread object −

threading.thread.start()

Example

let's see the below example, that demonstrates how to start a new thread in Python using the start() method.

from threading import Thread
from time import sleep

def my_function(arg):
   for i in range(arg):
      print("child Thread running", i)
      sleep(0.5)
thread = Thread(target = my_function, args = (10, ))
thread.start()
print("thread finished...exiting")

When the above code is executed, it produces the following result

child Thread running 0
thread finished...exiting
child Thread running 1
child Thread running 2
child Thread running 3
child Thread running 4
child Thread running 5
child Thread running 6
child Thread running 7
child Thread running 8
child Thread running 9

Example

Here is another example demonstrating the working of the start() method. You can observe that, by not calling the start() method on thread2, it remains inactive and does not begin execution.

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)
      print_time(self.name, self.counter)
      print("Exiting " + self.name)

def print_time(threadName, counter):
   while counter:
      time.sleep(1)
      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)
thread3 = MyThread(3, "Thread-3", 3)

# Start new Threads
thread1.start()
thread3.start()

print("Exiting Main Thread")

The above code will produce the following output −

Starting Thread-1
Starting Thread-3
Exiting Main Thread
Thread-1: Mon Jun 24 18:24:59 2024
Exiting Thread-1
Thread-3: Mon Jun 24 18:24:59 2024
Thread-3: Mon Jun 24 18:25:00 2024
Thread-3: Mon Jun 24 18:25:01 2024
Exiting Thread-3