Canceling tasks in Python

In asynchronous programming, it is often necessary to cancel tasks that are no longer needed or that are taking too long to complete. Canceling tasks in Python requires a good understanding of asynchronous programming concepts and techniques, as well as knowledge of the built-in asyncio module and other related libraries. In this tutorial, we will explore the basics of canceling tasks in Python, including the different ways to cancel tasks, handling exceptions, and best practices to follow.

Creating a Long-Running Task :

• In order to demonstrate how to cancel a long-running asynchronous task, we need to create a long-running task first.
• Here's an example of a co-routine that waits for five seconds before returning
• Code Sample : 

async def long_operation():
   print('Long operation started')
   await asyncio.sleep(5)
   print('Long operation completed')
   return 'Long operation result'
   
   
   

How to cancel an Asynchronous Task in python :

• We can cancel an asynchronous task using the   cancel()  method of the task object.
• Here's an example of how to create a task for our  long_operation()  co-routine and then cancel it after three seconds :
• Code Sample : 

import asyncio

async def long_operation():
   print('Long operation started')
   await asyncio.sleep(5)
   print('Long operation completed')
   return 'Long operation result'

async def main():
   print('Main started')
   long_op_task = asyncio.create_task(long_operation())
   await asyncio.sleep(3)
   long_op_task.cancel()
   print('Long operation cancelled')
   try:
       await long_op_task
   except asyncio.CancelledError:
       print('Long operation was cancelled')
   else:
       print('Long operation result:', long_op_task.result())
   print('Main completed')


asyncio.run(main())

Explanation : 

• In the above example, we define a co-routine,  long_operation() , that waits for five seconds before returning.
• We also define a  main()  co-routine that creates a task for  long_operation()  and then cancels it after three seconds using the  cancel()  method of the task object.
• We use a try/except block to catch the  CancelledError  that is raised when the task is cancelled.
• When we run this program, it will execute the  long_operation()  co-routine in the background, but cancel it after three seconds.
• The program will then print "Long operation cancelled" and "Long operation was cancelled".