Python asyncio stop task

Stopping asyncio Tasks in Python: A Comprehensive Guide

Asynchronous programming has become increasingly popular for Python developers, especially for those working with web frameworks and networking applications. The async/await syntax and asyncio library are common tools used to implement asynchronous programming in Python. However, it is essential to understand how to stop asyncio tasks effectively to avoid problems with performance and resource allocation.

In this article, we will explore practical techniques for stopping asyncio tasks in Python. We will discuss why it is necessary to end tasks appropriately, various methods to stop asyncio tasks, and the considerations you should keep in mind when implementing these methods.

Why Do We Need to Stop Asyncio Tasks?

Asyncio is a cooperative multitasking framework, meaning that it relies on the cooperation of other tasks to operate effectively. If a task is not explicitly terminated, it will continue to execute, even if it has finished its job, effectively hogging resources and slowing down the application. Therefore, it is crucial to terminate asyncio tasks to prevent them from consuming excessive resources and degrading performance.

How to Stop Asyncio Tasks?

There are several methods to stop asyncio tasks in Python, depending on the situation and the task’s state. Let’s take a look at some of the most widely used methods:

Using Task.cancel() Method

The most straightforward method to stop an asyncio task is to use the Task.cancel() method. It sends a cancellation request to the task and sets its cancelled attribute to True . The task can then catch the CancelledError exception and perform any necessary cleanup operations.

import asyncio async def my_coroutine(): try: while True: print('Still working. ') await asyncio.sleep(1) except asyncio.CancelledError: print('Cancelled!') async def main(): task = asyncio.create_task(my_coroutine()) await asyncio.sleep(5) task.cancel() await task asyncio.run(main()) 

The my_coroutine() function is an infinite loop that prints a message every second. After waiting for five seconds, we cancel the task using Task.cancel() . The output will look like this:

Still working. Still working. Still working. Still working. Still working. Cancelled! 

Note how the CancelledError exception is caught inside the my_coroutine() function, allowing us to perform any necessary cleanup operations.

Using Task.cancelled() Attribute

Sometimes, we want to check whether a task has been cancelled successfully. We can use the Task.cancelled() attribute to do this. It returns True if the task has been cancelled, and False otherwise.

import asyncio async def my_coroutine(): try: while True: print('Still working. ') await asyncio.sleep(1) except asyncio.CancelledError: print('Cancelled!') async def main(): task = asyncio.create_task(my_coroutine()) await asyncio.sleep(5) task.cancel() await asyncio.sleep(1) print(f'Task cancelled: ') await task asyncio.run(main()) 

After four seconds of execution, the Task.cancelled() method returns True , indicating that the task has been cancelled. The output will look like this:

Still working. Still working. Still working. Still working. Still working. Cancelled! Task cancelled: True 

Using asyncio.wait_for() Function

The asyncio.wait_for() function allows us to set a timeout for an asyncio task. If the specified time elapses, the function raises a asyncio.exceptions.TimeoutError exception, and the task is stopped.

import asyncio async def my_coroutine(): try: while True: print('Still working. ') await asyncio.sleep(1) except asyncio.CancelledError: print('Cancelled!') async def main(): task = asyncio.create_task(my_coroutine()) try: await asyncio.wait_for(task, timeout=5) except asyncio.exceptions.TimeoutError: task.cancel() await task asyncio.run(main()) 

The my_coroutine() function is the same as before, but this time we set a five-second timeout using asyncio.wait_for() . If the task takes more than five seconds to execute, the function raises a TimeoutError , and we cancel the task with Task.cancel() . The output will look like this:

Still working. Still working. Still working. Still working. Still working. Cancelled! 

Using asyncio.shield() Function

The asyncio.shield() function allows us to protect a task from cancellation. It creates a new asyncio task that executes the original task and shields it from cancellation requests. If the original task is cancelled, the shielded task will continue to run.

import asyncio async def my_coroutine(): try: while True: print('Still working. ') await asyncio.sleep(1) except asyncio.CancelledError: print('Cancelled!') async def main(): task = asyncio.create_task(my_coroutine()) shielded_task = asyncio.shield(task) await asyncio.sleep(5) task.cancel() await shielded_task asyncio.run(main()) 

The my_coroutine() function is the same as before, but this time we create a shielded task using asyncio.shield() . After five seconds, we cancel the original task, but the shielded task will continue to run. The output will look like this:

Still working. Still working. Still working. Still working. Still working. Cancelled! 

Considerations When Stopping Asyncio Tasks

When stopping an asyncio task, certain considerations must be kept in mind to avoid potential issues. Here are some essential considerations:

• Always use the appropriate method to stop a task, depending on the situation.
• Make sure that the task’s code catches the CancelledError exception and performs any necessary cleanup operations.
• If a task is blocking, consider using a separate thread or process to perform the blocking operation.
• Use shields and timeouts judiciously and only when necessary.
• Be aware that cancelling a task can have unintended consequences, such as data corruption or inconsistent internal state.

Conclusion

Stopping asyncio tasks in Python is a critical aspect of writing efficient and scalable asynchronous applications. In this article, we explored various methods to stop asyncio tasks, such as using Task.cancel() , Task.cancelled() , asyncio.wait_for() , and asyncio.shield() . We also discussed essential considerations to keep in mind when implementing these methods. By following these best practices, Python developers can ensure that their asyncio tasks are stopped effectively, avoiding issues with performance and resource allocation.

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Cancelling Tasks

Summary: in this tutorial, you’ll learn how to cancel a long-running asynchronous operation that may take forever to complete.

The following statement uses the await statement to wait for a task to be complete:

task = asyncio.create_task(coroutine()) result = await taskCode language: Python (python)

However, if the coroutine() took forever, you would be stuck waiting for the await statement to finish without obtaining any result. Additionally, you would have no way to stop it if you wanted to.

To resolve this, you can cancel the task using the cancel() method of the Task object. If you cancel a task, it’ll raise the CancelledError exception when you await it. For example:

import asyncio from asyncio import CancelledError async def call_api(message, result=1000, delay=3): print(message) await asyncio.sleep(delay) return result async def main(): task = asyncio.create_task( call_api('Calling API. ', result=2000, delay=5) ) if not task.done(): print('Cancelling the task. ') task.cancel() try: await task except CancelledError: print('Task has been cancelled.') asyncio.run(main())Code language: Python (python)

Cancelling the task. Task has been cancelledCode language: Python (python)

First, the call_api() coroutine prints a message, delays 3 seconds, and returns the result.

Second, create a new task using the create_task() function and pass the call_api() coroutine. The task will take 5 seconds to complete:

task = asyncio.create_task( call_api('Calling API. ', result=2000, delay=5) )Code language: Python (python)

Third, check if the task is not done by calling the done() method and cancel the task using the cancel() method:

if not task.done(): print('Cancelling the task. ') task.cancel()Code language: Python (python)

Finally, wait for the task to be completed using the await keyword. Since the task has been canceled, the CancelledError exception is raised:

try: await task except CancelledError: print('Task has been cancelled.')Code language: Python (python)

If you want to check every second if a task has been completed and cancel it if an amount of time has passed, you can use a while loop:

import asyncio from asyncio import CancelledError async def call_api(message, result=1000, delay=3): print(message) await asyncio.sleep(delay) return result async def main(): task = asyncio.create_task( call_api('Calling API. ', result=2000, delay=5) ) time_elapsed = 0 while not task.done(): time_elapsed += 1 await asyncio.sleep(1) print('Task has not completed, checking again in a second') if time_elapsed == 3: print('Cancelling the task. ') task.cancel() break try: await task except CancelledError: print('Task has been cancelled.') asyncio.run(main())Code language: Python (python)

In this example, the while loop checks if the task has been completed every second and cancels the task once the elapsed time reaches 3 seconds.

Summary

• Use the cancel() method of the Task object to cancel a task
• await a canceled task will raise a CancelledError exception.

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