13.11. AsyncIO Shield

• Shielding from Cancellation

• Awaitable asyncio.shield(aw)

• Protect an awaitable object from being cancelled.

In Python, asyncio is a module that provides tools for writing asynchronous code using coroutines. The asyncio.shield() function is a utility function that is used to protect a coroutine from being cancelled.

When a coroutine is cancelled in Python, it raises a CancelledError `` exception. This can be problematic if the coroutine is in the middle of performing an important task that should not be interrupted. The asyncio ``.shield() function can be used to prevent the coroutine from being cancelled until it has completed its task.

Here is an example of how to use asyncio.shield():

>>> import asyncio
>>>
>>> async def my_coroutine():
...     await asyncio.sleep(1)
...     print('Task complete')
>>>
>>> async def main():
...     task = asyncio.create_task(my_coroutine())
...     await asyncio.sleep(0.5)
...     asyncio.shield(task)
...     print('Task shielded')
>>>
>>> asyncio.run(main())
Task shielded

In this example, the my_coroutine function is a coroutine that sleeps for 1 second and then prints "Task complete!". The main function creates a task from my_coroutine and then sleeps for 0.5 second before calling asyncio.shield() on the task. The print statement after asyncio.shield() is executed immediately, while the coroutine continues to run in the background.

If the coroutine were to be cancelled before it completes its task, the asyncio.shield() function would prevent the cancellation from occurring until the task is finished. This ensures that the task is completed even if the coroutine is cancelled prematurely.

13.11.1. Example

>>> import asyncio
>>>
>>>
>>> async def work():
...     return 'done'
>>>
>>>
>>> async def main():
...     try:
...         res = await asyncio.shield(work())
...     except asyncio.CancelledError:
...         res = None
>>>
>>>
>>> asyncio.run(main())