{"id":4147,"date":"2022-07-18T07:31:03","date_gmt":"2022-07-18T07:31:03","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=4147"},"modified":"2023-06-03T07:59:39","modified_gmt":"2023-06-03T07:59:39","slug":"python-asyncio-wait_for","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-concurrency\/python-asyncio-wait_for\/","title":{"rendered":"Python asyncio.wait_for()"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn how to use the asyncio.wait_for()<\/code> function to wait for a coroutine to complete with a timeout.<\/p>\n\n\n\n

Introduction to the Python asyncio.wait_for() function #<\/a><\/h2>\n\n\n\n

In the previous tutorial, you learned how to cancel a task that is in progress by using the cancel()<\/a><\/code> method of the Task<\/code> object.<\/p>\n\n\n\n

To wait for a task to complete with a timeout, you can use the asyncio.wait_for()<\/code> function. The asyncio.wait_for()<\/code> function waits for a single task to be completed with a timeout.<\/p>\n\n\n\n

When a timeout occurs, the asyncio.wait_for()<\/code> function cancels the task and raises the TimeoutError<\/code> exception. Otherwise, it returns the result of the task. For example:<\/p>\n\n\n

import<\/span> asyncio\nfrom<\/span> asyncio.exceptions import<\/span> TimeoutError\n\n\nasync<\/span> def call_api(message, result=1000<\/span>, delay=3<\/span>):\n    print(message)\n    await<\/span> asyncio.sleep(delay)\n    return<\/span> result\n\n\nasync<\/span> def main():\n    task = asyncio.create_task(\n        call_api('Calling API...'<\/span>, result=2000<\/span>, delay=5<\/span>)\n    )\n\n    MAX_TIMEOUT = 3<\/span>\n    try<\/span>:\n        await<\/span> asyncio.wait_for(task, timeout=MAX_TIMEOUT)\n    except TimeoutError:\n        print('The task was cancelled due to a timeout'<\/span>)\n\nasyncio.run(main())\n<\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Calling API...\nThe task was cancelled due to a timeout<\/code><\/span><\/pre>\n\n\n
How it works<\/p>\n\n\n\n
First, define a call_api()<\/code> coroutine that takes 3 seconds to complete by default:<\/p>\n\n\n
async<\/span> def call_api(message, result=1000<\/span>, delay=3<\/span>):\n    print(message)\n    await<\/span> asyncio.sleep(delay)\n    return<\/span> result<\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, create a task that wraps the call_api coroutine and takes 5 seconds to complete:<\/p>\n\n\n
task = asyncio.create_task(\n    call_api('Calling API...'<\/span>, result=2000<\/span>, delay=5<\/span>)\n)<\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, use the asyncio.wait_for()<\/code> function to wait for the task to complete with a timeout of 3 seconds. Since the task takes 5 seconds to complete, a timeout will occur and a TimeoutError<\/code> will be raised:<\/p>\n\n\n
MAX_TIMEOUT = 3<\/span>\ntry<\/span>:\n    await asyncio.wait_for(task, timeout=MAX_TIMEOUT)\nexcept TimeoutError:\n    print<\/span>('The task was cancelled due to a timeout'<\/span>)<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Shielding a task from cancellation #<\/a><\/h2>\n\n\n\n
Sometimes, you may want to inform users that a task is taking longer than expected after a certain amount of time but not cancel the task when a timeout is exceeded.<\/p>\n\n\n\n
To do that, you can wrap the task with the asyncio.shield()<\/code> function. The asyncio.shield()<\/code> prevents the cancellation of a task. For example:<\/p>\n\n\n
import<\/span> asyncio\nfrom<\/span> asyncio.exceptions import<\/span> TimeoutError\n\n\nasync<\/span> def call_api(message, result=1000<\/span>, delay=3<\/span>):\n    print(message)\n    await<\/span> asyncio.sleep(delay)\n    return<\/span> result\n\n\nasync<\/span> def main():\n    task = asyncio.create_task(\n        call_api('Calling API...'<\/span>, result=2000<\/span>, delay=5<\/span>)\n    )\n\n    MAX_TIMEOUT = 3<\/span>\n    try<\/span>:\n        await<\/span> asyncio.wait_for(asyncio.shield(task), timeout=MAX_TIMEOUT)\n    except TimeoutError:\n        print('The task took more than expected and will complete soon.'<\/span>)\n        result = await<\/span> task\n        print(result)\n\nasyncio.run(main())\n<\/code><\/span>Code language:<\/span> JavaScript<\/span> (<\/span>javascript<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Calling API...\nThe task took more than expected and will complete soon.\n2000<\/code><\/span><\/pre>\n\n\n
In this example, the task takes 5 seconds to complete. Once the timeout is 3 seconds, the TimeoutEror<\/code> exception is raised. However, the task is not canceled due to the asyncio.shield()<\/code> function.<\/p>\n\n\n\n
In the exception handling section, we await<\/code> for the task to be completed and print out the result.<\/p>\n\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
    \n
  • Use asyncio.wait_for()<\/code> function to wait for a task with a timeout.<\/li>\n\n\n\n
  • Use asyncio.shield()<\/code> function to prevent the cancellation of a task after a timeout.<\/li>\n<\/ul>\n
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