在python里创建一个任务(Task)实例

与事件循环进行交互，最基本的方式就是任务，任务封装了协程和自动跟踪它的状态。任务是Future类的子类，所以其它协程可以等待任务完成，或当这些任务完成获取返回结果。

在这里通过create_task()函数来创建一个任务实例，然后事件循环就运行这个任务，直到这个任务返回为止：

import asyncio

async def task_func():
 print('in task_func')
 return 'the result'

async def main(loop):
 print('creating task')
 task = loop.create_task(task_func())
 print('waiting for {!r}'.format(task))
 return_value = await task
 print('task completed {!r}'.format(task))
 print('return value: {!r}'.format(return_value))

event_loop = asyncio.get_event_loop()
try:
 event_loop.run_until_complete(main(event_loop))
finally:
 event_loop.close()

结果输出如下：

creating task
waiting for <Task pending coro=<task_func() running at D:\work\csdn\python_Game1\example\asyncio_create_task.py:4>>
in task_func
task completed <Task finished coro=<task_func() done, defined at D:\work\csdn\python_Game1\example\asyncio_create_task.py:4> result='the result'>
return value: 'the result'

补充知识：python里创建任务执行一半时取消任务执行

下例子来演示创建任务执行一半时取消任务执行，这时会抛出异常CancelledError，同时也提供了一个机会来删除占用资源等等：

import asyncio

async def task_func():
 print('in task_func, sleeping')
 try:
   await asyncio.sleep(1)
 except asyncio.CancelledError:
   print('task_func was canceled')
   raise
 return 'the result'

def task_canceller(t):
 print('in task_canceller')
 t.cancel()
 print('canceled the task')

async def main(loop):
 print('creating task')
 task = loop.create_task(task_func())
 loop.call_soon(task_canceller, task)
 try:
   await task
 except asyncio.CancelledError:
   print('main() also sees task as canceled')

event_loop = asyncio.get_event_loop()
try:
 event_loop.run_until_complete(main(event_loop))
finally:
 event_loop.close()

结果输出如下：

creating task
in task_func, sleeping
in task_canceller
canceled the task
task_func was canceled
main() also sees task as canceled

来源：https://blog.csdn.net/caimouse/article/details/77895432