How to use Python’s asyncio common functions?

Definition of coroutine

Need to use async def statement

What coroutine can do:

1. Wait for a future result

2 , wait for another coroutine (produce a result or raise an exception)

3. Produce a result to the coroutine that is waiting for it

4. Throw an exception to the coroutine that is waiting for it Program

Running of the coroutine

Call the coroutine function, the coroutine will not start running, it just returns a coroutine object

There are two ways to run the coroutine object Method:

1. Use await to wait for it in another already running coroutine

2. Plan its execution through the ensure_future function

Only when the loop of a certain thread is running, the coroutine can run

The following example:

First get the default loop of the current thread, and then transfer the coroutine object Handed to loop.run_until_complete, the coroutine object will then be run in the loop

loop = asyncio.get_event_loop()
loop.run_until_complete(do_some_work(3))

run_until_complete is a blocking call, and it will not return until the coroutine is finished running

The parameter is a future, but what we pass to it is a coroutine object. It does an internal check and wraps the coroutine object into a future through the ensure_future function

We can write like this:

loop.run_until_complete(asyncio.ensure_future(do_some_work(3)))

Multiple coroutines running

Multiple coroutines run in a loop. In order to hand over multiple coroutines to the loop, you need to use the asyncio.gathrefunction

loop.run_until_complete(asyncio.gather(do_some_work(1), do_some_work(3)))

Or store the coroutine object in the list first, which is more common.

loop = asyncio.get_event_loop() #获取当前线程loop
coros_list = []
for i in range(2000):
    coros_list.append(main(i))
loop.run_until_complete(asyncio.gather(*coros_list))

gather plays the role of aggregation, packaging multiple futures into a single future, because loop.run_until_complete only accepts a single future.

About loop.close()

Simply speaking, as long as the loop is not closed, it can still run. :

loop = asyncio.get_event_loop() #获取当前线程loop
loop.run_until_complete(do_some_work(loop, 1))
loop.run_until_complete(do_some_work(loop, 3))
loop.close()

But if it is closed, it can no longer run:

loop = asyncio.get_event_loop() #获取当前线程loop
loop.run_until_complete(do_some_work(loop, 1))
loop.close()
loop.run_until_complete(do_some_work(loop, 3))  # 此处异常

Callback

Joining the coroutine is an IO read operation. After he finishes reading the data, We would like to be notified for further processing of the data. This can be achieved by adding callbacks to the future

def done_callback(futu):
    print('Done')
futu = asyncio.ensure_future(do_some_work(3))
futu.add_done_callback(done_callback)
loop.run_until_complete(futu)

Event loop

The event loop will run asynchronous tasks and callbacks, perform network IO operations, and run child processes

From asyncio event loop In the policy document, we learned that event loop policy is a process global object that controls the management of all event loops in the process.

The global policy of the process defines the meaning of the context controlled by the policy, and manages separate event loops in each context. The context defined by the default policy is the current thread, which means that different threads are Different contexts, therefore different event loops.

Get the event loop

asyncio.get_running_loop() # 返回当前os线程中正在运行的事件循环
asyncio.get_event_loop() # 获取当前事件循环
asyncio.set_event_loop(loop) # 获取当前事件循环
asyncio.new_event_loop() # 创建并返回一个新的事件循环对象

asyncio.get_event_loop()

If:

• The current thread is Main thread

• The current thread has not started event loop

Calling the asyncio.get_event_loop() method will generate a new default event loop and set it For the current thread's event loop.

At this time, get_event_loop() is equivalent to:

loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)

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