Asyncio is just one way to do async in Python, but it is an important one. Getting started with asyncio is difficult because of the profusion of coroutines, tasks and futures, how they differ and how to use them. 

Find out more in this extract from my new book Programmer's Python: Async

Programmer's Python:
Async
Threads, processes, asyncio & more

Is now available as a print book: Amazon

Contents

1)  A Lightning Tour of Python.

2) Asynchronous Explained

3) Processed-Based Parallelism
         Extract 1 Process Based Parallism
4) Threads
         Extract 1 -- Threads
5) Locks and Deadlock

6) Synchronization

7) Sharing Data
        Extract 1 - Pipes & Queues

8) The Process Pool
        Extract 1 -The Process Pool 1 ***NEW!

9) Process Managers

10) Subprocesses

11) Futures
        Extract 1 Futures, 

12) Basic Asyncio
        Extract 1 Basic Asyncio

13) Using asyncio
        Extract 1 Asyncio Web Client
14) The Low-Level API
       Extract 1 - Streams & Web Clients
Appendix I Python in Visual Studio Code

So far we have looked at processes as a way of increasing the speed of CPU-bound programs, and threads as a way of increasing the speed of I/O-bound programs. In the following chapters the emphasis changes to using a single thread to speed up I/O-bound processes. This uses an event queue or some other form of cooperative scheduling-based asynchronous programming. The basic idea is that you can use a single thread more efficiently if you simply arrange for it to do something else instead of just waiting for I/O to complete. That is, if you have a set of tasks that are I/O-bound then a single thread can manage all of them if you allow it to run other tasks while waiting for others to complete I/O.

Some are of the opinion that the alternative of allocating n threads, one to each I/O bound task, is actually slower than sharing a single thread between them all. This is certainly true for Python with the GIL restricting threads to one per Python interpreter. If the GIL is removed in the future it would still be likely that one thread for all I/O-bound tasks is going to be faster than one thread per task. There are examples of Python asyncio programs handling thousands of network connections with few problems but clearly what the limits are in any particular case depends on the task and the machine.

The key to keeping the thread busy is the event queue. This is a queue of tasks waiting to be run and the scheduler selects a task to run. This then uses the thread until it has to wait for something when it releases the thread back to the event queue and another task is selected to run on the thread. The task that had to wait is added back into the event queue and gets a chance to run when it has finished waiting. This way the single thread always has a task to keep it occupied. Notice that if the thread empties the queue then it just waits for something to do and this is the only time the thread waits.

In the rest of this chapter the focus is on using the asyncio module and this single-threaded multi-tasking approach is a different mindset to the earlier approaches using multiple threads or processes. Not only does it introduce new approaches, it also introduces new problems. It is also worth realizing that asyncio is focused on network operations rather than being a general purpose single thread asynchronous module. In particular, it isn’t an event processing system of the sort you would find as part of a typical GUI such as Tkinter or Qt. This doesn’t stop it from being used as a general purpose approach to async, but the main application in the mind of its creators is to handle network connections.

In this account of basic asyncio we only use the high-level API. This is the part that programmers using, rather than extending, asyncio should restrict themselves to. The deeper low-level API, which is the subject of Chapter 14, should only be used to create frameworks based on asyncio. Notice that many accounts of asyncio were written before the high-level API was complete and so tend to use low-level functions. Even worse, many examples and tutorials mix the use of high- and low-level functions simply because they haven’t caught up with best practices.

In book but not included in this extract

• Callbacks, Futures and Await

Coroutines

The main idea in sharing a single thread is the event loop, a basic cooperative scheduler. This is simply a queue of tasks that will be run on the thread as and when it can. However, this relies on the idea that a function can be suspended and restarted from where it was forced to wait. In a multi-threaded environment this is nothing special because the thread can just be suspended and restarted by the operating system. In a single-threaded environment the thread has to save its current state, start or resume work on another function and restore the state when it returns to the previous function. A function that can be suspended and restarted in this way is generally called a “coroutine”.

Python originally supported coroutines via generators and yield and yield from. However, support for this was removed in Python 3.10 and trying to understand coroutines via generators is no longer particularly useful. For the rest of this chapter generator coroutines are ignored.

A modern Python coroutine is created using the async keyword:

async def myCo():
    print("Hello Coroutine World")
    return 42

if you call myCo it doesn’t execute its code, instead it returns a coroutine object which can execute the code. This is very similar to a generator returning a generator object, but you cannot run a coroutine object directly. You have to run it in association with an event loop. To do this you can use low-level functions to create a loop and then submit it. However, it is much easier to use the asyncio.run method which creates and manages the event loop without you having knowing anything about it:

import asyncio
async def myCo():
    print("Hello Coroutine World")
    return 42


myCoObject=myCo()
result= asyncio.run(myCoObject)
print(result)

This runs the coroutine object and displays:

Hello Coroutine World
42

Instead of passing the coroutine object, the asyncio.run call is usually written as a single action:

result= asyncio.run(myCo())

Also notice that you can pass parameters to the coroutine:

import asyncio
async def myCo(myValue):
    print("Hello Coroutine World")
    return myValue
result= asyncio.run(myCo(42))
print(result)

It is also important to realize that asyncio.run runs myCo at once and the thread doesn’t return until myCo is completed. While running myCo an event loop is started and if the thread is freed it starts running any tasks queued before returning to myCo. In this sense the call to asyncio.run is where the asynchronous part of your program starts and you can think of it as starting the asynchronous main program.