Overview of Async IO in Python 3.7

Python 3’s asyncio module provides fundamental tools for implementing asynchronous I/O in Python. It was introduced in Python 3.4, and with each subsequent minor release, the module has evolved significantly.

This tutorial contains a general overview of the asynchronous paradigm, and how it’s implemented in Python 3.7.

Blocking vs Non-Blocking I/O

The problem that asynchrony seeks to resolve is blocking I/O.

By default, when your program accesses data from an I/O source, it waits for that operation to complete before continuing to execute the program.

with open('myfile.txt', 'r') as file:
    data = file.read()
    # Until the data is read into memory, the program waits here
print(data)

The program is blocked from continuing its flow of execution while a physical device is accessed, and data is transferred.

Network operations are another common source of blocking:

# pip install --user requests
import requests

req = requests.get('https://www.stackabuse.com/')

#
# Blocking occurs here, waiting for completion of an HTTPS request
#

print(req.text)

In many cases, the delay caused by blocking is negligible. However, blocking I/O scales very poorly. If you need to wait for 10¹⁰ file reads or network transactions, performance will suffer.

Multiprocessing, Threading, and Asynchrony

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