Python async

The async keyword defines an asynchronous function in Python. Asynchronous functions let a program handle multiple tasks concurrently without waiting for each to finish, improving efficiency and enabling non-blocking execution.

Syntax

import asyncio

async def function_name(parameters):
  # Function body
  await #awaitable code

In the syntax:

• import asyncio: Imports Python’s built-in library for running and managing asynchronous tasks.
• async: The async keyword is declared before def to define an asynchronous function (coroutine).
• await: Used inside an async function to pause execution until the awaited coroutine or awaitable object completes.

Parameters:

Same as a regular function, any number of positional or keyword arguments.

Return value:

Returns a coroutine object, which must be awaited (using await) to get the actual result.

Note: If an async function doesn’t contain any await statements, it will still be valid but won’t perform any asynchronous operations.

Example 1

In this example, the async function hello() prints the first half of ‘Hello world!’, waits 3 seconds, and then prints the second half:

import asyncio

async def hello():
   print("Hello")
   await asyncio.sleep(3) # Pauses for 3 seconds
   print("world!")

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Here is the output of this code:

Hello
world!

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Example 2

In this example, the brew_tea and make_toast async functions start running at the same time. make_toast finishes first in 3 seconds, while brew_tea takes 5 seconds to complete. The program waits for both tasks to finish, then prints their results together after a total of 5 seconds:

import asyncio

async def brew_tea():
  print("Start brewing tea")
  await asyncio.sleep(5) # Pauses for 5 seconds
  return "The tea is ready!"

async def make_toast():
  print("Start making toast")
  await asyncio.sleep(3) # Pauses for 3 seconds
  return "The toast is ready!"

async def make_breakfast():
  batch = asyncio.gather(brew_tea(), make_toast())
  tea_complete, toast_complete = await batch
  print(tea_complete)
  print(toast_complete)

result = asyncio.run(make_breakfast())
print(result)

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The output of this code is:

Start brewing tea
Start making toast
The tea is ready!
The toast is ready!
None

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