Python __aenter__() Magic Method – Be on the Right Side of Change

object.__aenter__(self)

💡 Summary: Python’s __aenter__() magic method is semantically identical to __enter__() but is used for asynchronous and parallel programming. Python calls the __aenter__() magic method when starting an async with block whereas the __aexit__() method is called when leaving it. An object that implements both __aenter__() and __aexit__() methods is called an asynchronous context manager.

import asyncio


class MyAsyncContextManager:

    async def __aenter__(self):
        print("ENTER async with")

    async def __aexit__(self, *args):
        print("EXIT async with")


async def run():
    async with MyAsyncContextManager() as cm:
        print("BODY async with")


asyncio.get_event_loop().run_until_complete(run())

Output:

ENTER async with
BODY async with
EXIT async with

We define a custom class MyAsyncContextManager that defines the __aenter__() and __aexit__() magic methods to make it an asynchronous context manager, i.e., allowing it to be used in an async with statement.
You define an asynchronous function run() that holds the asynchronous with environment and run it using the asyncio.get_event_loop().run_until_complete() function. The reason for this is that you need to use the async with block within an asynchronous function (using the expression async def to define it).

We call this a “Dunder Method” for “Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.