Show how Context Managers do not correctly support suspended execution

async_context.py

import asyncio

class CM:
    mode = 0

    def __init__(self, m):
        self.m = m

    def __enter__(self):
        self.old = CM.mode
        CM.mode = self.m

    def __exit__(self, *_):
        CM.mode = self.old
        

async def long_op():
    with CM('long'):
        print(f'- - mode {CM.mode}')
        await asyncio.sleep(3)
        print(f'- - mode {CM.mode} done')

async def short_op():
    with CM('short'):
        for i in range(5):
            print(f'{i} - mode {CM.mode}')
            await asyncio.sleep(1)
            print(f'{i} - mode {CM.mode} done')
    

async def main():
    await asyncio.gather(short_op(),
                         long_op())

asyncio.run(main())

This is how your example can be fixed with the contextvars module:

import asyncio
import contextvars

cm_mode = contextvars.ContextVar('cm_mode', default=None)

class CM:

    def __init__(self, m):
        self.m = m

    @property
    def mode(self):
        return cm_mode.get()

    def __enter__(self):
        self.old = cm_mode.get()
        cm_mode.set(self.m)
        return self

    def __exit__(self, *_):
        cm_mode.set(self.old)


async def long_op():
    with CM('long') as cm:
        print(f'- - mode {cm.mode}')
        await asyncio.sleep(3)
        print(f'- - mode {cm.mode} done')

async def short_op():
    with CM('short') as cm:
        for i in range(5):
            print(f'{i} - mode {cm.mode}')
            await asyncio.sleep(1)
            print(f'{i} - mode {cm.mode} done')


async def main():
    await asyncio.gather(short_op(),
                         long_op())

asyncio.run(main())