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exactly -

The problem is that there are three logical states that need three different actions

1. if nothing done yet: start async computation

2. if async computation in progress: wait for it

3. return result as computaton done

Atomicity in the sense that no other Python code can run during a setdefault, is not relevant.

Waiting for something that is running in another loop actually isn't easy either; I'm not sure that Python really allows it - won't you get an error:

   The future belongs to a different loop than the one specified as the loop argument

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import asyncio import threading future = None def p(msg): print(f"{msg}: {threading.get_ident()}, event loop: {id(asyncio.get_running_loop())}") async def one_time_setup(): p('one_time_setup start') await asyncio.sleep(1) p('one_time_setup done') async def maybe_initialize(): global future if not future: future = asyncio.create_task(one_time_setup()) p('waiting start') await future p('waiting done') def worker(): new_loop = asyncio.new_event_loop() asyncio.set_event_loop(new_loop) new_loop.run_until_complete(maybe_initialize()) number_of_threads = 2 for _ in range(number_of_threads): threading.Thread(target=worker).start()

RuntimeError: Task <Task pending coro=<maybe_initialize() running at test.py:20> cb=[_run_until_complete_cb() at /Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/asyncio/base_events.py:158]> got Future <Task pending coro=<one_time_setup() running at test.py:11> cb=[<TaskWakeupMethWrapper object at 0x10cafdeb8>()]> attached to a different loop