Python: How to call an async function from a synchronous one with an active event loop?

Question

Python

There is a function call stack:

[main async func0] -> [sync func1] -> [sync func2] -> [async func3]

How to call async func3 from func2 if func1 knows nothing about async?
func2 is passed to func1 as an interface implementation.

If you do this:

from asgiref.sync import async_to_sync

def func2():
  result = async_to_sync(func3)()

then we get an error:

RuntimeError: You cannot use AsyncToSync in the same thread as an async event loop - just await the async function directly.

But I can't call directly via await, because func1 knows nothing about async.

Answer 1

Wrap the asynchronous callback in a synchronous wrapper (maybe a lambda) that calls asyncio.create_task() on the target coroutine, and pass this wrapper as a callback.
If you're in control of func2, it's even easier, put a call to create_task() there.
Now, if you need to wait for the result of the execution, then it will be more difficult - but you can shove the processing of the result into the asynchronous part.