Another possibility, if it's a matter of passing callback parameter that must be sync where you wish it could be an async function, is that the callback can be

Another possibility, if it's a matter of passing callback parameter that must be sync where you wish it could be an async function, is that the callback can be the `put_nowait()` method of an async queue. You can then have a background async task that pulls from that queue and does async stuff. This works well if the callback is just there to notify you about a result (which is the most common case in fairness) but doesn't really work if the callback is meant to synchronously determine and return a result that is then used within the function that's calling the callback.