[ttfkam]

Threads are for doing your own work in parallel. Async is for waiting on others to do their work in parallel.

Your own work would be some CPU-intensive operations you can logically divide and conquer.

Others' work would be waiting for file I/O from the OS, waiting for a DB result set following a query, waiting for a gRPC response, etc.

Conceptually quite distinct, and there are demonstrated advantages and drawbacks to each. Right tool for right job and all that.

[jvanderbot]

All correct. An additional comment is that, when I was coming up, parallelism in its many forms was of the variety "I need to do this job in parallel" or "I need to handle exactly 32 concurrent workers". After web & such, it was common to just think of paralellism as "I declare this one method as returning a promise" and then "async def", which semantically is very different than managing threads. As pointed out, it's now more like "This function is basically a server for any and all uncontrolled calls from elsewhere".