This is only delightful when you are doing synchronous computation or you have async/await in the language. Until Loom is available, using this same pattern with CompletableFuture breaks in the face of try-with-resources.

Also I don't think it's possible in Python using await since await is a prefix keyword so in your example .groupBy is not a method on Future so it won't work.

I love ReactiveX as much as anyone but these issues cheese me off.

Rust has await as a postfix operator so this pattern works flawlessly when mixing async and sync code.