There is excellent support for Jest, I'm guessing with Python and the rest of the languages that are translated to JavaScript you would be able use it too, but

There is excellent support for Jest, I'm guessing with Python and the rest of the languages that are translated to JavaScript you would be able use it too, but it's easier to go with TypeScript, you don't really want suprises in Workers as you are billed for them. Using Rust makes sense for WASM, but Python is not worth it if you ask me. On the client you can do whatever of course.