I’ve recently been exploring the exciting new world of asynchronous I/O libraries in Python 3 – specifically asyncio and curio. These two libraries make some different design choices. This is an essay that I wrote to try to explain to myself what those differences are and why I think they matter, and distill some principles for designing event loop APIs and asynchronous libraries in Python. This is a quickly changing area and the ideas here are very much still under development, so this text probably assumes all kinds of background knowledge and possibly that you live inside my head – but maybe you’ll find it interesting anyway. I’d love to hear what you think or discuss further.