1000 threads was a very specific use case I probably shouldn't have generalized from where I needed to match the number of Python threads running in a web server to a Java process running on the same host using the same number of threads. They were mostly idle.

There's no reason Python should fall over at any number of threads. You just usually end up either running out of memory or (more likely) saturate a single CPU core well before that number of threads.

Without consulting my notes I can't recall why I didn't use gevent on that project.