Ask HN: Intermediate Python learning resources?

So I've completed Codecademy's course on Python, I have some experience fiddling with Flask and putting together random Python scripts. Generally, when I want to build something that I've never built before, I look up how to do it on Stackoverflow and manage to understand most of the things.

How can I take my knowledge to the next level?

Free learning resources are preferred. Hopefully ones you have used yourself when in my position.

Thanks!