Real Python is a website to improve your python skills in byte sized content that can even be received as a newsletter every week.
The site has a lot of good tutorials.
HackerRank can be used to test your knowledge against challenges, and also learn by competing.
This is an excellent resource for intermediate level developers who want to learn more about the features of the Python language and standard library.
It is presented as a lump of information in a standard textbook format, so it's not suitable for the easily bored.
The official Python documentation includes information about the builtins, core datatypes, standard library, and some inner workings of Python.
It is suitable as a reference but also contains a tutorial on the language itself here