The same way you build mastery in anything: practice.<p>Write lots of Python (or whatever language you want to be proficient in) code. Every so often, go back to what you wrote before and review it.<p>Read lots of Python code. Tinker with it, add features, change things, see how what you do affects the program.<p>And study -- read lots about coding in Python. Interact with Python devs. That sort of thing.
The following links have helped me upgrade my python skills. Also i do spaced repetition with new python skills i have acquired which has done wonders to retaining new concepts.
* <a href="https://www.pythonmorsels.com/" rel="nofollow">https://www.pythonmorsels.com/</a>
* <a href="https://github.com/norvig/pytudes">https://github.com/norvig/pytudes</a>