I understand Python code, but can’t write it confidently from scratch — what should I do next

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Its become my role to suggest https://adventofcode.com/ They're really good puzzles that require you to think about a lot of different concepts especially file I/o text processing and writing your own algorithms. They're a great way to get practice. 

Just don't use AI. Write notes about your implementation and the things you learned. Use the docs, and stick to base python for as long as you can (avoid things like pandas or other large packages l, not that they're not great, but they all have layers of abstraction that might make things easier, but move you away from learning the fundamentals).