Since there are no type annotations to help your understanding of the code, you need to keep a lot in your head, as opposed to statically typed languages.
Also, we do have type annotations now. They’re not universally used, and definitely not required, but mypy and friends can go a long way toward finding type issues even without them.
OK, so I don't have every corner of the stdlib memorized, especially the odd deprecated batteries-included stuff like playing sounds on a Sun workstation. Nor do I have the extension API docs committed to memory, as you can use the language a lot without having to write those things yourself.
But the core of Python, like the language itself and the most common parts of the stdlib that are actually used in 95% of Python code, I have a strong grip on. In particular, the actual language is pretty simple.