> What are emphatically not pretty or useful are Python’s leading underscores to loosely enforce encapsulation.

Python will not prevent anyone from doing something dumb. It'll just force them to acknowledge that by forcing them to use a convention. As a library writer, I'm free to change or remove anything that starts with an underscore because, if someone else is depending on it, frankly, they had it coming. I can assume everyone who uses my library is a responsible adult and I can treat them as that.

> And why are some classes (int, str, float) allowed to be lower case

Because they are part of the language like `else` or `def`. Only Guido can do that. ;-)