I think _private has always been a convention in Python, though I'd say most Python is not so rigorous about it. I don't see why it couldn't be applied to modules.

I honestly love when I see a package do stuff like this: it's very clear then what is public interface, and I should consider usable (without sin) and what is supposed to be an internal detail.

Same with the modules: then it is very clear that the re-export of those names in __init__.py is where they're meant to be consumed, and the other modules are just for organizational purposes, not API purposes.

_Todo is then a private type.

Very clean.

I tend to do the same, some colleagues as well, so I guess this is some common pattern.

The way I see it there are two schools:

- The whitelist school: You write everything without _ prefix, then you whitelist what you want accessible with __all__.

- The explicit school: You forget about all and just use _ for symbols, modules, etc.

I find the latter more readable and consistent (can be applied to attributes, free functions, modules...

Yup, you(/sibling comments) have it correct, it's to mark it as private.

Not sure where I got it from, it just seems clean. I don't think I see this super frequently in the ecosystem at large, although anything I've had a hand in will tend to use this style!