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...