If you expose everything, someone will write code that depends against it and make it harder for you to improve or change it in the future. Tossing in roadblocks helps guide users to the better long-term path.

It doesn't really matter for small teams, but it's essential for large ones without aggressive (and difficult) enforcement of contract between team members.

> It doesn't really matter for small teams

Doesn't even have to be your team. We encountered this by way of a major library, celery.

I forget all the details, but celery was importing something else (name started with a "k", I think), and that was importing an underscore-prefixed function from uuid. Somewhere between python versions 2.7.8 and 2.7.13, the implementation of uuid changed and that import failed.

`foo.__a` accessible only inside `Foo`

I think it works.

It's not all that inaccessible. Try:

    class C:
      def __init__(self):
        self.__private = 'Is it private?'
    
    c = C()
    c._C__private