Python style question: Support class as inner class?

Question

I like to use the structures built into the language to organize my code. But there is one situation where I can't stay consistent, simply because I don't see a definitive best way of doing it. It's regarding support classes, that is, classes that are exclusively used internally by another class: Do I make them inner classes, or separate classes.

Inner classes:

class Complicated:
    class Utility1:
        pass
    class Utility2:
        pass
    pass

Separate classes:

class Complicated:
    pass

class Utility1:
    pass

class Utility2:
    pass

Inner classes has the advantage of being scoped inside the only class that uses them. But the problem is that I get less space to write code due to indentation.

Outer classes have neither the advantage nor the disadvantage. I am tired of always spending some mental energy whenever I write support classes, wondering about this silly issue.

My question is whether anyone with a substantial python experience on their back can advise as to whether there is a best practice regarding this? Even if the answer is that "it depends", it is appreciated if it comes with someone more experienced than myself.

Answer 1

I would suggest

class Complicated:
    pass

class _Utility1:
    pass

class _Utility2:
    pass

and put all this in a module of its own. The leading _ indicates that the utility classes are meant for internal use only (and for what it's woth, they won't get imported by from module import *; but I don't like this anyway).

Edit: Citing from PEP 8:

Classes for internal use have a leading underscore in addition.