Is it a convention to prefix private classes with underscores?

Question

I have seen code in which functions/constants are prefixed with underscores. My understanding is that this indicates that they are not to be used directly. Can I do this with classes ?

class _Foo(object):
    pass

class __Bar(object):
    pass

Answer 1

Better only use one _. This indicates that a name is private within a module.

It is not imported with the catch-all from <module> import *, and it has some other features such as "preferred destruction".

From here:

If __all__ is not defined, the set of public names includes all names found in the module’s namespace which do not begin with an underscore character ('_').

From here:

Starting with version 1.5, Python guarantees that globals whose name begins with a single underscore are deleted from their module before other globals are deleted.

Double-underscore starting class members are name-mangled.

Answer 2

Yes; the single underscore usage is endorsed by PEP8 for internal-use classes.

I don't believe the double underscore usage will have any real effect most of the time, since it's used to active name mangling for class attributes, and generally a class isn't an attribute of another class (granted, it can be, in which case Python will happily mangle the name for you.)

Answer 3

Yes, and this is not only a convention. When you import * from this module, names starting with underscore will not be imported.