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As far the implementation goes, since the `__dir__` overload support in the `dir()` builtin ignores instance dictionaries, this could be implemented in module.__dir__ (or object.__dir__) rather than directly in the builtin.
Alternatively, it could use a different attribute name rather than having `__dir__` be a list of strings on instances, and a protocol method on classes.
If we went with a different name, then I think `__public__` would be a reasonable option, since the purpose of the attribute is to make it easy to programmatically distinguish public attributes from non-public ones (independently of the established leading underscore convention, which doesn't work well for transitive module references).