python - Export decorator that manages __all__

Question

A proper Python module will list all its public symbols in a list called __all__. Managing that list can be tedious, since you'll have to list each symbol twice. Surely there are better ways, probably using decorators so one would merely annotate the exported symbols as @export.

How would you write such a decorator? I'm certain there are different ways, so I'd like to see several answers with enough information that users can compare the approaches against one another.

Answer 1

You could simply declare the decorator at the module level like this:

__all__ = []

def export(obj):
    __all__.append(obj.__name__)
    return obj

This is perfect if you only use this in a single module. At 4 lines of code (plus probably some empty lines for typical formatting practices) it's not overly expensive to repeat this in different modules, but it does feel like code duplication in those cases.