def public(thing, value=None):
    if isinstance(thing, str):
        mdict = sys._getframe(1).f_globals
        name = thing
        mdict[name] = thing  # no need for retyping! ;)
    else:
        mdict = sys.modules[thing.__module__].__dict__
        name = thing.__name__
    dunder_all = mdict.setdefault('__all__', [])
    dunder_all.append(name)
    return thing

@public
def baz(a, b):
    return a+ b

public('CONST1', 3)

CONST2 = 4

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On the down side, you know somebody is going to @public a class' method -- how do we check for that?