> work off its __name__ and __module__

Why is __module__ required? It seems to me this should only operate on the current module. 

I added a prototype to Python/bltinmodule.c that gets or creates the __all__ list from the current globals (i.e. PyEval_GetGlobals). It accepts at most one positional argument and any number of keyword arguments. It adds the positional argument's __name__ to __all__, sets it in globals, and returns a reference for use as a decorator. The keyword argument dict is used to update globals and extend __all__. 

    Python 3.6.0a0 (default:3eec7bcc14a4+, Mar 24 2016, 20:40:52) 
    [GCC 4.8.4] on linux
    Type "help", "copyright", "credits" or "license" for more
    information.
    >>> @public
    ... def foo():
    ...     pass
    ... 
    >>> def bar(): pass
    ... 
    >>> public(bar, spam=1, eggs=2)
    <function bar at 0x7efe96ca1048>
    >>> __all__
    ['foo', 'spam', 'eggs', 'bar']
    >>> foo, bar
    (<function foo at 0x7efe96c8af28>, <function bar at 0x7efe96ca1048>)
    >>> spam, eggs
    (1, 2)

Maybe it should be generalized to handle multiple positional arguments. Currently it's an error:

    >>> public(foo, bar)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: public expected at most 1 arguments, got 2

The positional argument must have a __name__ that's a string:

    >>> public('CONST')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: 'str' object has no attribute '__name__'
    >>> class C:
    ...     __name__ = 1
    ... 
    >>> public(C())
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: __name__ must be a string

If it's used to decorate a method definition, it stores a reference to the function in the module's globals. That's not very useful, but at least it won't lead to an error with a star import.