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A discussion on comp.lang.python about prettying up the "if __name__ == 'main__'" idiom led to a suggestion that a decorator could simple register the main function using atexit.register. That looks like it will work, but leaves open the possibility that while main() is running via atexit._run_exitfuncs, other exit functions might be registered.
As currently defined (at least in the Python version with 2.7), I think everything will work fine. Still, the behavior of adding new exit functions during exit is not defined. Would be kind of nice if this behavior was blessed, and then mentioned in the documentation.

From a post by Ian Kelly (https://mail.python.org/pipermail/python-list/2014-November/681073.html)
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In fact it seems the behavior does differ between Python 2.7 and Python 3.4:
$ cat testatexit.py
import atexit
@atexit.register
def main():
 atexit.register(goodbye)
@atexit.register
def goodbye():
 print("Goodbye")
$ python2 testatexit.py
Goodbye
Goodbye
$ python3 testatexit.py
Goodbye