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