If you run attached file w/ 3.2 and 3.3 (and later) versions, you'll notice that the new version of parser doesn't handle empty argument list:

$ python3.2 test.py
usage: test.py [-h] {demo} ...
test.py: error: too few arguments
$ python3.3 test.py
Namespace()

Everything is naturally failing in 3.3 version as the execution continues with the empty Namespace. I suspect this is due to the issue10424 that removed explicit check for positionals.