argparse store false if unspecified

The store_true option automatically creates a default value of False.

Likewise, store_false will default to True when the command-line argument is not present.

The source for this behavior is succinct and clear: http://hg.python.org/cpython/file/2.7/Lib/argparse.py#l861

The argparse docs aren't clear on the subject, so I'll update them now: http://hg.python.org/cpython/rev/49677cc6d83a

With

import argparse
parser=argparse.ArgumentParser()
parser.add_argument('-auto', action='store_true', )
args=parser.parse_args()
print(args)

running

% test.py

yields

Namespace(auto=False)

So it appears to be storing False by default.

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