Path argument validation in argparse

[yashpatel6]

It's not a critical point, but for input paths I generally use something like this extant_file type rather than str.

def extant_file(x):
    """
    'Type' for argparse - checks that file exists but does not open.
    """
    if not os.path.exists(x):
        # Argparse uses the ArgumentTypeError to give a rejection message like:
        # error: argument input: x does not exist
        raise argparse.ArgumentTypeError("{0} does not exist".format(x))
    return x

Originally posted by @nwiltsie in #88 (comment)