Converts a path into its absolute path and verifies that it exists or throws an exception.

def to_existing_absolute_path(string):
    """Converts a path into its absolute path and verifies that it exists or throws an exception."""
    value = os.path.abspath(string)
    if not os.path.exists( value ) or not os.path.isdir( value ):
        msg = '"%r" is not a valid path to a directory.' % string
        raise argparse.ArgumentTypeError(msg)
    return value