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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def existing_path(string):
    '''"Convert" a string to a string that is a path to an existing file.'''
    if not os.path.exists(string):
        msg = 'path {0!r} does not exist'.format(string)
        raise argparse.ArgumentTypeError(msg)
    return string