def _interpret_arg_mode(arg, default='auto'):
    """
      >>> _interpret_arg_mode("Str")
      'string'
    """
    if arg is None:
        arg = default
    if arg == 'auto' or arg == 'eval' or arg == 'string':
        return arg
    rarg = str(arg).strip().lower()
    if rarg in ['eval', 'evaluate', 'exprs', 'expr', 'expressions',
        'expression', 'e']:
        return 'eval'
    elif rarg in ['strings', 'string', 'str', 'strs', 'literal', 'literals',
        's']:
        return 'string'
    elif rarg in ['auto', 'automatic', 'a']:
        return 'auto'
    elif rarg == 'error':
        return 'error'
    else:
        raise ValueError(
            'Invalid arg_mode=%r; expected one of eval/string/auto' % (arg,))
