def is_int(str):
    """ Checks if input string is a number rather than a letter

    Parameters
    ----------
    str: str
        string to test

    Returns
    -------
    answer: bool
        returns True if string is a number; False if string is not
    """
    answer = False
    try:
        int(str)
    except ValueError:
        return answer
    answer = True
    return answer