def is_string_integer(string: str):
    """
    This function takes in a character value and checks if it is a valid integer.

    All entries must be strings of length 1

    :param string: the string to be analyzed
    :type string: str
    :return: true or false, depending on if the character is a valid integer
    :rtype: bool
    """
    assert isinstance(string, str)
    assert len(string) == 1
    try:
        int(string)
    except ValueError:
        return False
    else:
        return True