def text_length_validator(value, values):
    """ Provides a validator function that a valid string for the display
    commands of the Keithley. Raises a TypeError if value is not a string.
    If the string is too long, it is truncated to the correct length.

    :param value: A value to test
    :param values: The allowed length of the text
    """
    if not isinstance(value, str):
        raise TypeError('Value is not a string.')
    return value[:values]
