Internal function that is used for update the number of active stars (that define notebook
    difficulty level)

    ----------
    Parameters
    ----------
    star_code : str
        String with the HTML code to be changed.

    number_stars : int
        Number of stars that will be active.

    Returns
    -------
    out : str
        It is returned a string with the HTML code after updating the number of active stars.

def _set_star_value(star_code, number_stars):
    """
    Internal function that is used for update the number of active stars (that define notebook
    difficulty level)

    ----------
    Parameters
    ----------
    star_code : str
        String with the HTML code to be changed.

    number_stars : int
        Number of stars that will be active.

    Returns
    -------
    out : str
        It is returned a string with the HTML code after updating the number of active stars.

    """

    for star in range(1, 6):
        if star <= number_stars:
            star_code = star_code.replace("fa fa-star " + str(star), "fa fa-star "
                                                                             "checked")
        else:
            star_code = star_code.replace("fa fa-star " + str(star), "fa fa-star")

    return star_code