Internal function that is used for generation of the page where notebooks are organized by
    difficulty level.

    ----------
    Parameters
    ----------
    notebook_object : notebook object
        Object of "notebook" class where the body will be created.

    dict_by_difficulty : dict
        Global Dictionary that groups Notebooks names/files by difficulty level.

def _generate_notebook_by_difficulty_body(notebook_object, dict_by_difficulty):
    """
    Internal function that is used for generation of the page where notebooks are organized by
    difficulty level.

    ----------
    Parameters
    ----------
    notebook_object : notebook object
        Object of "notebook" class where the body will be created.

    dict_by_difficulty : dict
        Global Dictionary that groups Notebooks names/files by difficulty level.

    """

    difficulty_keys = list(dict_by_difficulty.keys())
    difficulty_keys.sort()
    for difficulty in difficulty_keys:
        markdown_cell = STAR_TABLE_HEADER
        markdown_cell = _set_star_value(markdown_cell, int(difficulty))
        for notebook_file in dict_by_difficulty[str(difficulty)]:
            split_path = notebook_file.split("/")
            notebook_type = split_path[-2]
            notebook_name = split_path[-1].split("&")[0]
            notebook_title = split_path[-1].split("&")[1]
            markdown_cell += "\n\t<tr>\n\t\t<td width='20%' class='header_image_color_" + \
                             str(NOTEBOOK_KEYS[notebook_type]) + "'><img " \
                             "src='../../images/icons/" + notebook_type.title() +\
                             ".png' width='15%'>\n\t\t</td>"
            markdown_cell += "\n\t\t<td width='60%' class='center_cell open_cell_light'>" + \
                             notebook_title + "\n\t\t</td>"
            markdown_cell += "\n\t\t<td width='20%' class='center_cell'>\n\t\t\t<a href='" \
                             "../" + notebook_type.title() + "/" + notebook_name + \
                             "'><div class='file_icon'></div></a>\n\t\t</td>\n\t</tr>"

        markdown_cell += "</table>"

        # ==================== Insertion of HTML table in a new Notebook cell ======================
        notebook_object["cells"].append(nb.v4.new_markdown_cell(markdown_cell))