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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def extend_webfont_settings(webfont_settings):
 """ Validate a webfont settings and optionally fill missing ``csspart_path`` option. Args: webfont_settings (dict):
 Webfont settings (an item value from ``settings.ICOMOON_WEBFONTS``). Returns: dict: Webfont settings """

    if not webfont_settings.get('fontdir_path', False):
        raise IcomoonSettingsError(("Webfont settings miss the required key "
                                    "item 'fontdir_path'"))

    if not webfont_settings.get('csspart_path', False):
        webfont_settings['csspart_path'] = None

    return webfont_settings