converts a style_dict to an xlsxwriter format dict

        Parameters
        ----------
        style_dict : style dictionary to convert
        num_format_str : optional number format string

def convert(cls, style_dict, num_format_str=None):
        """
        converts a style_dict to an xlsxwriter format dict

        Parameters
        ----------
        style_dict : style dictionary to convert
        num_format_str : optional number format string
        """

        # Create a XlsxWriter format object.
        props = {}

        if num_format_str is not None:
            props['num_format'] = num_format_str

        if style_dict is None:
            return props

        if 'borders' in style_dict:
            style_dict = style_dict.copy()
            style_dict['border'] = style_dict.pop('borders')

        for style_group_key, style_group in style_dict.items():
            for src, dst in cls.STYLE_MAPPING.get(style_group_key, []):
                # src is a sequence of keys into a nested dict
                # dst is a flat key
                if dst in props:
                    continue
                v = style_group
                for k in src:
                    try:
                        v = v[k]
                    except (KeyError, TypeError):
                        break
                else:
                    props[dst] = v

        if isinstance(props.get('pattern'), str):
            # TODO: support other fill patterns
            props['pattern'] = 0 if props['pattern'] == 'none' else 1

        for k in ['border', 'top', 'right', 'bottom', 'left']:
            if isinstance(props.get(k), str):
                try:
                    props[k] = ['none', 'thin', 'medium', 'dashed', 'dotted',
                                'thick', 'double', 'hair', 'mediumDashed',
                                'dashDot', 'mediumDashDot', 'dashDotDot',
                                'mediumDashDotDot',
                                'slantDashDot'].index(props[k])
                except ValueError:
                    props[k] = 2

        if isinstance(props.get('font_script'), str):
            props['font_script'] = ['baseline', 'superscript',
                                    'subscript'].index(props['font_script'])

        if isinstance(props.get('underline'), str):
            props['underline'] = {'none': 0, 'single': 1, 'double': 2,
                                  'singleAccounting': 33,
                                  'doubleAccounting': 34}[props['underline']]

        return props