code: def get_font(self, font):
          
          font = {'a': 0, 'b': 1}.get(font, font)
          if not six.text_type(font) in self.fonts:
              raise NotSupported(
                  '"{}" is not a valid font in the current profile'.format(font))
          return font
docs: Return the escpos index for `font`. Makes sure that
          the requested `font` is valid.