code: def best_fit_font_size(cls, text, extents, max_size, font_file):
          
          line_source = _LineSource(text)
          text_fitter = cls(line_source, extents, font_file)
          return text_fitter._best_fit_font_size(max_size)
docs: Return the largest whole-number point size less than or equal to
          *max_size* that allows *text* to fit completely within *extents* when
          rendered using font defined in *font_file*.