code: def update_preview(self, index=None, scheme_name=None):
          
          text = ('"""A string"""\n\n'
                  '# A comment\n\n'
                  '# %% A cell\n\n'
                  'class Foo(object):\n'
                  '    def __init__(self):\n'
                  '        bar = 42\n'
                  '        print(bar)\n'
                  )
          show_blanks = CONF.get('editor', 'blank_spaces')
          update_scrollbar = CONF.get('editor', 'scroll_past_end')
          if scheme_name is None:
              scheme_name = self.current_scheme
          self.preview_editor.setup_editor(linenumbers=True,
                                           markers=True,
                                           tab_mode=False,
                                           font=get_font(),
                                           show_blanks=show_blanks,
                                           color_scheme=scheme_name,
                                           scroll_past_end=update_scrollbar)
          self.preview_editor.set_text(text)
          self.preview_editor.set_language('Python')
docs: Update the color scheme of the preview editor and adds text.
  
          Note
          ----
          'index' is needed, because this is triggered by a signal that sends
          the selected index.