docs: def _print_single_file(self, path, apps_models):
          """
          Print apps_models which contains a list of 2-tuples containing apps and their models
          into a single file.
          """
          if path:
              outfile = codecs.open(path, 'w', encoding='utf-8')
              self._print = lambda s: outfile.write(s + '\n')
          self._print(self._diagram_start)
          for app, app_models in apps_models:
              self._print_app(app, app_models)
          self._print(self._diagram_end)
          if path:
              outfile.close()