code: def select_template_name(template_name_list, using=None):
      
      if not isinstance(template_name_list, tuple):
          template_name_list = tuple(template_name_list)
  
      try:
          return _cached_name_lookups[template_name_list]
      except KeyError:
          # Find which template of the template_names is selected by the Django loader.
          for template_name in template_name_list:
              try:
                  get_template(template_name, using=using)
              except TemplateDoesNotExist:
                  continue
              else:
                  template_name = six.text_type(template_name)  # consistent value for lazy() function.
                  _cached_name_lookups[template_name_list] = template_name
                  return template_name
  
          return None
docs: Given a list of template names, find the first one that exists.