docs: def get_default_driver(self):
          """
          Get the default cache driver name.
  
          :rtype: str
  
          :raises: RuntimeError
          """
          if 'default' in self._config:
              return self._config['default']
  
          if len(self._config['stores']) == 1:
              return list(self._config['stores'].keys())[0]
  
          raise RuntimeError('Missing "default" cache in configuration.')