code: def apply_flat(self, config, namespace_separator='_', prefix=''):
          # type: (Dict[str, Any], str, str) -> None
          
          self._init_flat_pointers()
          for key_stack, (container, orig_key) in self._flat_pointers.items():
              flat_key = '{prefix}{joined_key}'.format(prefix=prefix, joined_key=namespace_separator.join(key_stack))
              if flat_key in config:
                  container[orig_key] = config[flat_key]
docs: Apply additional configuration from a flattened dictionary
  
          This will look for dictionary items that match flattened keys from base_config and apply their values on the
          current configuration object.
  
          This can be useful for applying configuration from environment variables and flat configuration file formats
          such as INI files.