code: def _effective_filename(self):
          # type: () -> str
          
          # same logic for the configuration filename. First, check if we were
          # initialized with a filename...
          config_filename = ''
          if self.filename:
              config_filename = self.filename
  
          # ... next, take the value from the environment
          env_filename = getenv(self.env_filename_name)
          if env_filename:
              self._log.info('Configuration filename was overridden with %r '
                             'by the environment variable %s.',
                             env_filename,
                             self.env_filename_name)
              config_filename = env_filename
  
          return config_filename
docs: Returns the filename which is effectively used by the application. If
          overridden by an environment variable, it will return that filename.