code: def from_envvar(self, variable_name):
          
          config_file = os.environ.get(variable_name)
          if not config_file:
              raise RuntimeError(
                  "The environment variable %r is not set and "
                  "thus configuration could not be loaded." % variable_name
              )
          return self.from_pyfile(config_file)
docs: Load a configuration from an environment variable pointing to
          a configuration file.
  
          :param variable_name: name of the environment variable
          :return: bool. ``True`` if able to load config, ``False`` otherwise.