code: def env_resolver(cls, var_name: str, env_name: str = None,
                       default: Any = _NONE) -> 'Configuration':
          
  
          cls.resolver(var_name)(
              partial(
                  _env_resolver, var_name=var_name, env_name=env_name,
                  default=default))
          return cls()
docs: Method for configuring environment resolver.
  
          :param var_name: Variable name
          :param env_name: An optional environment variable name. If not set\
              haps looks for `HAPS_var_name`
          :param default: Default value for variable. If it's a callable,\
              is called before return. If not provided\
              :class:`~haps.exceptions.UnknownConfigVariable` is raised
          :return: :class:`~haps.config.Configuration` instance for easy\
                    chaining