class template:
    def __init__(self) -> None:
        self.__default = set()
        self.__sections = dict()

    def _add_key(self, key: str, section: str | None = None) -> None:
        if section is None:
            self.__default.add(key)
            return
            
        if not section in self.__sections:
            self.__sections[section] = set()

        self.__sections[section].add(key)
        
    @property
    def default_keys(self) -> set:
        return self.__default.copy()

    @property
    def sections_names(self) -> set:
        return set(self.__sections.keys())

    def get_section_keys(self, section: str) -> set:
        if not section in self.__sections:
            raise RuntimeError("Section " + section + " not exists.")

        return self.__sections[section].copy()