How to use the dataclasses._FIELDS function in dataclasses

def __str__(self):
        fields = getattr(self, _FIELDS, None)
        if fields is None:
            return ""

        fields = [f for f in fields.values() if f._field_type is _FIELD]
        fields_str = ', '.join(f"{f.name}={getattr(self, f.name)}" for f in fields)
        return f"{self.__class__.__qualname__}({fields_str})"

def _dataclass__str__(self):
    fields = getattr(self, _FIELDS, None)
    if fields is None:
        return ""

    fields = [f for f in fields.values() if f._field_type is _FIELD]
    fields_str = ', '.join(f"{f.name}={getattr(self, f.name)}" for f in fields)
    return f"{self.__class__.__qualname__}({fields_str})"

def defaults(self):
        d = {}
        for ancestor in self.cls.__mro__[-1:0:-1]:
            if is_dataclass(ancestor):
                for field in getattr(ancestor, _FIELDS).values():
                    d[field.name] = field.default
        for name in self.__get_fields(recursive=False):
            d[name] = self.namespace.get(name, MISSING)
        return d