Атрибут __slots__

Пример кода из модуля ipaddress

class _IPAddressBase:

    """The mother class."""

    __slots__ = ()

    @property
    def exploded(self):
        """Return the longhand version of the IP address as a string."""
        return self._explode_shorthand_ip_string()

    @property
    def compressed(self):
        """Return the shorthand version of the IP address as a string."""
        return str(self)


@functools.total_ordering
class _BaseAddress(_IPAddressBase):

    """A generic IP object.
    This IP class contains the version independent methods which are
    used by single IP addresses.
    """

    __slots__ = ()

    def __int__(self):
        return self._ip

    def __eq__(self, other):
        try:
            return (self._ip == other._ip
                    and self._version == other._version)
        except AttributeError:
            return NotImplemented

    def __lt__(self, other):
        if not isinstance(other, _BaseAddress):
            return NotImplemented
        if self._version != other._version:
            raise TypeError('%s and %s are not of the same version' % (
                             self, other))
        if self._ip != other._ip:
            return self._ip < other._ip
        return False

class _BaseV4:

    """Base IPv4 object.
    The following methods are used by IPv4 objects in both single IP
    addresses and networks.
    """

    __slots__ = ()
    _version = 4
    # Equivalent to 255.255.255.255 or 32 bits of 1's.
    _ALL_ONES = (2**IPV4LENGTH) - 1
    _DECIMAL_DIGITS = frozenset('0123456789')

    # the valid octets for host and netmasks. only useful for IPv4.
    _valid_mask_octets = frozenset({255, 254, 252, 248, 240, 224, 192, 128, 0})

    _max_prefixlen = IPV4LENGTH
    # There are only a handful of valid v4 netmasks, so we cache them all
    # when constructed (see _make_netmask()).
    _netmask_cache = {}

    def _explode_shorthand_ip_string(self):
        return str(self)


class IPv4Address(_BaseV4, _BaseAddress):

    """Represent and manipulate single IPv4 Addresses."""

    __slots__ = ('_ip', '__weakref__')

    def __init__(self, address):

        """
        Args:
            address: A string or integer representing the IP
              Additionally, an integer can be passed, so
              IPv4Address('192.0.2.1') == IPv4Address(3221225985).
              or, more generally
              IPv4Address(int(IPv4Address('192.0.2.1'))) ==
                IPv4Address('192.0.2.1')
        Raises:
            AddressValueError: If ipaddress isn't a valid IPv4 address.
        """