Private mutable state

foo.py

@attrs(frozen=True)
class Foo(object):
    """
    Something Foo-y
    """

    @attrs(frozen=False)
    class _State(object):
        """
        Internal mutable state for Foo.
        """

        thing = attrib(init=False)

    # Attribute assigned via init
    _path = attrib(validator=optional(instance_of(Path)))

    # Private attribute to store internal state
    _state = attrib(default=_State(), init=False)

    def __attrs_post_init__(self):
        self._state.thing = thingFromPath(self._path)