Used to set the attribute a single time using the given function.

def _setable_set_(name, self, func):
        "Used to set the attribute a single time using the given function."

        setattr(self._attr_data_, name, func())

        if hasattr(self._attr_func_, name):
            delattr(self._attr_func_, name)

        setattr(
            type(self),
            name,
            property(
                functools.partial(self._simple_get_, name)
            )
        )