Right, they shouldn't be just aliases, but Constant subclasses with __new__ which return Constant and __instancecheck__ which checks the type of the value. And the Constant class should have writable properties n and s. All these operations should emit a deprecation warning at runtime. Even this doesn't preserve perfect compatibility. issubclass(type(node), Num) will not work, and compile(Num('123')) will raise en exception in the Num constructor instead of compile().