> Actually, it's not enough. If you can mutate the class variables (attributes), then you can always create a class that doesn't have these invariants enforced. The only way that could completely prevent invariants in the class being disrespected would be to have a strong, possibly dependent, type system.
You can completely avoid it if the invariants are expressed as validation methods, and mutation of state isn't direct but through a mechanism initially updates shadow state and guarantees that validators are run and failed validations result in both the original state being preserved and an error being signalled, and only successful validation results in state updates.
A sufficiently robust type system is superior than this kind of runtime check in all kinds of ways, but is not the only way to avoid invalid state with mutable objects.
You can completely avoid it if the invariants are expressed as validation methods, and mutation of state isn't direct but through a mechanism initially updates shadow state and guarantees that validators are run and failed validations result in both the original state being preserved and an error being signalled, and only successful validation results in state updates.
A sufficiently robust type system is superior than this kind of runtime check in all kinds of ways, but is not the only way to avoid invalid state with mutable objects.