Regarding the constraints, the article also makes a very important point: it's not all or nothing, you can have _some_ invariants and pre/post conditions easily

Regarding the constraints, the article also makes a very important point: it's not all or nothing, you can have _some_ invariants and pre/post conditions easily. I.e. size of arrays or "nonblankness" of strings, or balanced invoices. The simple fact of thinking about the constraints, will help design code.