This one is less painful for the devs to break, if they need to, because it only works 99% of the time. If you rely on this behaviour for correctness, rather th

This one is less painful for the devs to break, if they need to, because it only works 99% of the time. If you rely on this behaviour for correctness, rather than performance, it will set you right pretty quickly. If you do want to take advantage of this, maybe you can go a level deeper: dig into Python's behaviour when allocating memory for strings, and figure out in what circumstances you can be guaranteed to get the same id back. E.g. maybe if you create a 28-character string, there will always be room to append 4 more.