Is python's “set” stable?

The question arose when answering to another SO question (there).

When I iterate several times over a python set (without changing it between calls), can I assume it will always return elements in the same order? And if not, what is the rationale of changing the order ? Is it deterministic, or random? Or implementation defined?

And when I call the same python program repeatedly (not random, not input dependent), will I get the same ordering for sets?

The underlying question is if python set iteration order only depends on the algorithm used to implement sets, or also on the execution context?