Is python’s “set” stable?

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There’s no formal guarantee about the stability of sets. However, in the CPython implementation, as long as nothing changes the set, the items will be produced in the same order. Sets are implemented as open-addressing hashtables (with a prime probe), so inserting or removing items can completely change the order (in particular, when that triggers a resize, which reorganizes how the items are laid out in memory.) You can also have two identical sets that nonetheless produce the items in different order, for example:

>>> s1 = {-1, -2}
>>> s2 = {-2, -1}
>>> s1 == s2
True
>>> list(s1), list(s2)
([-1, -2], [-2, -1])

Unless you’re very certain you have the same set and nothing touched it inbetween the two iterations, it’s best not to rely on it staying the same. Making seemingly irrelevant changes to, say, functions you call inbetween could produce very hard to find bugs.

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