I don't know if this works the same for sets, but for dicts, this is the semantics that everyone has wanted for a long time. It makes doctests (and similar things) easier to write, reduces a source of nondeterministic failure, and removes a wart that everyone had to learn:

>>> {"foo": 1, "bar": 2, "baz": 3}
{'baz': 3, 'foo': 1, 'bar': 2}
>>>

It's essentially the same reason as why Python specifies that expressions are (in most cases) evaluated from left to right. User don't realize that f()+g() might call g() before f(), and write code that assumes f() is called first -- the language should not disappoint them, optimization opportunities be damned.

On Mon, Dec 16, 2019 at 4:23 AM Petr Viktorin <encukou@gmail.com> wrote:

On 2019-12-16 06:00, Larry Hastings wrote:
[...]
>
> These are all performance concerns. As I mentioned previously in this
> thread, in my opinion we should figure out what semantics we want for
> the object, then implement those, and only after that should we worry
> about performance. I think we should decide the question "should set
> objects maintain insertion order?" literally without any consideration
> about performance implications.

Then this thread is missing arguments saying *why* ordered dicts are
actually better, semantics-wise.

Originally, making dicts ordered was all about performance (or rather
memory efficiency, which falls in the same bucket.) It wasn't added
because it's better semantics-wise.
Here's one (very simplified and maybe biased) view of the history of dicts:

* 2.x: Dicts are unordered, please don't rely on the order.
* 3.3: Dict iteration order is now randomized. We told you not to rely
on it!
* 3.6: We now use an optimized implementation of dict that saves memory!
As a side effect it makes dicts ordered, but that's an implementation
detail, please don't rely on it.
* 3.7: Oops, people are now relying on dicts being ordered. Insisting on
people not relying on it is battling windmills. Also, it's actually
useful sometimes, and alternate implementations can support it pretty
easily. Let's make it a language feature! (Later it turns out
MicroPython can't support it easily. Tough luck.)

By itself, "we already made dicts do it" is not a great argument in the
set *semantics* debate.
Of course, it may turn out ordering was a great idea semantics-wise as
well, but if I'm reading correctly, so far this thread has one piece of
anectodal evidence for that.
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