[nerdponx]

The problem with Python here is that CPython is not only the reference implementation but the de-facto specification. So dicts are still "supposed to be" unordered collections, but now dicts must also preserve insertion order as per the docs and the reference implementation, so now all alternative implementations must also conform to this even if it doesn't make sense for them to conform to it, or they must specifically choose to be non-comformant on this point.

Of course in this case, the order-preserving optimization was actually first implemented by an alternative implementation (PyPY), but I don't think that changes the issue.

[lemagedurage]

Since Python 3.7 preservering insertion-order is part of the language specification.

"the insertion-order preservation nature of dict objects has been declared to be an official part of the Python language spec."

https://docs.python.org/3/whatsnew/3.7.html

[nerdponx]

Right, but that's kind of my point. Adding it to the language spec now creates an additional and frankly somewhat unnecessary point of compliance for other implementations. Python is already so damn big and complicated, my opinion is that we shouldn't makes its spec even more complicated, even if its reference implementation adds more features like this.