On 16Dec2019 0417, Petr Viktorin wrote:
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.)
For the record, we missed out on a very memory efficient "frozendict"
implementation because it can't maintain insertion order - Yury is
currently proposing it as FrozenMap in PEP 603.
https://discuss.python.org/t/pep-603-adding-a-frozenmap-type-to-collections/2318
Codifying semantics isn't always the kind of future-proof we necessarily
want to have :)
Cheers,
Steve
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