[Petr Viktorin <encu...@gmail.com>] > ... > 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.
As I tried to flesh out a bit in a recent message, the original "compact dict" idea got all the memory savings, but did _not_ maintain insertion order. Maintaining insertion order too complicated deletions (see recent message), but was deliberately done because people really did want "ordered dicts". > 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.) A very nice summary! My only quibble is as above: the "compact dict" implementation doesn't maintain insertion order naturally, _unless_ there are no deletions (which is, e.g., true of dicts constructed to pass keyword arguments). The code got hairier to maintain insertion order in the presence of mixing insertions and deletions. _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/TRVOCOLQGSOM2OLLAI3UPRCTFIKIWWH6/ Code of Conduct: http://python.org/psf/codeofconduct/
