[Python-Dev] Guarantee ordered dict literals in v3.7? (original) (raw)

David Mertz mertz at gnosis.cx
Tue Nov 7 10:21:21 EST 2017

On Nov 6, 2017 9:11 PM, "Raymond Hettinger" <raymond.hettinger at gmail.com> wrote:

On Nov 6, 2017, at 8:05 PM, David Mertz <mertz at gnosis.cx> wrote: I strongly opposed adding an ordered guarantee to regular dicts. If the implementation happens to keep that, great. Maybe OrderedDict can be rewritten to use the dict implementation. But the evidence that all implementations will always be fine with this restraint feels poor, and we have a perfectly good explicit OrderedDict for those who want that.

I think this post is dismissive of the value that users would get from having reliable ordering by default.

Dismissive seems like an overly strong word. I recognize I disagree with Raymond on best official semantics. Someone else points out that if someday an "even more efficient unordered dict" is discovered, user-facing "dict" doesn't strictly have to be the same data structure as "internal dict". The fact they are is admittedly an implementation detail also.

I've had all those same uses about round-tripping serialization that Raymond mentions. I know the standard work arounds (which are not difficult, but DO require a little extra code if we don't have order).

But like Raymond, I make most of my living TEACHING Python. I feel like the extra order guarantee would make teaching slightly harder. I'm sure he feels contrarily. It is true that with 3.6 I can no longer show an example where the dict display is oddly changed when printed. But then, unordered sets also wind up sorting small integers on printing, even though that's not a guarantee.

Ordering by insertion order (possibly "only until first deletion") is simply not obvious to beginners. If we had, hypothetically, a dict that "always alphabetized keys" that would be more intuitive to them, for example. Insertion order feels obvious to us experts, but it really is an extra cognitive burden to learners beyond understanding "key/Val association".

Having worked with Python 3.6 for a while, it is repeatedly delightful to encounter the effects of ordering. When debugging, it is a pleasure to be able to easily see what has changed in a dictionary. When creating XML, it is joy to see the attribs show in the same order you added them. When reading a configuration, modifying it, and writing it back out, it is a godsend to have it written out in about the same order you originally typed it in. The same applies to reading and writing JSON. When adding a VIA header in a HTTP proxy, it is nice to not permute the order of the other headers. When generating url query strings for REST APIs, it is nice have the parameter order match documented examples.

We've lived without order for so long that it seems that some of us now think data scrambling is a virtue. But it isn't. Scrambled data is the opposite of human friendly.

Raymond

P.S. Especially during debugging, it is often inconvenient, difficult, or impossible to bring in an OrderedDict after the fact or to inject one into third-party code that is returning regular dicts. Just because we have OrderedDict in collections doesn't mean that we always get to take advantage of it. Plain dicts get served to us whether we want them or not. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20171107/a39b759d/attachment.html>

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