[Python-Dev] Guarantee ordered dict literals in v3.7?

[Chris Barker]

On Tue, Nov 7, 2017 at 7:21 AM, David Mertz <mertz at gnosis.cx> wrote:

> But like Raymond, I make most of my living TEACHING Python.
>

and I make least of my living TEACHING Python ( :-) ),and:


> I feel like the extra order guarantee would make teaching slightly harder.
>

I can't understand how this possibly makes python (or dicts) harder to
teach -- you can simply say: "dicts insertion order is preserved" or not
mention it at all -- I think most people kind of expect it to be preserved,
which is why I (used to )always bring up the lack-of-ordering of dicts
early on -- but I suspect I simply won't bother mentioning it if it's
decided as a language feature.

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.
>

Exactly! I have a really hard time deciding how to handle this --
explaining that ordering is not guaranteed, but not being able to
demonstrate it! And frankly, my students are all going to forget what I
"explained" soon enough, and replace it with their experience -- which will
be that dicts retain their order.

But then, unordered sets also wind up sorting small integers on printing,
> even though that's not a guarantee.
>

but it's not hard to make an easy example with order not preserved -- jsut
start with a non order example:

In [6]: s = {3,7,4}

In [7]: s

Out[7]: {3, 4, 7}

or use other types...

And  "set" is a mathematical concept that has no oder, whereas the
"dictionary" metaphor DOES have order...

> Ordering by insertion order (possibly "only until first deletion") is
> simply not obvious to beginners.
>

the "only until first deletion" part is really hard -- I hope we don't go
that route. But I don't think insertion-order is non-obvious --
particularly with literals.


> 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".
>

again, I don't think so -- I kind of agree if dicts did not preserve order
in practice -- demonstrating that right out of the gate does help make the
"key/Val association" clear -- but if you can't demonstrate it, I think
we're looking at more confusion...

Maybe I'll ask my students this evening -- this is the first class I'm
teaching with py3.6 ....

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.
>
>
exactly!

-CHB

-- 

Christopher Barker, Ph.D.
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