[Python-Dev] Python 3.6 dict becomes compact and gets a private version; and keywords become ordered

[Nathaniel Smith]

On Fri, Sep 9, 2016 at 11:39 AM, Barry Warsaw <barry at python.org> wrote:
> On Sep 09, 2016, at 01:08 PM, Elvis Pranskevichus wrote:
>
>>Are there any downsides to explicitly specifying that all dicts are ordered?
>>People will inevitably start relying on this behaviour, and this will
>>essentially become the *de-facto* spec, so alternative Python implementations
>>will have to follow suit anyway.
>
> It *might* make sense to revisit this once 3.5 is no longer maintained at all,
> but I think Guido's exactly right in his analysis.  If people start relying on
> all dicts being ordered now, their code won't be compatible with both 3.5 and
> 3.6, and I think it's important to emphasize this to developers.

I feel like I'm missing something here... by this reasoning, we should
*never* change the language spec when new features are added. E.g. if
people use async/await in 3.5 then their code won't be compatible with
3.4, but async/await are still part of the language spec. And in any
case, the distinction between "CPython feature" and "Python
language-spec-guaranteed feature" is *extremely* arcane and
inside-basebally -- it seems really unlikely that most users will even
understand what this distinction means, never mind let it stop them
from writing CPython-and-PyPy-specific code. Emphasizing that this is
a new feature that only exists in 3.6+ of course makes sense, I just
don't understand why that affects the language spec bit.

(OTOH it doesn't matter that much anyway... the language spec is
definitely a useful thing, but it's largely aspirational in practice
-- other implementations target CPython compatibility more than they
target language spec compatibility.)

-n

-- 
Nathaniel J. Smith -- https://vorpus.org