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

Nathaniel Smith njs at pobox.com
Sat Sep 10 19:41:28 EDT 2016

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 It might make sense to revisit this once 3.5 is no longer maintained at all, but I think Guido's exactly right in his analysis.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

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