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

Nathaniel Smith njs at pobox.com
Sun Nov 5 19:31:05 EST 2017

On Nov 5, 2017 2:41 PM, "Paul Ganssle" <pganssle at gmail.com> wrote:

I think the question of whether any specific implementation of dict could
be made faster for a given architecture or even that the trade-offs made by
CPython are generally the right ones is kinda beside the point. It's
certainly feasible that an implementation that does not preserve ordering
could be better for some implementation of Python, and the question is
really how much is gained by changing the language semantics in such a way
as to cut off that possibility.

The language definition is not nothing, but I think it's easy to
overestimate its importance. CPython does in practice provide ordering
guarantees for dicts, and this solves a whole bunch of pain points: it
makes json roundtripping work better, it gives ordered kwargs, it makes it
possible for metaclasses to see the order class items were defined, etc.
And we got all these goodies for better-than-free: the new dict is faster
and uses less memory. So it seems very unlikely that CPython is going to
revert this change in the foreseeable future, and that means people will
write code that depends on this, and that means in practice reverting it
will become impossible due to backcompat and it will be important for other
interpreters to implement, regardless of what the language definition says.

That said, there are real benefits to putting this in the spec. Given that
we're not going to get rid of it, we might as well reward the minority of
programmers who are conscientious about following the spec by letting them
use it too. And there were multiple PEPs that went away when this was
merged; no one wants to resurrect them just for hypothetical future
implementations that may never exist. And putting it in the spec will mean
that we can stop having this argument over and over with the same points
rehashed for those who missed the last one. (This isn't aimed at you or
anything; it's not your fault you don't know all these arguments off the
top of your head, because how would you? But it is a reality of mailing
list dynamics that rehashing this kind of thing sucks up energy without
producing much.)

MicroPython deviates from the language spec in lots of ways. Hopefully this
won't need to be another one, but it won't be the end of the world if it is.

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