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

Nick Coghlan ncoghlan at gmail.com
Sun Nov 5 22:07:51 EST 2017

On 6 November 2017 at 06:50, Peter Ludemann via Python-Dev <python-dev at python.org> wrote:

Isn't ordered dict also useful for **kwargs?

3.6 already made this semantic change for **kwargs and class execution namespaces - it just left the door open to having implementations meet those requirements by way of substituting in collections.OrderedDict for those use cases, rather than using a regular builtin dictionary.

So we're also already imposing the requirement on conformant Python implementations to have a reasonably performant insertion-ordered mapping implementation available.

The open questions around insertion ordering thus relate to what user expectations should be for:

It's that last category which is particularly problematic now, as in CPython 3.6, the dicts used for these operations actually are insertion ordered, so round trips from disk, through a CPython builtin dict, and then back to disk will be order preserving, whereas in previous versions there was no guarantee that that would be the case (and hash randomisation meant the key reordering wasn't even deterministic).

While such operations were already order preserving in PyPy (since their switch to an insertion-ordered builtin dict implementation served as prior art for the subsequent switch in CPython), we know from experience that it's incredibly easy for even things that are nominally implementation details of CPython to become expected behaviour for Python users (e.g. there's still plenty of code out there that relies on the use of an eager refcounting memory management strategy to handle external resources properly).

That means our choices for 3.7 boil down to:

When we did the "insertion ordered hash map" availability review, the main implementations we were checking on behalf of were Jython & VOC (JVM implementations), Batavia (JavaScript implementation), and MicroPython (C implementation). Adding IronPython (C# implementation) to the mix gives:

Since the round-trip behaviour that comes from guaranteed order preservation is genuinely useful, and we're comfortable with folks switching to more specialised containers when they need different performance characteristics from what the builtins provide, elevating insertion order preservation to a language level requirements makes sense.

Cheers, Nick.

[1] https://blog.golang.org/go-maps-in-action (search for "Iteration Order") [2] https://docs.oracle.com/javase/8/docs/api/java/util/LinkedHashMap.html [3] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map [4] https://docs.microsoft.com/en-us/dotnet/api/system.collections.specialized.ordereddictionary?view=netframework-4.7.1

-- Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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