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

[Nick Coghlan]

On 13 September 2016 at 12:37, Tim Delaney <timothy.c.delaney at gmail.com> wrote:
> Personally I expect all Python 3.6 implementations will have
> order-preserving dict as that's the easiest way to achieve the existing
> guarantees.

Not all Python 3 implementation will be able to afford the memory hit
that comes from doing that relative to their current approaches (e.g.
MicroPython), and others may be relying on a 3rd party VM for their
core data structures which may not offer a hash map with these
characteristics (VOC and the JVM, Batavia and JavaScript runtimes)

Using collections.OrderedDict selectively may not impose too large a
memory or performance hit, but using it pervasively likely would.

> And that enough code will come to depend on an order-preserving
> dict that eventually the decision will be made to retrospectively guarantee
> the semantics.

We explicitly want to discourage that though, as one of the "alternate
deployment targets" we'd like folks to retain compatibility with at
the library and framework level is single-source 2/3 deployments. Most
incompatibilities are splashy ones that can be detected easily just by
testing on older versions, but this one can be a bit hard to pick up
if you don't already know to check for it.

The benefit of making the official stance be that
dict-ordering-as-the-default-behaviour is an implementation detail, is
that it puts the burden of maintaining compatibility on library and
framework developers, and application developers that support "bring
your own Python runtime" deployments, *not* on interpreter
implementers.

Cheers,
Nick.

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