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

[Paul Moore]

On 7 November 2017 at 06:39, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 7 November 2017 at 09:23, Chris Barker <chris.barker at noaa.gov> wrote:
>> in short -- we don't have a choice (unless we add an explicit randomization
>> as some suggested -- but that just seems perverse...)
>
> And this is the key point for me: "choosing not to choose" is
> effectively the same as standardising the feature, as enough Python
> code will come to rely on CPython's behaviour that most alternative
> implementations will feel obliged to start behaving the same way
> CPython does (with MicroPython being the potential exception due to
> memory usage constraints always winning over algorithmic efficiency
> concerns in that context).

Personally, I think that having an ordered implementation and then
deliberately breaking that ordering is pretty silly. Not least because
we chose not to do that for 3.6. So we're left with the simple
question of whether we make the behaviour required in the
documentation (which is basically where this thread started). I see 3
options. First, we maintain the status quo, treat it as a CPython/PyPy
implementation detail, and accept that this means that people will
expect it - resulting in pressure on alternative implementations to
conform, but without a language spec to support them. Second, we
document the requirement in the language spec, requiring alternative
implementations to either implement it or document it as a way in
which they don't confirm to the language spec (which is unattractive
for them, as "implements the official Python language spec" is a
selling point). Or third, we could document it as an optional
behaviour, that language implementations are expected to implement if
possible, and if they can't they should document the variance. That is
to some extent the best of both worlds, in that it allows
implementations to claim conformance with the spec while still not
implementing the feature if it causes them problems to do so - they
just have to document that they don't implement this optional but
recommended behaviour. The downside of this option is that there's no
precedent for it in the Python spec (it's basically C's
"implementation defined behaviour", which is something Python hasn't
needed this far).

Paul