[Python-Dev] PEP 468 (Ordered kwargs)

[Armin Rigo]

Hi all,

On 24 January 2015 at 11:50, Maciej Fijalkowski <fijall at gmail.com> wrote:
> I would like to point out that we implemented rhettingers idea in PyPy
> that makes all the dicts ordered by default and we don't have any
> adverse performance effects (in fact, there is quite significant
> memory saving coming from it). The measurments on CPython could be
> different, but in principle OrderedDict can be implemented as
> efficiently as normal dict.

I would like to add that http://bugs.python.org/issue16991 is the same
as today's dicts with an additional doubly-linked list for the order.
I'm unsure why you would do that after the 2012 thread started by
Raymond Hettinger, but anyway, don't conclude from only this that in
the CPython case ordered dictionaries would be slower and bigger.  My
guess is that, with a simple port of what is now in PyPy, they would
not be (but of course no-one can be sure at this point).  Let's say,
if you could imagine that CPython's dictionaries, tomorrow, are always
magically fully ordered, then would it still be a bad idea?

If such a discussion would resurface (soon or "one day"), and if other
related issues are resolved (like what to do in Jython and
IronPython), and if the conclusion would tentatively turn out
positive... then, provided there would at that point still be no
"Raymond-style" implementation of dicts, I would volunteer to port
PyPy's one to CPython[1].  As you may have guessed I don't consider
this particularly likely to occur, but it is a standing offer
nevertheless :-)


A bientôt,

Armin.


[1]  Someone could also do such a port for the goal of getting an
alternate `odictobject.c`.  He would be welcome to #pypy to get some
help from the PyPy guys, including me --- but my offer above doesn't
apply in this case.  I want to remove a thorn in the foot of
python-dev discussing about the language; I'm not really interested in
contributing to the `collections.OrderedDict` type.