[Python-Dev] Ordering keyword dicts (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Sun May 19 15:01:48 CEST 2013

On Sat, 18 May 2013 22:47:35 -0700 Guido van Rossum <guido at python.org> wrote:

On Sat, May 18, 2013 at 10:27 PM, Raymond Hettinger <raymond.hettinger at gmail.com> wrote: > BTW, I'm +1 on the idea for ordering keyword-args. It makes > it easier to debug if the arguments show-up in the order they > were created. AFAICT, no purpose is served by scrambling them > (which is exacerbated by the new randomized hashing security feature).

I'm slow at warming up to the idea. My main concern is speed -- since most code doesn't need it and function calls are already slow (and obviously very common :-) it would be a shame if this slowed down function calls that don't need it noticeably. An observation is that it's only necessary to preserve order if the function definition uses **kwds. AFAIK we currently don't know if this is the case when the call is made though, but perhaps the information could be made available to the call site somehow. There are also many special cases to consider; e.g. using **kwds in the call where kwds is an unordered dict, or calls from C, or calls to C. But maybe someone considers this a challenge and comes up with a patch? The benefits to some use cases would be obvious.

The main use case seems to be the OrderedDict constructor itself. Otherwise, I can't think of any situation where I would've wanted it.

Changing keyword arguments to be an OrderedDict without impacting performance in all the cases you mentioned (and without breaking C-level compatibility) would be a real, tough challenge.

Regards

Antoine.

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