[Python-Dev] More compact dictionaries with faster iteration

Raymond Hettinger raymond.hettinger at gmail.com
Sun May 19 07:27:36 CEST 2013

On May 15, 2013, at 4:32 AM, Christian Tismer <tismer at stackless.com> wrote:

> What is the current status of this discussion?
> I'd like to know whether it is a considered alternative implementation.

As far as I can tell, I'm the only one working on it (and a bit slowly at that).
My plan is to implement it for frozensets to see how it works out.

Frozensets are a nice first experiment for several reasons:
* The current implementation is cleaner than dictionaries
   (which have become more complicated due to key-sharing).
* It will be easy to benchmark (by racing sets vs frozen sets)
   for an apples-to-apples comparison.
* There is no need to have a list-like over-allocation scheme
   since frozensets can't grow after they are created. 
   That will guarantee a significant space savings and
   it will simplify the coding.
* I wrote the code for setobject.c so I know all the ins-and-outs. 

> There is also a discussion in python-ideas right now where this
> alternative is mentioned, and I think especially for small dicts
> as **kwargs, it could be a cheap way to introduce order.

The compaction of keys and values into a dense array was
intended to save space, improve cache performance, and
improve iteration speed.  The ordering was just a side-effect
and one that is easily disturbed if keys ever get deleted.

So a compacted dict might be a cheap way to introduce order
for kwargs, but it would need special handling if the user decided
to delete keys.

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).

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