"Ordered" dicts

[Martin Miller]

mekstran at scl.ameslab.gov wrote:
> > Lots and lots of people want ordered dicts it seems. Or at least, they
> > want
> > to be able to access their dictionary keys in order.
> > [snipped lots of examples, nice pro-con lists, etc.]
> >  What do y'all think?
>
> I'll second the need for this. Although, what can also be useful as a
> further extension (at least I needed this capability for a project I
> worked on) is the ability to not only have the keys sorted, but have them
> arranged in an arbitrary order (I had parsed some XML data, and each
> "object" represented in the XML had certain fields, which I was storing in
> a dictionary, but I needed each object to iterate that dictionary in the
> same order, preferably the order they occured in the file).
>
> I accomplished this through subclassing dict to create a
> SpecialSortedDict, overriding keys(), values(), the iterxxx() functions,
> and any others relevant so that it had an associated key order list. It
> would return keys in the specified order (ignoring any in the order list
> that weren't in the dictionary), an then any remaing keys were sorted at
> the end.
>
> I don't know if that'd be a useful enhancement to Chris's idea, or if it's
> a really obscure corner case. But I thought I'd throw it out there.

The need to have an ordered dictionaries is fairly common, IMHO. Having
one with keys sorted alphabetically is just a specific type of
ordering.

Russell Owen of the University of Washington has long had a class that
supports exactly these capabilities in his free RO Python package
(along with a number of  other useful things), see
<http://www.astro.washington.edu/owen/ROPython.html>.

Owen's 'OrderedDict' class is [also] implemented by subclassing dict.

To avoid continued reinvention of this wheel, I'd also vote to have
this functionality be at least included in a standard module, if not
built-in.

They say necessity is the mother of invention, therefore a lack of
standards or built-ins must be the mother of reinvention. ;-)

-Martin