Why are there no ordered dictionaries?

[Tom Anderson]

On Tue, 22 Nov 2005, Christoph Zwerschke wrote:

> One implementation detail that I think needs further consideration is in 
> which way to expose the keys and to mix in list methods for ordered 
> dictionaries.
>
> In Foord/Larosa's odict, the keys are exposed as a public member which 
> also seems to be a bad idea ("If you alter the sequence list so that it 
> no longer reflects the contents of the dictionary, you have broken your 
> OrderedDict").
>
> I think it would be probably the best to hide the keys list from the public, 
> but to provide list methods for reordering them (sorting, slicing etc.).

I'm not too keen on this - there is conceptually a list here, even if it's 
one with unusual constraints, so there should be a list i can manipulate 
in code, and which should of course be bound by those constraints.

I think the way to do it is to have a sequence property (which could be a 
managed attribute to prevent outright clobberation) which walks like a 
list, quacks like a list, but is in fact a mission-specific list subtype 
whose mutator methods zealously enforce the invariants guaranteeing the 
odict's integrity.

I haven't actually tried to write such a beast, so i don't know if this is 
either of possible and straightforward.

tom

-- 
When I see a man on a bicycle I have hope for the human race. --
H. G. Wells