Why are there no ordered dictionaries?

[Fuzzyman]

Alex Martelli wrote:
> Fuzzyman <fuzzyman at gmail.com> wrote:
>    ...
> > > - the internal keys list should be hidden
> >
> > I disagree. It is exposed so that you can manually change the order
> > (e.g. to create a "sorted" dict, rather than one ordered by key
> > insertion).
> >
> > What do you *gain* by hiding it ?
>
> Freedom of implementation, of course.  E.g., I can perhaps get better
> performance by keeping a red-black tree of (insertiontime,key) pairs, so
> for example deleting a key is O(log(N)) rather than O(N) as it would
> have to be if the sequence had to be kept as a Python list.
>
> What ELSE do you ever gain by enforcing abstraction and information
> hiding?  Conceptual integrity and implementation freedom...
>

The poster I was replying to inferred that we should hide it to protect
him from breaking it. :-)

Your reason is much more valid than the one I inferred. (And probably
worth chanign the code for).

>
> > > - list methods should be mixed in instead
> >
> > Hmm... I did consider this. Which list methods would you consider
> > appropriate ?
> >
> > The difficulty is that integers are valid keys for a dictionary.
> > Perhaps a subclass that uses integers as indexes instead...
>
> What about allowing slicing, taking advantage of the fact that slices
> are NOT valid dictionary keys?

Hmm... ok, simple enough to implement. What would anyone *use* it for
though ?

> Presumably sort and reverse methods are also desired.
>

Yeah - good idea. Insert is also good - I can't remember if we provide
this or not.

Thanks


Fuzzyman
http://www.voidspac.org.uk/python/index.shtml

> > You can always perform list operations on the ``sequence`` attribute of
> > course.
>
> But NOT having to expose .sequence as a real mutable list whose changes
> affect ordering has many advantages, as above noticed.
> 
> 
> Alex