[Zope-dev] difference between OOSet and OOTreeSet?

[Gary Poster]

On Mar 1, 2007, at 3:18 PM, Chris Withers wrote:

> Gary Poster wrote:
>>> Traceback (most recent call last):
>>>   File "<stdin>", line 1, in ?
>>> TypeError: argument to reversed() must be a sequence
>> The fact that this works for the OOSet is an implementation accident.
>> As discussed elsewhere in this thread, sets are not sequences.   
>> The fact that the elements are ordered is an implementation detail  
>> (although given that these sets are essentially the keys of a  
>> BTree, it's unsurprising that they are).
>> You could argue that reversed(OOSet((1,2,3))) should raise an  
>> exception.  IMO, why bother.
>
> *grunt*
>
> Okay, so I want a persistent, ordered sequence which is quick to  
> find items in and which doesn't re-store the whole sequence when an  
> item is inserted or removed.
>
> What should I be using?

Ordered, as in sorted?  Or ordered, as in user-determined order?

If sorted, use BTreeSet (or the keys of a BTree).  Even though my  
assertion is right semantically for "Set", this is a BTreeSet, and I  
don't see this behavior changing ever.  No, it isn't a sequence, so  
`reversed` won't work, but `list` will always give you the same  
order.  "Set" does not match the  "doesn't re-store the whole  
sequence when an item is inserted or removed" requirement.

If user-determined (like a list) then you are currently out of luck.   
I actually have a implementation of this (which I think is cool) with  
some tests, but I regard it as experimental and am refactoring it in  
my spare (ha) time.  (I would not mind you looking at what I have,  
but I don't particularly recommend it yet for production, so I expect  
it is useless to you.)

In both cases, IMO you'll want your data to be of homogenous types.

Gary