Interface of the set classes

[Pierre Barbier de Reuille]

Ok, I first want to stress that I looked through the newsgroups archives 
(on Google) for an answer to my question, but I didn't find it.

It's about the interface of the set classes as defined in the PEP 218.
I don't understand why the sets has an interface of mapping object 
without associated value and not an interface of sequence ?

For me, sets are first sequences. I use set where lists are inefficient 
or inadapted, for example when I need to remove redundancy and that I 
don't care about the positions of my elements. So I happen to switch 
from list to sets depending of the data involved (for example if the 
data are non-redundant, lists can be more efficient, but if they are, 
they become too expensive). And that's just impossible to do with Python 
sets ...

Then, why can't you at all access sets element by position ? As Python 
iterator are so poor (mainly, you cannot copy them ...) you have things 
you cannot do without positionnal iteration (typically when you need to 
store the current position and go on the iteration to restart from the 
saved position later on).

Please, if this discussion already exists give me the pointer ... and 
don't tell me "because of implementation if over dictionnary" ... I 
don't think that can be a good answer :o)

Thanks,

Pierre Barbier de Reuille