insert unique data in a list

[mattia]

Il Sun, 13 Dec 2009 16:37:20 +0000, mattia ha scritto:

> How can I insert non-duplicate data in a list? I mean, is there a
> particular option in the creation of a list that permit me not to use
> something like:
> def append_unique(l, val):
>     if val not in l:
>         l.append(val)
> 
> Thanks,
> Mattia

Ok, so you all suggest to use a set. Now the second question, more 
interesting. Why can't I insert a list into a set? I mean, I have a 
function that returns a list. I call this function several times and 
maybe the list returned is the same as another one already returned. I 
usually put all this lists into another list. How can I assure that my 
list contains only unique lists? Using set does'n work (i.e. the python 
interpreter tells me: TypeError: unhashable type: 'list')...