Re: insert unique data in a list

[Gary Herron]

mattia wrote:
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')...
  

Sets can contain *only* hashable objects, but lists are not hashable 
(since they are mutable).

Perhaps, you could convert your list to a tuple first -- tuples *are* 
hashable.

>>> s = set()
>>> l = [1,2,3]
>>> s.add(l)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
>>> s.add(tuple(l))
>>> s
set([(1, 2, 3)])

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