Standard ways to get union, intersection, difference of lists?

Mickel Grönroos wrote:
[color=blue]
> Are there any standard list methods for getting the intersection and
> difference of two lists? (The union is easy ("list1.extend(list2)"),
> unless you want it to contain unique values.)[/color]

If you have Python 2.3 available, why not use a set?
[color=blue][color=green][color=darkred]
>>> import sets
>>> s1 = sets.Set([1, 2, 3])
>>> s2 = sets.Set([3, 4])
>>> s1.intersection(s2)[/color][/color][/color]
Set([3])[color=blue][color=green][color=darkred]
>>> s1.difference(s2)[/color][/color][/color]
Set([1, 2])[color=blue][color=green][color=darkred]
>>> s2.difference(s1)[/color][/color][/color]
Set([4])

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>>>>> "Mickel" == Mickel Grönroos <mickel@csc.fi> writes:
[color=blue]
> Hi! Are there any standard list methods for getting the
> intersection and difference of two lists? (The union is easy
> ("list1.extend(list2)"), unless you want it to contain unique
> values.)[/color]

[snip]
[color=blue]
> I realize I could quite easily implement this myself, but I was
> hoping for a built-in solution.[/color]

You might want to take a look at the sets module in Python 2.3. It's
not exactly what you ask for since sets.Sets are not lists - but they
do support these operations and can easily be converted to or from
lists.

I know lots of people hate using filter and lambda calls but this will give you a union between 2 lists, example:

[COLOR=Green]list1=range(5) # [0,1,2,3,4]
list2=range(3,7) # [3,4,5,6]
union=list1+filter(lambda x:x not in list1,list2)
# union = [0,1,2,3,4,5,6]
[/COLOR]

Intersection is just as easy

[COLOR=Green]
intersection=filter(lambda x:x in list1,list2)
# intersection=[3,4]
[/COLOR]

Difference is the last thing you wanted...
[COLOR=Green]
difference=filter(lambda x:x not in list2,list1)
# difference=[0,1,2]
[/COLOR]

And my set vocabulary is rusty but the distinct elements or those from both lists that are NOT in common (un-intersection?) would be:

[COLOR=Green]
distinct=filter(lamba x:x not in list2,list1)+filter(lambda x:x not in list1,list2)
# distinct=[0,1,2,5,6]
[/COLOR]

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Xoanan