complementary lists?

[Arnaud Delobelle]

Kay Schluehr <kay.schluehr at gmx.net> writes:

> On 29 Apr., 05:41, Ross <ross.j... at gmail.com> wrote:
>> If I have a list x = [1,2,3,4,5,6,7,8,9] and another list that is a
>> subset of x:  y = [1,4,7] , is there a quick way that I could return
>> the complementary subset to y.... z=[2,3,5,6,8,9] ?
>>
>> The reason I ask is because I have a generator function that generates
>> a list of tuples and I would like to divide this list into
>> complementary lists.
>
> z = [u for u in x if u not in y]
>
> or
>
> z = [u for u in x if u not in set(y)]

The above will evaluate set(y) for each element in x.

s = set(y)
z = [u for u in x if u not in s]

-- 
Arnaud