Most pythonic means to find out if an element from one list is not a member of another list

Question

Possible Duplicate:
Get difference from 2 lists. Python

I have two lists

rt = [1,2,3]
dp = [1,2]

What is the most pythonic way to find out that in the rt list that 3 is not a element of the dp list?

The answer depends on whether the elements are hashable/sortable — John La Rooy, Jul 10 '12 at 04:42

GWW · Accepted Answer · 2012-07-10T04:40:43.930

7

>>> rt = [1,2,3]
>>> dp = [1,2]

You can use sets:

>>> set(rt) - set(dp)
set([3])

Or a list comprehension:

>>> [x for x in rt if x not in dp]
>>> [3]

EDIT: jamylak pointed out you could use a set to improve the efficiency of membership lookup:

>>> dp_set = set(dp)
>>> [x for x in rt if x not in dp_set]
>>> [3]

edited Jul 10 '12 at 04:40

answered Jul 10 '12 at 04:34

GWW

2

To be most efficient you could make `dp_set = set(dp)` and check for membership out of that in your list comp. – jamylak Jul 10 '12 at 04:38
It works on both strings and numbers. I used the list comprehension. – Tampa Jul 10 '12 at 04:54
@Tampa: Just keep jamylak's suggestion in mind if efficiently is important to you. – GWW Jul 10 '12 at 05:15
Well..I liked that too of jamylak. That solution is now in my toolkit. – Tampa Jul 10 '12 at 05:29

score 3 · Answer 2 · edited Jul 10 '12 at 04:41

3

If they are both sets you can do this:

set(rt) - set(dp)

edited Jul 10 '12 at 04:41

jamylak

answered Jul 10 '12 at 04:34

matcauthon

score 3 · Answer 3 · answered Jul 10 '12 at 04:40

3

Any of these will work:

set(rt).difference(set(dp))

OR

[i for i in rt if i not in dp]

OR

set(rt) - set(dp)

answered Jul 10 '12 at 04:40

inspectorG4dget

score 2 · Answer 4 · answered Jul 10 '12 at 04:32

2

You are probably looking for one of these:

>>> rt = [1,2,3]
>>> dp = [1,2]
>>> set(rt).issubset(dp)
False
>>> 3 in dp
False

answered Jul 10 '12 at 04:32

jamylak

score 1 · Answer 5 · answered Jul 10 '12 at 04:36

1

Sounds like you may want set subtraction:

>>> rt = [1,2,3]
>>> dp = [1,2]
>>> set(rt) - set(dp)
set([3])

answered Jul 10 '12 at 04:36

Jeff Bradberry

score 0 · Answer 6 · answered Jul 10 '12 at 04:33

0

Kind of ambiguous what you want. You mean you want to check each element of rt against dp?

for num in rt:
    if num in dp:
        print(num, 'is in dp!')
    else:
        print(num, 'is not in dp!')

answered Jul 10 '12 at 04:33

Dubslow

6 Answers6