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?

Answer 1

>>> 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]