If order is not important and you don't need to worry about duplicates then you can use set intersection:
>>> a = [1,2,3,4,5]>>> b = [1,3,5,6]>>> list(set(a) & set(b))[1, 3, 5]
Using list comprehensions is a pretty obvious one for me. Not sure about performance, but at least things stay lists.
[x for x in a if x in b]
Or "all the x values that are in A, if the X value is in B".
If you convert the larger of the two lists into a set, you can get the intersection of that set with any iterable using intersection():
a = [1,2,3,4,5]b = [1,3,5,6]set(a).intersection(b)