Python list intersection efficiency: generator or filter()?

Your solution has a complexity of O(m*n), where m and n are the respective lengths of the two lists. You can improve the complexity to O(m+n) using a set for one of the lists:

s = set(list1)
result = [x for x in list2 if x in s]

In cases where speed matters more than readability (that is, almost never), you can also use

result = filter(set(a).__contains__, b)

which is about 20 percent faster than the other solutions on my machine.

Neither of these. The best way is to use sets.

list1 = [1,2,3,4]
list2 = [3,4,5,6]
result = set(list1).intersection(list2)

Sets are iterable, so no need to convert the result into anything.