How can I compare two lists in python and return matches

Question

I want to take two lists and find the values that appear in both.

a = [1, 2, 3, 4, 5]
b = [9, 8, 7, 6, 5]

returnMatches(a, b)

would return

Answer 1

Use set.intersection(), it's fast and readable.

>>> set(a).intersection(b)
set([5])