compare two lists in python and return indices of matched values

Question

For two lists a and b, how can I get the indices of values that appear in both? For example,

a = [1, 2, 3, 4, 5]
b = [9, 7, 6, 5, 1, 0]

return_indices_of_a(a, b)

would return [0,4], with (a[0],a[4]) = (1,5).

Answer 1

The best way to do this would be to make b a set since you are only checking for membership inside it.

>>> a = [1, 2, 3, 4, 5]
>>> b = set([9, 7, 6, 5, 1, 0])
>>> [i for i, item in enumerate(a) if item in b]
[0, 4]

Answer 2

def return_indices_of_a(a, b):
  b_set = set(b)
  return [i for i, v in enumerate(a) if v in b_set]