python：用另一个列表中的项目索引替换列表项

Question

I have 2 lists, for example say

list1 = ['abc', 'xyz', 'cat', 'xyz', 'abc', 'pqr', 'dog']
list2 = ['xyz', 'dog', 'pqr', 'abc', 'cat']

I want to create a new list by replacing list1 with indexes of the elements in list2 like

list3 = [3, 0, 4, 0, 3, 2, 1]

I should mention here that I actually get list2 from list1 by

list2 = list(set(list1))

So list2 has no duplicates and has all the elements of list1 .

I want to know the fastest way I can get list3 in a pythonic way. I have tried the 2 ways so far:

1> basic .index way

list3 = [list2.index(item) for item in list1]

2> using a dict with elements as keys and indexes as values

d = {list2[i]:i for i in range(len(list2))}
list3 = [d[item] for item in list1]

Answer 1

If you have a concerned about looking up duplicate values, just create an index first:

>>> idx={e:list2.index(e) for e in list2}

Alternatively:

>>> idx={e:n for n,e in enumerate(list2)}

Then:

>>> [idx[e] for e in list1]
[3, 0, 4, 0, 3, 2, 1]

Answer 2

idx = dict(zip(list2, range(len(list2))))
list(map(idx.get, list1))
# [3, 0, 4, 0, 3, 2, 1]