Find common elements of two lists in Python

The intersection of two lists is the items that appear in both. Here we find the people who are members of both clubs.

The program

python_club = ["Ada", "Linus", "Grace", "Alan"]
chess_club = ["Hedy", "Grace", "Ada", "Margaret"]

chess_members = set(chess_club)
both = [name for name in python_club if name in chess_members]

print("In both clubs:", both)

In both clubs: ['Ada', 'Grace']

How it works

• The list comprehension keeps each name from python_club that is also in the chess club, so the result is in the order of the first list.
• set(chess_club) is made once, before the loop. Checking in on a set is fast, even for a big set. Checking in on a list looks at the items one by one, which gets slow when both lists are long.

Another way: the & operator

If you do not need the order, turn both lists into sets and use &:

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

common = set(a) & set(b)
print(sorted(common))

[3, 4, 5]

• A set has no duplicates, so 5 appears once even though it is twice in a.
• A set has no fixed order. For strings, the order can even change each time you run the program. sorted() gives a list in a predictable order.

Related programs: Set operations.