Set operations in Python: union, intersection, difference

A set holds unique values with no order. Python has four operators that combine two sets, the same operations you may know from maths.

The program

python

python_club = {"Ada", "Linus", "Grace", "Alan"}
chess_club = {"Grace", "Alan", "Hedy", "Margaret"}

print("In either club:", sorted(python_club | chess_club))
print("In both clubs: ", sorted(python_club & chess_club))
print("Only Python:   ", sorted(python_club - chess_club))
print("Only one club: ", sorted(python_club ^ chess_club))

output

In either club: ['Ada', 'Alan', 'Grace', 'Hedy', 'Linus', 'Margaret']
In both clubs:  ['Alan', 'Grace']
Only Python:    ['Ada', 'Linus']
Only one club:  ['Ada', 'Hedy', 'Linus', 'Margaret']

How it works

a | b or a.union(b) is the union: everything that is in a or b.
a & b or a.intersection(b) is the intersection: only what is in both.
a - b or a.difference(b) is the difference: what is in a but not in b.
a ^ b or a.symmetric_difference(b) is the symmetric difference: what is in one set but not in both.
A set has no order, and the order Python prints a set of strings in can change from one run to the next. sorted() turns each result into a list in alphabetical order, so the output is always the same.
a - b and b - a are different: chess_club - python_club would give Hedy and Margaret.

Methods accept any iterable

The operators need a set on both sides. The method versions also accept a list or tuple:

python

python_club = {"Ada", "Linus", "Grace", "Alan"}
new_members = ["Hedy", "Ada", "Hedy"]

print(sorted(python_club.union(new_members)))

output

['Ada', 'Alan', 'Grace', 'Hedy', 'Linus']

Ada and Hedy appear only once in the result, because a set never holds the same value twice.