A set is an unordered collection of unique, hashable items with O(1) membership tests — and real set algebra.

Python

a = {1, 2, 3, 4}
b = {3, 4, 5}
print(a | b)          # {1, 2, 3, 4, 5}
print(a & b)          # {3, 4}
print(a - b)          # {1, 2}
print(a ^ b)          # {1, 2, 5}
print({1, 2} <= a)    # True

Operator	Method	Meaning
`a \| b`	`a.union(b)`	In either
`a & b`	`a.intersection(b)`	In both
`a - b`	`a.difference(b)`	In `a` but not `b`
`a ^ b`	`a.symmetric_difference(b)`	In exactly one
`a <= b`	`a.issubset(b)`	Every item of `a` is in `b`

Adding & removing

Python

s = set()                  # {} would be an empty *dict*
s.add(3); s.add(3); s.add(1)
s.discard(99)              # no error if missing
print(sorted(s), len(s))   # [1, 3] 2

When to reach for a set

Python

xs = [3, 1, 3, 2, 1]
print(list(dict.fromkeys(xs)))                   # [3, 1, 2]
def has_duplicate(nums):
    return len(set(nums)) != len(nums)
print(has_duplicate(xs), has_duplicate([1, 2]))  # True False