Sets

A set is an unordered collection with no duplicates. It uses braces {}.

numbers = {1, 2, 3, 3, 2}
print(numbers)   # {1, 2, 3} — duplicates removed

Add and remove

numbers.add(4)
numbers.discard(1)

No indexing

Sets aren’t ordered, so numbers[0] errors. Iterate with for:

for n in numbers:
    print(n)

Set operations

a = {1, 2, 3}
b = {3, 4, 5}

print(a | b)   # union:        {1, 2, 3, 4, 5}
print(a & b)   # intersection: {3}
print(a - b)   # difference:   {1, 2}

Common use: remove duplicates

items = [1, 1, 2, 3, 3]
unique = list(set(items))   # [1, 2, 3]

Empty set

empty = set()   # {} creates a dict, not a set