Python sets — unique collections and set operations

Sets

A set is an unordered collection of unique, hashable items. It automatically removes duplicates and provides fast O(1) membership testing.

a = {1, 2, 3, 4}
b = {3, 4, 5, 6}

# dedup a list:
unique = set([1, 2, 2, 3, 3])
print(unique)  # {1, 2, 3}

Set Operations

print(a | b)   # {1,2,3,4,5,6}  union
print(a & b)   # {3,4}          intersection
print(a - b)   # {1,2}          difference (in a, not b)
print(a ^ b)   # {1,2,5,6}      symmetric difference

Modifying Sets

a.add(10)
a.remove(1)     # raises KeyError if absent
a.discard(99)   # safe — no error if absent
print(3 in a)   # True  — fast membership test

Frozenset

A frozenset is an immutable set — it is hashable and can be used as a dictionary key. Create one with frozenset([1, 2, 3]). Sets do not support indexing or slicing because they have no defined order.

Sets shine for membership testing and deduplication. If you have a list of IDs and need to check whether a new ID already exists, converting the list to a set first and then testing membership is dramatically faster than looping. The set difference operator is useful for finding what changed between two states: new_ids - old_ids gives you added items, old_ids - new_ids gives removed ones. Remember that sets do not preserve insertion order, so if order matters, use a list with a set as a parallel lookup structure.