After this lesson, you will be able to:

• Create sets and explain why duplicate values appear only once.
• Add or safely discard individual set values.
• Check membership without relying on a set's element order.
• Combine and compare sets with union, intersection, and difference.

A set stores unique values. Duplicate entries collapse into one element, so sets are useful for tags, permissions, and discovering which identifiers are missing. Sets do not have meaningful positions: never depend on iteration order or attempt to read items[0] from a set.

Use {value1, value2} to make a nonempty set and set() to make an empty one ({} creates an empty dictionary). add(value) inserts one value. discard(value) removes a value if present and does nothing if it is absent; unlike remove(value), it does not raise an error when missing.

Membership uses value in items. Set operations answer useful questions: a | b combines all unique values; a & b finds shared values; a - b keeps values present only in a. These operations return new sets rather than modifying the operands.

1. Duplicates disappear

The repeated identifier counts as a single member.

regions = {"us", "eu", "us"}
print(len(regions))
print("eu" in regions)

Output:

2
True

2. Add and safely discard values

discard() is safe even if the value is already missing.

roles = {"reader"}
roles.add("editor")
roles.discard("owner")
print(len(roles))
print("editor" in roles)

Output:

2
True

3. Compare two groups

Intersection counts shared values; union counts everything present in either set.

online = {"api", "worker", "cache"}
monitored = {"cache", "mailer"}
print(len(online & monitored))
print(len(online | monitored))

Output:

1
4

4. Find what's missing

Difference keeps the values that have not been granted.

required = {"read", "write", "deploy"}
granted = {"read", "deploy"}
print(len(required - granted))

Output:

1

1. {} is an empty dictionary, not an empty set.
2. A set has no positional indexing and no promised output order.
3. append() belongs to lists, not sets.
4. remove() raises KeyError for an absent element; use discard() when absence is acceptable.
5. Set difference has a direction: a - b is not the same as b - a.