Sets, and Choosing the Right Structure

Why this matters

A set is a collection with no duplicates and no order. It is the right tool whenever you ask “is this in the group?” or “what do these two groups share?”. Removing repeats from a list is one short line.

The idea

seen = {"a", "b", "a"}     # duplicates vanish: {'a', 'b'}
seen.add("c")
print("a" in seen)         # True, and very fast
print(set([1, 1, 2, 3, 3]))  # {1, 2, 3}

Careful: {} on its own is an empty dictionary. An empty set is set().

Sets can be combined like Venn diagrams:

a = {"ann", "bo", "cy"}
b = {"bo", "cy", "di"}
print(a & b)   # in both          {'bo', 'cy'}
print(a | b)   # in either        {'ann', 'bo', 'cy', 'di'}
print(a - b)   # only in a        {'ann'}

Because sets have no order, print may show items in any order. Sort when you need a predictable result: sorted(a & b).

Which structure should I use?

You need…	Use
An ordered collection you will change, with repeats allowed	list
A fixed group of related values (a point, a record)	tuple
To look something up by a name or label	dict
Only uniqueness, or fast “is it in here?” and group maths	set

Try it

Remove the duplicates from [3, 1, 3, 2, 1] and print the result sorted.

Your turn

Two lines are typed in: the guests at Alice’s party, then the guests at Ben’s party (names separated by spaces). Print who went to both, who went to only the first, who only the second, and everyone, each sorted alphabetically with ", " between names. If nobody is in a group, print the word nobody instead.

Both: bo, cy
Only first: ann
Only second: di
Everyone: ann, bo, cy, di

Recap

• A set keeps unique items with no order; set(list) removes duplicates.
• & (both), | (either) and - (difference) combine sets.
• Pick the structure by the question you ask: order, grouping, lookup, or uniqueness.

Common mistakes

• {} is a dict, not a set. Use set() for an empty one.
• Expecting order or indexing a set (s[0] is a TypeError).
• Putting a list inside a set. Items must be unchangeable: use a tuple.