lesson 39 Dictionaries & Sets · beginner

Sets: distinct values and comparisons

A set holds each value once. set(items) removes repeats, and intersection, union and difference compare two groups. A set has no order, so sort it before you print it.

You'll be able to

Make a set from a literal or from a list with set(...), and test membership
Answer group questions with intersection, union and difference
Print a set reliably with sorted, and remember that {} is an empty dictionary

The idea

Some questions are about groups, not individual items: *which customers ordered in both months?*, *which tags do two films share?* A set is a collection of distinct values. A repeated value is kept once, and in on a set is very fast.

Write a set with curly braces and no colons: {"noir", "heist"}. set(items) builds one from a list and drops the repeats. Three methods compare two sets:

a.intersection(b): the values in both.
a.union(b): the values in either.
a.difference(b): the values in a but not in b.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · Tags two films share

first = {"thriller", "noir", "slow burn", "twist ending"}
second = {"noir", "twist ending", "heist"}
print(sorted(first.intersection(second)))
print(sorted(first.union(second)))
print(sorted(first.difference(second)), sorted(second.difference(first)))
print(len(first), "noir" in first)

In data work, sets usually come from lists. An order history repeats customers, so set(customer_ids) gives each customer once. Comparing two months then answers retention questions directly.

example-2.py · Returning, new and lapsed customers

january = ["C00164", "C00800", "C04248", "C00800", "C00164"]
february = ["C00800", "C01338", "C00800", "C00077"]
jan = set(january)
feb = set(february)
print(len(january), "orders,", len(jan), "customers in January")
print("returning:", sorted(jan.intersection(feb)))
print("new in February:", sorted(feb.difference(jan)))
print("lapsed:", sorted(jan.difference(feb)))
print(type({}), type(set()))

When to use it: to remove repeats when order doesn't matter, to test membership in a large group, and to compare two groups. Mistakes to avoid:

Printing a set and expecting a fixed order.
Writing {} for an empty set.
Using a set when you need the first-seen order or the repeats. That is a job for a list.

Review

A set keeps each value once. set(items) removes repeats, and in on a set is fast.
intersection gives both, union gives either, and difference gives one but not the other, which depends on the direction.
A set has no order, so print sorted(s). {} is an empty dictionary and set() is an empty set.
Next: dictionaries and lists inside dictionaries, the shape of real records.

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