Sets

Big idea

A set is a bag of unique values with no order. It removes duplicates automatically and answers "is this in here?" very fast.

Making a set

colors = {"red", "green", "red", "blue", "green"}
print(len(colors))
print("red" in colors)
print(sorted(colors))

output
3
True
['blue', 'green', 'red']

A set has no positions, so colors[0] is an error, and printing a set may show its items in any order. sorted(...) gives you a list in a fixed order when you need one.

An empty set is set() — because {} already means an empty dictionary (next lesson).

Removing duplicates

The most common use: turn a list into a set to drop repeats.

visits = ["ana", "ben", "ana", "cy", "ben", "ana"]
unique = set(visits)
print(len(visits))
print(len(unique))

6
3

Adding and removing

tags = set()
tags.add("python")
tags.add("python")     # already there — nothing happens
tags.discard("java")   # remove if present; no error if not

Set maths

Sets can be combined like Venn diagrams:

chess = {"ana", "ben", "cy"}
drama = {"cy", "dee", "ana"}
print(sorted(chess & drama))
print(sorted(chess | drama))
print(sorted(chess - drama))

output
['ana', 'cy']
['ana', 'ben', 'cy', 'dee']
['ben']

• a & b — intersection: in both.
• a | b — union: in either.
• a - b — difference: in a but not b.

Fast membership

Checking x in some_list looks at items one by one. Checking x in some_set jumps almost straight to the answer, however big the set is — so when you'll check membership many times, use a set.

word = "banana"
print(len(word))

output
3

Watch it

Think of it like

A guest list at the door. Each name is on it once, the order doesn't matter, and the bouncer can tell you instantly whether someone's on it.

Key terms

• Set — An unordered collection of unique values: {1, 2, 3}.
• Union / intersection / difference — |, &, -: combining sets.
• Membership test — x in s; very fast for sets.