Sets and Dictionaries

Use sets for unique values and dictionaries for named values.

14 min read 10 quiz questions

Sets keep unique values

A set stores unique values. Create one with braces or set(). Sets do not keep a useful item order, so sort them before printing examples.

Example 1: create and change a set

colors = {'red', 'blue', 'red'}
colors.add('green')
colors.remove('blue')
print(sorted(colors))

The repeated 'red' is kept only once. add() adds a value. remove() removes a value and raises an error if it is missing.

Set operations

Example 2: compare sets

a = {1, 2, 3}
b = {3, 4}
print(sorted(a | b))
print(sorted(a & b))
print(sorted(a - b))

Operation	Meaning	Example
a \| b	Union: values in either set	{1, 2} \| {2, 3}
a & b	Intersection: shared values	{1, 2} & {2, 3}
a - b	Difference: values only in a	{1, 2} - {2, 3}

Dictionaries connect keys to values

A dictionary maps a unique key to a value. Use square brackets when a key must exist. Use get() when a missing key should return a fallback instead of an error.

Example 3: read, add, update, and delete

student = {'name': 'Ivy', 'score': 8}
print(student['name'])
print(student.get('level', 'beginner'))
student['score'] = 10
student['city'] = 'Lima'
del student['city']
print(student)

Assigning to a new key adds it. Assigning to an existing key updates it. del removes a key and its value.

Example 4: loop through a dictionary and nest one

student = {'name': 'Ivy', 'score': 10}
print(list(student.keys()))
print(list(student.values()))
for key, value in student.items():
    print(key + ':', value)
profile = {'user': {'name': 'Ivy', 'active': True}}
print(profile['user']['name'])

Key points

Sets keep each value only once.
Sort a set before printing it when order matters.
Set operators compare groups of values.
Dictionaries map keys to values.
get() can safely handle a missing dictionary key.

Worked example

A libary app records who entered today; duplicates collapse in a set. Marks live in a dictionary keyed by student.

Example

visitors = ["Mim", "Rafi", "Mim"]
unique = set(visitors)
marks = {"Mim": 78, "Rafi": 64}
print(len(unique), marks["Mim"])

How it works, step by step

set() keeps one copy of each value.
Dictionary lookup uses the KEY in square brackets.
len(set) counts distinct visitors: 2.

References & Further Reading

Related entries from PyLabs's own reference library.

add() (Add a single element to the set.)
union() (Return a new set with elements from this set and all others.)
get() (Look up a value safely without raising a KeyError if missing.)
items() (Return a dynamic view of all (key, value) pairs in the dictionary.)

Your notes

Practice exercise

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Create fruit = {'apple': 3, 'pear': 2}. Add 'banana' with value 5, then print fruit['banana'].

Expected output: 5

# Write your code below

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