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CHAPTER 13 — Dictionaries: Storing Information With Keys - Printable Version

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CHAPTER 13 — Dictionaries: Storing Information With Keys - Leejohnston - 11-15-2025

Chapter 13 — Dictionaries: Storing Information With Keys
Dictionaries let you store information in a structured, searchable way — just like a mini database.

A dictionary stores values using keys, not indexes.

Example:

Code:
person = {
    "name": "Mia",
    "age": 14,
    "city": "Liverpool"
}

Keys are like labels. 
You don’t search by number — you search by name.

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13.1 Creating a Dictionary

Code:
car = {
    "brand": "Tesla",
    "model": "Model 3",
    "year": 2020
}

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13.2 Accessing Values

Use square brackets:

Code:
print(car["brand"])    # Tesla

Or safer:

Code:
print(car.get("year"))

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13.3 Adding New Key–Value Pairs

Code:
car["colour"] = "red"

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13.4 Changing Values

Code:
car["year"] = 2024

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13.5 Removing Items

Remove a specific key:

Code:
car.pop("model")

Remove the last added item:

Code:
car.popitem()

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13.6 Checking if a Key Exists

Code:
if "brand" in car:
    print("Brand found!")

---

13.7 Looping Through Dictionaries

Loop through keys:

Code:
for key in car:
    print(key)

Loop through values:

Code:
for value in car.values():
    print(value)

Loop through both:

Code:
for key, value in car.items():
    print(key, value)

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13.8 Nested Dictionaries

Dictionaries can contain other dictionaries:

Code:
students = {
    "001": {"name": "Sam", "score": 88},
    "002": {"name": "Ella", "score": 93}
}

Access:

Code:
print(students["001"]["score"])

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13.9 Dictionary vs List — When to Use Which

Use a list when: 
• order matters 
• you just store items 
• you want easy loops 

Use a dictionary when: 
• you need labelled data 
• you search by name/key 
• you want structured info 

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13.10 Real-World Example — User Profiles

Code:
profile = {
    "username": "astro_john",
    "posts": 120,
    "reputation": 87,
    "member_since": "2024-01-12"
}

This is why dictionaries are everywhere online.

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13.11 Mini Project — Contact Book

Create a dictionary storing:

• name 
• phone number 
• email 

Then print a formatted contact card.

Example:

Code:
Name: Mia
Phone: 07700 123456
Email: mia@example.com

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13.12 Challenge — Student Grades Manager

Ask user for:

• student name 
• 3 test scores 

Store in a dictionary like:

Code:
{
  "name": "Ella",
  "scores": [82, 91, 88],
  "average": 87
}

Then print:

• name 
• each score 
• final average 

BONUS: 
Mark the grade:

• A (≥ 90) 
• B (≥ 80) 
• C (≥ 70) 
• D (≥ 60) 
• F (< 60) 

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13.13 Chapter Summary

• dictionaries store data using keys 
• access values with dictionary["key"] 
• .get(), .keys(), .values(), .items() are essential 
• dictionaries can contain lists and other dictionaries 
• use dictionaries for structured, labelled data 

Next:
Chapter 14 — Functions: Reusable Code Blocks

This is where your programs become modular, organised, and professional.

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Written and Compiled by Lee Johnston — Founder of The Lumin Archive