Dictionaries

Store and access data using key-value pairs for fast and organized data management.

Part of the free Python course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn in This Lesson

1. What Are Dictionaries?

A dictionary stores data in key-value pairs . Instead of accessing items by position (like lists), you access them by a unique key name.

Key Characteristics

• Key-value pairs — Each item has a name (key) and data (value)
• Fast lookups — Access any value instantly by its key
• Unique keys — Each key can only appear once
• Mutable — You can add, change, or remove items
• Ordered — Maintains insertion order (Python 3.7+)

2. Lists vs Dictionaries

Feature

List

Dictionary

Access by

Index (0, 1, 2...)

Key name

Syntax

[item1, item2]

Best for

Ordered collections

Named/labeled data

Example use

List of names

User profile info

3. Creating Dictionaries

Method

Example

Description

Empty dictionary

my_dict = {}

Start with nothing

With data

Key-value pairs

Using dict()

dict(name="Alice")

Constructor function

4. Accessing Values

There are two ways to access dictionary values:

Syntax

If Key Missing

Square brackets

dict["key"]

KeyError (crashes)

.get() method

dict.get("key")

Returns None (safe)

.get() with default

dict.get("key", "default")

Returns your default

5. Adding and Changing Items

Dictionaries are mutable — you can add new key-value pairs or update existing ones using the same syntax:

6. Removing Items

What It Does

del dict["key"]

Delete a specific key

del person["age"]

.pop("key")

Remove and return the value

age = person.pop("age")

.popitem()

Remove last inserted item

last = person.popitem()

.clear()

Remove all items

person.clear()

7. Checking if a Key Exists

Use in to check if a key exists before accessing it:

8. Looping Through Dictionaries

Loop Type

What You Get

Keys only

Each key

for key in dict:

Values only

Each value

for val in dict.values():

Both

Key and value

for key, val in dict.items():

9. Useful Dictionary Methods

What It Returns

.keys()

All keys

person.keys()

.values()

All values

person.values()

.items()

All key-value pairs

person.items()

.get(key)

Value or None

person.get("name")

.update(dict2)

Merges dict2 into dict

.copy()

A copy of the dict

new = person.copy()

len(dict)

Number of pairs

len(person)

10. Nested Dictionaries

Dictionaries can contain other dictionaries (or lists) as values. This is useful for organizing complex data.

11. Dictionary Comprehension (Advanced)

Like list comprehension, you can create dictionaries in one line:

12. Common Mistakes to Avoid

Mistake

Problem

Solution

Accessing missing key

dict["missing"]

Use .get() or check with in

Using list as key

Keys must be immutable (str, int, tuple)

Using square brackets

["key", "value"]

Use curly braces

Forgetting the colon

Use

Duplicate keys

Each key must be unique (last value wins)

13. Practical Examples

Example 1: User Profile

Example 2: Word Counter

Example 3: Simple Contact Book

Summary: Quick Reference

Operation

Create

Access (unsafe)

person["name"]

Access (safe)

Add/Change

dict["key"] = val

person["age"] = 25

Remove

Check exists

"key" in dict

"name" in person

Loop

for k, v in dict.items():

for k, v in person.items():

Length

Lesson 8 done — you can now model real-world data in Python!

Dictionaries are how Python stores structured data — user profiles, API responses, config settings. You know how to create, access, update, loop, and nest them safely.

🚀 Up next: File Handling — learn to save data permanently so it survives after your program closes.

Practice quiz

Continue this course

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