In Python, a dictionary is a powerful, flexible, and widely-used data structure that allows you to store data in key-value pairs.

In this tutorial, you’ll learn:

• What dictionaries are

• How to create and access them

• Common methods and operations

• Nesting dictionaries

• Tips and pitfalls

What Is a Dictionary?

A dictionary in Python:

• Stores data in key-value pairs

• Is unordered (in versions <3.7)

• Is mutable

• Keys must be unique and immutable

• Values can be of any type

Example:

person = {
    "name": "Alice",
    "age": 30,
    "is_active": True
}

✅ Creating Dictionaries

Using Curly Braces

user = {"username": "john_doe", "email": "[email protected]"}

Using the dict() Constructor

user = dict(username="john_doe", email="[email protected]")

Creating Empty Dictionary

empty_dict = {}

Accessing Dictionary Values

print(user["username"])  # john_doe

Using get() to Avoid KeyError

print(user.get("email"))          # [email protected]
print(user.get("phone", "N/A"))   # N/A

✏️ Modifying Dictionaries

Add or Update a Key

user["age"] = 25  # Adds if not exists, updates if exists

Remove Items

del user["email"]             # Deletes key
user.pop("age")               # Removes and returns value
user.clear()                  # Empties the dictionary

Looping Through Dictionaries

Loop Through Keys

for key in user:
    print(key, user[key])

Loop Through Values

for value in user.values():
    print(value)

Loop Through Key-Value Pairs

for key, value in user.items():
    print(f"{key}: {value}")

Common Dictionary Methods

Nested Dictionaries

Dictionaries can contain other dictionaries.

student = {
    "name": "Tom",
    "grades": {
        "math": 90,
        "science": 85
    }
}

print(student["grades"]["science"])  # 85

Dictionary Comprehensions

You can create dictionaries using a concise syntax:

squares = {x: x**2 for x in range(5)}
print(squares)  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

Full Example: Contact Book

contacts = {}

# Add new contact
contacts["Alice"] = {"phone": "123-456-7890", "email": "[email protected]"}
contacts["Bob"] = {"phone": "555-555-5555", "email": "[email protected]"}

# Access
print("Alice's email:", contacts["Alice"]["email"])

# Update
contacts["Bob"]["phone"] = "000-000-0000"

# Remove
del contacts["Alice"]

# Display
for name, info in contacts.items():
    print(f"{name}: {info}")

Tips for Working with Dictionaries

• Use get() to prevent crashes from missing keys.

• Use dictionary comprehension for dynamic dictionary creation.

• Keys must be immutable (e.g., strings, numbers, tuples).

• Use update() to merge two dictionaries.

⚠️ Common Pitfalls

Summary

✅ Practice Exercise

Task: Create a program that counts the frequency of each word in a sentence.

sentence = "python is fun and python is easy"
words = sentence.split()
word_count = {}

for word in words:
    word_count[word] = word_count.get(word, 0) + 1

print(word_count)

Output:

{'python': 2, 'is': 2, 'fun': 1, 'and': 1, 'easy': 1}

What’s Next?

After mastering dictionaries, dive into:

• JSON and dictionary parsing

• Handling nested data structures

• Real-world examples like API responses and config files