🐍 Lesson 9: Python Dictionaries – Key-Value Pairs Made Easy

Welcome to Lesson 9! Today we’ll explore dictionaries — a Python data structure that stores information in key-value pairs. Dictionaries are extremely useful for organizing data efficiently.

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⭐ What You Will Learn in This Lesson

• How to create and access dictionaries
• Adding, updating, and removing key-value pairs
• Looping through dictionaries
• Practical use cases of dictionaries in Python

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👥 Who Is This Lesson For?

• Beginners who want to organize data efficiently
• Students learning Python data structures
• Anyone interested in JSON, APIs, or structured data

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📦 What Is a Dictionary?

A dictionary is like a real-life dictionary: each word (key) has a definition (value). In Python, it’s a collection of key-value pairs.

my_dict = {
    "name": "Alice",
    "age": 25,
    "country": "USA"
}
print(my_dict)

🧠 Output

{'name': 'Alice', 'age': 25, 'country': 'USA'}

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🧩 1. Accessing Values

Use the key to get its value:

print(my_dict["name"])     # Alice
print(my_dict.get("age"))  # 25

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🧩 2. Adding or Updating Values

my_dict["profession"] = "Developer"  # Add new key-value
my_dict["age"] = 26                   # Update existing value
print(my_dict)

🧠 Output

{'name': 'Alice', 'age': 26, 'country': 'USA', 'profession': 'Developer'}

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🧩 3. Removing Items

• pop(key) – removes a specific key
• del dict[key] – delete a specific key
• clear() – removes all items

my_dict.pop("country")
del my_dict["profession"]
print(my_dict)

🧠 Output

{'name': 'Alice', 'age': 26}

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🧩 4. Looping Through a Dictionary

# Loop through keys
for key in my_dict:
    print(key, ":", my_dict[key])

# Loop through key-value pairs
for key, value in my_dict.items():
    print(key, "->", value)

🧠 Output

name : Alice
age : 26
name -> Alice
age -> 26

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🧩 5. Why Use Dictionaries?

• Store related information together (e.g., a person’s details)
• Fast lookup using keys
• Ideal for JSON, APIs, and structured data

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🧪 Practice

1. Create a dictionary to store your favorite book’s title, author, and year.
2. Add a key for genre and assign a value.
3. Update the year of publication.
4. Print all keys and values using a loop.

Bonus: Try looping through both keys and values using items().

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❌ Common Mistakes

• Using incorrect keys or spelling mistakes
• Forgetting to use quotes around string keys
• Trying to access a key that doesn’t exist without get()

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❓ Frequently Asked Questions (FAQ)

1. Can dictionary keys be numbers?

Yes! Keys can be strings, numbers, or even tuples (as long as they are immutable).

2. Can I store lists or other dictionaries as values?

Absolutely. Dictionaries can store any type of value.

3. Are dictionaries ordered?

Starting Python 3.7+, dictionaries maintain insertion order.

4. Can I use a key more than once?

No. Each key must be unique. Assigning a value to an existing key updates it.

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🚀 What’s Next?

In the next lesson, you’ll learn about:

• Sets
• Unique items and membership testing
• Useful set operations

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➡ Next Lesson

Go to Lesson 10 →