Module 2 Lesson 4: Dictionaries

Lists and Tuples are great, but they have a limitation: search by position. If you have a list of 1,000 users, finding "Alex's email" requires knowing exactly where Alex is in that list. Dictionaries solve this problem by allowing you to store data as Key-Value Pairs.

Lesson Overview

In this lesson, we will cover:

• Creating Dictionaries: Using curly braces {}.
• Keys and Values: The "Label" and the "Data."
• Accessing Data: Looking up values by their keys.
• Modifying Dictionaries: Adding, updating, and removing entries.
• Nested Dictionaries: Data inside of data.

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1. Creating a Dictionary

A dictionary uses curly braces {}. Each entry is a key: value pair.

user = {
    "name": "Alex",
    "age": 25,
    "is_pro": True
}

• Key: Must be unique and immutable (usually a string).
• Value: Can be anything (string, number, list, or even another dictionary!).

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2. Accessing and Updating Data

You "look up" a value by using its key in square brackets.

# Access
print(user["name"]) # Output: Alex

# Update
user["age"] = 26

# Add a New Key
user["hobby"] = "Photography"

Pro Tip: If you try to access a key that doesn't exist, Python will crash. To avoid this, use the .get() method!

email = user.get("email", "Not Found")

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3. Dictionary Methods

Method	Description	Example
.keys()	Get all the labels	user.keys()
.values()	Get all the data	user.values()
.items()	Get pairs as tuples	user.items()
.pop(key)	Remove item by key	user.pop("age")
.clear()	Empty the dictionary	user.clear()

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4. Loops and Dictionaries

You can loop through a dictionary to see all its contents.

capitals = {"France": "Paris", "Spain": "Madrid", "Italy": "Rome"}

for country, city in capitals.items():
    print(f"The capital of {country} is {city}.")

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5. Nested Dictionaries

Dictionaries can hold other dictionaries. This is how complex data (like what you get from social media APIs) is structured.

employees = {
    "emp1": {"name": "Sara", "job": "Developer"},
    "emp2": {"name": "Bill", "job": "Manager"}
}

print(employees["emp1"]["name"]) # Output: Sara

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Practice Exercise: The Contact Book

Create a file named contacts.py.

1. Create a dictionary named contact with keys for name, phone, and email.
2. Add a new key-value pair for address.
3. Update the phone number to a new string.
4. Print out only the keys of the dictionary.
5. Use a for loop to print each piece of information in the format: "Label: Value".

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Quick Knowledge Check

1. What symbols do we use to create a Dictionary?
2. Can a Dictionary have two identical keys?
3. How do you safely access a key that might not exist?
4. What is the difference between a key and a value?

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Key Takeaways

• Dictionaries store data in key: value pairs.
• They are optimized for fast lookups using keys.
• Keys must be unique and immutable.
• Nesting allows you to model complex, real-world information.

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What’s Next?

We’ve covered the most structured ways to store data. But what if you just need a "bag" of unique items? In Lesson 5, we’ll look at Sets and learn how to handle unique collections!