What are Dictionaries?

A Dictionary lets you store information in key-value pairs. Instead of finding items by numbered positions like 0 or 1, you look up values using unique labels called keys.

Think of a dictionary like:

• A contact book (Name -> Phone Number)
• A restaurant menu (Dish Name -> Price)
• A user profile (Username -> Email Address)

Creating a Dictionary

You create a dictionary using curly braces {} with a colon : separating each key from its value:

user = {
    "name": "Alice",
    "age": 25,
    "is_admin": True
}

Accessing Values by Key

To read a value, write the dictionary name followed by the key label inside square brackets []:

user = {"name": "Alice", "age": 25}

print(user["name"])  # Output: Alice
print(user["age"])   # Output: 25

Adding and Updating Values

If you assign a value to a key that already exists, Python updates the value. If the key does not exist yet, Python creates a brand new key-value pair:

user = {"name": "Alice"}

# Update existing key
user["name"] = "Alice Smith"

# Add a brand new key
user["email"] = "alice@example.com"

print(user)
# Output: {'name': 'Alice Smith', 'email': 'alice@example.com'}

Removing Key-Value Pairs (del, pop)

• del my_dict["key"] deletes the key and value completely.
• .pop("key") deletes the key and returns the value so you can store or inspect it.

user = {"name": "Alice", "role": "editor", "points": 100}

# Delete points
del user["points"]

# Pop out role
user_role = user.pop("role")
print(f"Removed role: {user_role}")
print(user)  # Output: {'name': 'Alice'}

Common Beginner Mistakes

• KeyError on Missing Keys: Looking up a key that does not exist (user["phone"]) crashes Python.
• Fix: Use "phone" in user to check first, or use .get("phone") for safe lookup.

---

Quick Summary

• Dictionary Definition: A collection of key: value mappings enclosed in curly braces {...}.
• Key Rules: Keys must be unique and immutable (strings, integers, tuples); values can be any data type.
• Access & Mutation: Access using dict[key]; create or update values using dict[key] = new_value.
• Fast Lookup: Dictionaries use hash tables for lightning-fast $O(1)$ key lookups.

---

What's Next?

Let's explore dictionary inspection methods like .get(), .keys(), .values(), and .items() in the next lesson!

Dictionary Methods

Master safe key lookups with .get() and inspecting keys and values