Dictionaries

Store key-value pairs with dictionaries

Dictionaries — Key-Value Data

What Is a Dictionary?

A dictionary (dict) stores data in key-value pairs. Instead of numbered positions like a list, dictionaries use meaningful keys (usually strings) to label each value. Think of it like a real dictionary — you look up a word (key) to find its definition (value).

Syntax

python

person = {
    "name": "Alice",
    "age": 25,
    "city": "New York"
}

print(person["name"])     # "Alice" — access by key
print(person.get("age"))  # 25 — safer access (no error if missing)

Part	What it means
`{ }`	Curly braces create a dictionary
`"name"`	Key — like a variable name (unique identifier)
`:`	Separates key from value
`"Alice"`	Value — can be any type: string, number, list, even another dict
`,`	Separates key-value pairs

How It Works

Creating and Accessing

python

student = {
    "name": "Bob",
    "grades": [85, 92, 78],
    "active": True,
    "id": 12345
}

print(student["name"])          # "Bob"
print(student["grades"][0])     # 85 — value can be a list!
print(student.get("phone"))     # None — key doesn't exist, returns None safely
# print(student["phone"])       # ❌ KeyError! Use .get() for safety

Modifying Dictionaries

python

settings = {"theme": "dark", "volume": 80}

settings["volume"] = 50         # Update existing key
settings["language"] = "en"     # Add new key-value pair
del settings["theme"]           # Remove a key
print(settings)                 # {"volume": 50, "language": "en"}

Useful Dictionary Methods

python

capitals = {"USA": "Washington", "France": "Paris", "Japan": "Tokyo"}

print(capitals.keys())      # dict_keys(["USA", "France", "Japan"])
print(capitals.values())    # dict_values(["Washington", "Paris", "Tokyo"])
print(capitals.items())     # dict_items([("USA", "Washington"), ...])

� **Lists vs Dictionaries:** Lists are for ordered sequences you access by position. Dictionaries are for labeled data you access by name. Use a dict when you'd naturally describe items with names and values.

Why Use Dictionaries?

**User profiles** — name, email, age, preferences, address all labeled clearly

**Configuration** — theme, language, notifications on/off

**API responses** — JSON data from the web is almost always dictionaries

**Counters** — count occurrences: `counts["apple"] = counts.get("apple", 0) + 1`

**Lookup tables** — fast retrieval: `phonebook["Alice"]` → number

Common Mistakes

❌ Wrong	Why	✅ Right
`dict[0]` when keys are strings	Lists use index numbers, dicts use keys	`dict["name"]`
`dict = {}` then `dict["key"]` works but...	Avoid naming variables `dict` — shadows the built-in	Use `data = {}`
`student["phone"]` when key doesn't exist	Causes KeyError!	Use `student.get("phone")`
`{"name": "Alice", "name": "Bob"}`	Duplicate keys — second overwrites first	Keep keys unique

Your Turn!

Create a dictionary called movie with:

`"title"` — your favorite movie

`"year"` — when it was released

`"rating"` — your rating out of 10

Then print: "My favorite movie is {title} from {year}. I rate it {rating}/10."

💡Need a hint? (3 available)

#1Dicts use curly braces and key: value pairs

#2Access values with dict["key"]

#3Use .get("key") for safe access — returns None if missing

🔑Show solution (try it yourself first!)

movie = {"title": "Inception", "year": 2010, "rating": 9.5}
print(f"My favorite movie is {movie["title"]} from {movie["year"]}. I rate it {movie["rating"]}/10.")

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