Dictionaries

In the previous unit, we explored ranges, sets, and tuples. Now let's look at dictionaries, Python's way of storing data as key-value pairs.

What Is a Dictionary?

A dictionary stores items as key-value pairs. You look up values by their keys, not by position. This makes dictionaries fast for retrieving specific data.

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

Each key (like "name") maps to a value (like "Alice"). Keys must be unique and immutable.

Creating and Accessing

Create a dictionary with curly braces {}, separating keys and values with colons.

person = {"name": "Alice", "age": 25}
print(person["name"])  # Outputs: Alice

To change a value, assign to its key:

person["age"] = 26

To add a new key-value pair, just assign it:

person["city"] = "Boston"

Common Methods

Dictionaries have several useful methods.

person = {"name": "Alice", "age": 26, "city": "Boston"}

# Get all keys
print(person.keys())  # dict_keys(['name', 'age', 'city'])

# Get all values
print(person.values())  # dict_values(['Alice', 26, 'Boston'])

# Get key-value pairs as tuples
print(person.items())  # dict_items([('name', 'Alice'), ...])

# Safely get a value (returns None if key doesn't exist)
print(person.get("name"))  # Alice
print(person.get("unknown"))  # None

# Update with another dictionary
person.update({"city": "LA", "state": "CA"})

# Remove and return a value
city = person.pop("city")  # Returns "LA"

The get() method is especially useful because it won't raise an error if the key doesn't exist, unlike bracket notation.

Project: Command-Based Drawing

Let's use a dictionary to map commands to Turtle functions. The user types commands like "forward" or "left", and the dictionary tells us which function to call.

import turtle

screen = turtle.Screen()
screen.setup(width=800, height=600)
screen.bgcolor("white")

t = turtle.Turtle()

# Map commands to turtle functions
commands = {
    'forward': t.forward,
    'backward': t.backward,
    'left': t.left,
    'right': t.right,
    'color': t.color
}

while True:
    cmd = input("Command (or 'exit'): ")
    if cmd == 'exit':
        break

    if cmd not in commands:
        print("Unknown. Try:", list(commands.keys()))
        continue

    value = input("Value: ")
    if cmd != 'color':
        value = int(value)

    commands[cmd](value)

t.hideturtle()
turtle.done()

The dictionary commands maps strings like "forward" to actual functions like t.forward. When the user enters a command, we look it up in the dictionary and call the corresponding function with the provided value.

This pattern is powerful because it's easy to extend. Add 'pensize': t.pensize to the dictionary, and you've added a new command without changing any other code.

In the next unit, we'll learn how to read from and write to files in Python.