Lists & Dictionaries

A single variable holds one value. When you have a whole collection, such as the notes in a song or the squares a snake covers in a game, use a list or a dictionary.

Lists

A list holds values in order, inside square brackets:

empty_list = []
fruits = ["apple", "orange", "grape"]

Each item has a position, called its index. Python starts counting from 0:

fruits[0]     # "apple": the first item
fruits[2]     # "grape": the third item
fruits[-1]    # "grape": negative numbers count back from the end
fruits[3]     # IndexError! There is no fourth item

Changing a list:

fruits.append("banana")   # add to the end: ["apple", "orange", "grape", "banana"]
fruits.pop(0)             # remove the first item: ["orange", "grape", "banana"]
fruits.remove("grape")    # remove by value: ["orange", "banana"]
fruits[0] = "kiwi"        # replace an item: ["kiwi", "banana"]
len(fruits)               # 2: how many items there are
"kiwi" in fruits          # True: is this value in the list?

Where lists show up in the projects

In Snake, the snake's body is a list of positions. Each move, the new head position is appended to the end, and the tail is popped off the front.

Dictionaries

A dictionary (dict) stores values under keys instead of positions. Think of a real dictionary: you look up a word (the key) to find its meaning (the value). Dictionaries use curly braces and key: value pairs:

state_capitals = {
    "Alabama": "Montgomery",
    "Alaska": "Juneau",
    "Arizona": "Phoenix",
}

state_capitals["Alaska"]                      # "Juneau"
state_capitals["California"] = "Sacramento"  # add a new pair
state_capitals.pop("California")             # remove it again
state_capitals.keys()                        # all the keys
state_capitals.values()                      # all the values
len(state_capitals)                          # 3
"Texas" in state_capitals                    # False: checks the keys

Keys are usually strings, and values can be anything: numbers, strings, lists, even other dictionaries.

Where dictionaries show up in the lessons

When you ask a website's API for data (in Wi-Fi & APIs), the answer comes back as a dictionary. You pull out the part you want by its key, such as current["temperature_2m"].

The buzzer lesson also uses one to look up a musical note's frequency by name: NOTES["C"].

Next: If, For & While