Dictionaries

So far we have stored values in variables and in lists. A list is great when we have a bunch of values and we access them by their position (index 0, 1, 2, …). But what if we don't care about position and instead we care about a name?

Imagine Tommy Vercetti's phone book. He doesn't say "give me contact number 3". He says "give me Cortez's phone number". The name is the way he looks things up

This is exactly what a dictionary does in Python

phone_book = {
    "Cortez": "555-1234",
    "Lance": "555-5678",
    "Phil": "555-9999"
}

print(phone_book["Cortez"])

Output 555-1234

A dictionary is created with curly braces {}. Inside, we write key: value pairs separated by commas. The key is what we use to look things up (like the name), and the value is what we get back (like the phone number)

phone_book = {
    "Cortez": "555-1234",
    "Lance": "555-5678",
    "Phil": "555-9999"
}

print(phone_book["Lance"])
print(phone_book["Phil"])

Output

555-5678
555-9999

What happens if we ask for a key that doesn't exist?

phone_book = {
    "Cortez": "555-1234",
    "Lance": "555-5678"
}

print(phone_book["Sonny"])

Run it. You'll see a KeyError. Python is telling us: "I don't know any Sonny"

We can add a new entry or change an existing one very easily

phone_book = {
    "Cortez": "555-1234",
    "Lance": "555-5678"
}

phone_book["Tommy"] = "555-0000"
print(phone_book["Tommy"])

phone_book["Lance"] = "555-1111"
print(phone_book["Lance"])

Output

555-0000
555-1111

If the key doesn't exist, Python creates it. If it already exists, Python updates it. Simple

We can also remove an entry with del

phone_book = {
    "Cortez": "555-1234",
    "Lance": "555-5678"
}

del phone_book["Lance"]
print(phone_book)

Output

{'Cortez': '555-1234'}

Lance is gone. Sorry Lance :)

To check if a key exists before accessing it, use in

phone_book = {"Cortez": "555-1234"}

if "Cortez" in phone_book:
    print("Found Cortez!")
else:
    print("Cortez is not in the phone book")

Just like lists, dictionaries can hold any type of value: strings, numbers, booleans, even other lists or dictionaries

player = {
    "name": "Tommy Vercetti",
    "health": 100,
    "is_alive": True,
    "weapons": ["bat", "pistol", "shotgun"]
}

print(player["name"])
print(player["health"])
print(player["weapons"])

Mission: Crew Database

In the editor you're given four details about a crew member as variables: name, role, age, and station.

Do the following, in order:

Create a dictionary crew with three keys ("name", "role", and "age") using the name, role, and age variables as their values
Add a new key "station", using the station variable as its value
It's the crew member's birthday, so update age inside the dictionary by adding 1 to it (use the + operator)
Print the values for name, role, age, and station, each on its own line
We don't track rank yet, so if the key "rank" is not in the dictionary, print Rank: unknown

Output

For the values in the editor, the program prints something like

Tommy
Pilot
35
Laniakea
Rank: unknown

Change a value at the top and run again, the report follows.