Give Each Value a Label

A list is excellent when position is the useful way to find a value. The first item is at index 0, the second at index 1, and so on.

That becomes awkward when the values are different facts about one thing.

book = ["Kindred", "Octavia E. Butler", 1979]

What does position 1 mean? You have to remember. Someone reading book[1] has to search for the list’s definition or trust a comment.

You could split the facts into variables:

title = "Kindred"
creator = "Octavia E. Butler"
year = 1979

That is clear for one book. For twenty books, you need twenty sets of variable names.

A record with labels

A dictionary stores key-value pairs:

book = {
    "title": "Kindred",
    "creator": "Octavia E. Butler",
    "year": 1979,
}

Each key labels the meaning of its value:

Instead of asking for a position, you ask for a key:

print(book["creator"])

The brackets look like list indexing, but the lookup is different. A list answers, “What is at position 1?” A dictionary answers, “What value is stored under the key "creator"?”

Keys must be unique

A dictionary can store many values, but each key appears only once. If you assign another value to an existing key, that entry changes:

book["year"] = 1988

The dictionary still has one "year" key. Its value is now 1988.

Values do not need to be unique. Two different keys can both hold "unknown".

Dictionaries are mutable

Like a list, a dictionary can change after it is created. You can:

• add a new key-value pair;

• replace the value for an existing key;

• remove a pair;

• read and loop through its contents.

This chapter concentrates on adding, changing, reading, and looping. Chapter 10 will return to the errors that can happen when a program asks for a missing key.

Model the information, not its display

The dictionary holds facts:

book = {
    "title": "Kindred",
    "creator": "Octavia E. Butler",
    "year": 1979,
}

A function can decide how to display them:

def book_label(book):
    return f"{book['title']} by {book['creator']}"

That separation uses the function design from Chapter 8. The dictionary models the record. The function performs an operation with it.

place = {
    "name": "Kyoto",
    "country": "Japan",
}

print(place["country"])