Write Functions for Dictionary Records

Chapter 8 gave functions clear inputs and outputs. A dictionary can package several related inputs into one record.

book = {
    "title": "Kindred",
    "category": "novel",
    "year": 1979,
}

A formatting function can receive that record:

def catalog_line(item):
    return f"{item['title']} | {item['category']} | {item['year']}"

The parameter item holds one dictionary during the call. Bracket lookups read its required fields.

line = catalog_line(book)
print(line)

This prints:

Kindred | novel | 1979

The dictionary defines a record shape

catalog_line expects an item with three keys:

• "title"

• "category"

• "year"

That expected set of fields is the record’s shape. Python does not declare it separately here. The function and the data must agree.

This record works:

map_item = {
    "title": "City map",
    "category": "reference",
    "year": 2026,
}

This one is missing a required key:

incomplete = {
    "title": "City map",
    "year": 2026,
}

Calling catalog_line(incomplete) raises a KeyError for "category".

That is not a reason to use .get() everywhere. If every catalog item requires a category, the incomplete record should be fixed.

Keep responsibilities separate

The dictionary stores facts. The function decides how to format them. The caller decides whether to print, store, or compare the returned line.

line = catalog_line(book)
print(line)

This is the same boundary you practiced with strings and lists in Chapter 8, now applied to a structured record.

def item_summary(item):
    return f"{item['name']}: {item['stock']}"

record = {"name": "Compass", "stock": 8}
summary = item_summary(record)