Check Keys and Use get()

Bracket lookup is direct and clear when a key is required:

title = book["title"]

If the key is missing, Python raises a KeyError. Sometimes that is useful because a required field really is absent. Optional information needs a different plan.

Check membership first

The in operator checks dictionary keys:

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

if "creator" in book:
    print(book["creator"])
else:
    print("Creator missing")

in checks keys, not values. This expression:

"Kindred" in book

is False because "Kindred" is a value, not a key.

Use not in for the opposite check:

if "creator" not in book:
    print("Creator missing")

Ask with get()

The dictionary method .get() also reads a value:

format_name = book.get("format")

When the key is missing, .get() returns None instead of raising a KeyError.

Supply a second argument to choose a more useful fallback:

format_name = book.get("format", "unknown")
print(format_name)
# unknown

The dictionary is not changed. .get() only chooses what value the call returns.

Required or optional?

Choose based on the record’s meaning:

• Use book["title"] when every valid book record must have a title.

• Use book.get("format", "unknown") when format is optional and the caller has a sensible fallback.

• Use "creator" in book when the program needs to take a different action depending on whether the field exists.

Do not replace every bracket lookup with .get(). Silently accepting a missing required key can hide bad data. Safe lookup means making an intentional choice.

item = {"name": "Compass", "price": 12}
print(item.get("color", "not listed"))
print(len(item))