meet it

Type hints say what each value should be.

A is a note about types written into the code. def total(prices: list[int]) -> int: says prices should be a list of whole numbers, and the function hands back one. Variables can have hints too: name: str = "Sam".

Write dict[str, float] for a dictionary from text to decimals, and a like str | None when a value might be missing.

Python ignores hints when it runs. A such as mypy reads them and warns you about mistakes before your code ever runs. Your editor uses them too.

def find_price(menu: dict[str, int], item: str) -> int | None:
    if item in menu:
        return menu[item]
    return None

menu: dict[str, int] = {"tea": 180, "cake": 325}
print(find_price(menu, "cake"))
print(find_price(menu, "pie"))

To check for a missing value, write if price is None:, because is asks whether it is that exact object.

guess first

The hint says int. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

def double(n: int) -> int:
    return n * 2

print(double(4))
print(double("ha"))

your turn

Your turn. Add the missing hints.

Hint that scores is a list of whole numbers, and that the function returns a decimal number. The last line prints the hints Python stored.

Your code

press run to see what happens

fix it

The hint warned you, but it still crashes.

Broken code to fix

press run to see what happens

The hint on line 1 warns that stock_of can hand back None. Which item is not in stock?
Rulers are missing, so you add None to a number. Check the answer before adding it.
Store the answer in count, then add it only if count is not None:, then run.

nailed it

your code says what it means now. :D

name: type and -> type add hints to your code.
Python ignores hints. A type checker reads them for you.
X | None warns that a value might be missing.

new words in your collection

type hint
union type
type checker

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