and, or and not

Combining conditions

Three words let you build bigger conditions out of smaller ones:

• and — true only when both sides are true
• or — true when either side is true
• not — flips a boolean

if age >= 18 and has_ticket:
    print("Come in")

if day == "Saturday" or day == "Sunday":
    print("Weekend")

Python is lazy about these, which is more useful than it sounds. In a and b, if a is false it never looks at b — so if items and items[0] == "x": is safe even when items is empty.

Your turn: write can_ride(height, age) returning True only when the person is at least 120cm and at least 8 years old.

not, and readable conditions

not inverts:

if not logged_in:
    print("Please log in")

That reads better than if logged_in == False:, and it is what experienced Python programmers write.

When conditions get long, brackets make the grouping explicit. and binds tighter than or, so a or b and c means a or (b and c) — rarely what anyone intends. Put the brackets in.

Your turn: write needs_review(score, flagged) returning True when the score is below 40 or the item is flagged — but False for anything scoring 90 or more, no matter what.