Course 3.2Lesson 1 of 5

and, or and not

and, or and not join conditions. A and B is True only when both are True. A or B is True when at least one is True. not A turns True into False and False into True. In a long condition, Python does not first, then and, then or, and brackets change the order. Write each comparison in full: age >= 18 and age <= 60 is right, and age >= 18 and <= 60 is a SyntaxError.

about 35 min1 simulation2 practice questions

Lesson 1 of 5Show all lessons

Try the code and the simulations beside the lesson.

What you will be able to do

After this lesson you can

join two conditions with and and with or,
turn a condition around with not,
say what a combined condition gives for any values, and
use brackets when the order matters.

One condition is often not enough

To enter a club, a person must be 18 or older and must have a ticket. To get a discount, a person must be a student or a senior citizen. To go out to play, it must not be raining.

Each of these needs a combined condition. Python has three words for it: and, or and not. They work on values that are True or False, and they give a new True or False.

and: both must be True

A and B is True only when both A and B are True. If one of them is False, the answer is False. Think about the club again: if you are old enough but have no ticket, you cannot enter. The word and is strict.

or: at least one must be True

A or B is True when at least one of them is True. It is False only when both are False. The word or is generous. In everyday English, "or" sometimes means "one of them but not both" (tea or coffee). In Python, or also allows both.

not: turn it around

The three words in one table

Three cards. and means both must be True: True and True is True, True and False is False, False and False is False. or means at least one must be True: True or False is True, False or True is True, False or False is False. not turns it around: not True is False, and not False is True.

`A`	`B`	`A and B`	`A or B`	`not A`
`True`	`True`	`True`	`True`	`False`
`True`	`False`	`False`	`True`	`False`
`False`	`True`	`False`	`True`	`True`
`False`	`False`	`False`	`False`	`True`

Look at the column of and: only one True, in the first row. In the column of or: only one False, in the last row. This is the whole idea. A table like this is called a truth table, and in a later lesson you write them yourself.

Each side of and and or is a full comparison: age >= 10 and height >= 140, never age and height >= 140.

The order of the words

What does a or b and c mean? There is an order, like in arithmetic where multiplication goes before addition: not goes first, then and, then or. And brackets go before all of them.

A safe habit: when you mix and with or in one condition, always use brackets. It costs nothing, and a reader of your program (you, next week) does not need to remember the order.

Write each comparison in full

A very common mistake: age >= 18 and <= 60. In English we say "18 or more and 60 or less", but Python needs each comparison to have both sides: age >= 18 and age <= 60. You will see this again in the next lesson, with a shorter form.

Common mistakes

Forgetting the second side: x > 1 and < 5. Write x > 1 and x < 5.
Mixing and and or without brackets, and trusting your memory of the order.
Writing &&, || or ! from other languages. Python uses the English words.
Thinking that or means one but not both.
Using = instead of == inside the combined condition.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

Warm-up. Open the lab A truth table. Fill the answers of a truth table, and the checker shows the rows that are wrong.
Build. Open the problem Predict: and, or, not and the problem Is it a teenager?
Stretch. Open the problem The roller coaster. Write its rule as one condition, and then test it on the edges.