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Comparing values: ask a yes-or-no question

Before a program can decide anything, it has to ask a question. Is the oven hot enough? Are the cookies gone? A comparison asks, and Python answers True or False.

The big idea

A comparison checks two values against each other and gives back a yes-or-no value, called a boolean: True or False.

See it in code

1Start simple

>= means "greater than or equal to." Print a check and you can see the answer Python gets:

python
oven_temp = 300
ready_temp = 350
print(oven_temp >= ready_temp)
Is the oven hot enough yet?
False

300 isn't greater than or equal to 350, so the answer is False. Change oven_temp to 350 and the same line prints True.

2A step further

Two equals signs, ==, ask whether two things are the same. You can store the answer in a variable and give it a name that reads like a question:

python
cookies_left = 0
is_sold_out = cookies_left == 0
print("Sold out?", is_sold_out)

special = "brownies"
print(special == "cookies")
One check for numbers, one for words:
Sold out? True
False

== works on words as well as numbers. The words must match exactly, letter for letter, or the answer is False.

3In the bakery

The real job of a check is to steer the program. Here > compares what the customer wants with what's in the case:

python
cookies_left = 5
how_many = int(input("How many? "))

if how_many > cookies_left:
    print(f"Sorry, we only have {cookies_left}.")
else:
    print("Coming right up!")
A customer asks for 8, but only 5 are left:
How many? 8
Sorry, we only have 5.

how_many > cookies_left was True, so the first message ran. That's all an if needs: a check that answers yes or no.

The same idea, everywhere

Checks like these run constantly. A painting asks whether a row number is even. A game asks whether a ball has passed the wall, sixty times every second.

Try it yourself

Run the last program again and type 5 instead of 8. Which message do you get? Then change > to >= and type 5 once more. Why did the answer flip?

The common mistake

Using one equals sign when you mean two. = stores a value. == asks whether two values match. Writing if cookies_left = 0: stops the program, because Python expected a question there.

What it unlocks

Comparisons are the questions behind every if and else. Join two of them with and, or, not and your program can make careful choices.

Older, or want more depth? Read the Teens version →

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