Track 3 · Lesson 1 of 11

Making decisions with if

Run code only when a condition is true, using if, a colon and an indented block.

About 25 minFirst steps4 exercisesFree

You will be able to

Explain what a condition is and why it evaluates to True or False
Write an if statement with a colon and an indented block
Predict which lines run when a condition is true and when it is false
Store the result of a comparison in a variable and use it as a condition

Every program you have written so far runs the same lines every time. Real programs react to their data. A warehouse system warns you only when stock is low. A checkout adds a delivery fee only when the order is small. A thermostat switches on the heating only when the room is cold.

All of these are decisions: do something if a situation is true, and skip it otherwise. In Python you make that kind of decision with the if statement.

Conditions are True or False

A condition is an expression whose value is either True or False. These two values have their own type, bool (short for boolean). Comparisons are the most common conditions:

Operator	Meaning	Example	Value
`==`	equal to	`5 == 5`	`True`
`!=`	not equal to	`5 != 5`	`False`
`<`	less than	`3 < 10`	`True`
`>`	greater than	`3 > 10`	`False`
`<=`	less than or equal to	`10 <= 10`	`True`
`>=`	greater than or equal to	`9 >= 10`	`False`

Notice that "equal to" is two equals signs. A single = assigns a value to a variable; == asks a question.

Comparisons produce booleans

stock = 7
print(stock < 10)
print(stock == 10)
print(type(stock < 10))

is_low = stock < 10
print("Low stock?", is_low)

Try it: Change stock to 10 and run it again. Which lines change?

The if statement

An if statement has three parts:

if condition:
    line that runs only when the condition is True
    another line that runs only when the condition is True
line that always runs

The keyword if, then the condition.
A colon : at the end of that line.
An indented block: one or more lines pushed in by four spaces. These lines belong to the if.

Python uses indentation, not brackets, to decide which lines are inside the block. The first line that is back at the left edge is outside it, so it runs whatever the condition was.

A low-stock warning

stock = 4

if stock < 10:
    print("Warning: stock is low")
    print("Send a reorder request")

print("Stock check finished")

Try it: Set stock to 25. The two indented lines are skipped, but the last line still runs.

Read it line by line:

stock holds 4.
Python evaluates stock < 10, which is True, so it runs the indented block: both warning lines print.
The unindented print is not part of the block, so it runs either way.

With stock = 25 the condition is False. Python skips the whole indented block and jumps straight to the first line after it.

Predict the output

balance = 120
price = 150

print("Checking balance")
if balance >= price:
    print("Purchase approved")
    balance = balance - price
print("Balance:", balance)

The condition here is false. What does the program print?

Conditions from user input

Conditions become useful when the data is not fixed in the code. Combine input() and type conversion (from the previous track) with an if. Convert first: "9" < 10 is an error, because a string cannot be compared with a number.

Deciding based on what the user types

temperature = float(input("Body temperature in C: "))

has_fever = temperature >= 38.0
if has_fever:
    print("Fever detected. Please see the nurse.")

print("Reading recorded:", temperature)

Try it: Change the input to 37.2 (or run it and type a value). Then try exactly 38.0: does it count as a fever?

Guided practice · ●difficulty 1 of 5 · about 4 min

Low-stock alert

A shop wants a warning when an item is running out. The starter reads the number of units in stock.

If there are fewer than 10 units, print Reorder needed.
In every case, print Check complete as the last line.

Examples:

Input	Output
`4`	`Reorder needed` then `Check complete`
`25`	`Check complete`

Keep the input() line as it is.

Plain text editor

Ctrl+Enter run · Shift+Enter check · your code is saved in this browser as you type

Output

Run your code to see its output, or Check to test it against the requirements.

The solution becomes available after you pass, or after all three hints and two checks.

Challenge · ●●difficulty 2 of 5 · about 6 min

Library late fee

A library charges 1.50 SAR per day for late books. The program reads how many days late a book was returned. Books returned early are recorded with a negative number, and on-time books with 0.

Create a variable fee that starts at 0. If the book is late (more than 0 days), set fee to the days late times 1.5. Then print the fee with two decimal places:

Fee: 4.50 SAR

Input	Output
`3`	`Fee: 4.50 SAR`
`0`	`Fee: 0.00 SAR`
`-2`	`Fee: 0.00 SAR`

Plain text editor

Ctrl+Enter run · Shift+Enter check · your code is saved in this browser as you type

Output

Run your code to see its output, or Check to test it against the requirements.

The solution becomes available after you pass, or after all three hints and two checks.

Debugging · ●difficulty 1 of 5 · about 4 min

Fix the free-delivery check

An online grocer, Basket & Co, offers free delivery on orders of 200 SAR or more. Their developer wrote this program, but pressing Run gives a SyntaxError before anything is printed.

Fix the program so that:

orders of 200 SAR or more print You qualify for free delivery,
every order then prints the total, for example Order total: 185.50 SAR.

Input	Output
`250`	`You qualify for free delivery` then `Order total: 250.00 SAR`
`185.5`	`Order total: 185.50 SAR`

Plain text editor

Ctrl+Enter run · Shift+Enter check · your code is saved in this browser as you type

Output

Run your code to see its output, or Check to test it against the requirements.

The solution becomes available after you pass, or after all three hints and two checks.

Edge cases · ●●difficulty 2 of 5 · about 5 min

Store a condition in a variable

An airline gate checks carry-on bags. A bag may weigh at most 7 kg. The program reads the bag's weight in kilograms (it may have decimals).

Create a variable too_heavy that holds the boolean result of the comparison (True when the bag is over the limit).
Print Too heavy: True or Too heavy: False using that variable.
If the bag is too heavy, also print Please check this bag in.

Input	Output
`8.2`	`Too heavy: True` then `Please check this bag in`
`7`	`Too heavy: False`

Plain text editor

Ctrl+Enter run · Shift+Enter check · your code is saved in this browser as you type

Output

Run your code to see its output, or Check to test it against the requirements.

The solution becomes available after you pass, or after all three hints and two checks.

Summary

A condition is an expression whose value is True or False (type bool); comparisons like <, >= and == produce one.
if condition: runs the indented block below it only when the condition is True.
Indentation decides which lines belong to the block; the first unindented line runs every time.
You can store a comparison in a variable and use the variable as the condition.
Always test values exactly on the threshold, and just either side of it.