Making Decisions: if / elif / else

14 min 25 XP

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Lesson 5 of 7 · View course roadmap

Step 1

Learn the idea

Programs become intelligent when they can choose. Python decides with if, elif and else:

Comparison operators produce booleans: == (equal), != (not equal), <, >, <=, >=. Combine conditions with and, or, not.

Indentation is the syntax. The indented block under an if only runs when the condition is True. Python convention is 4 spaces.

if: checked first
elif: checked only if everything above was False (you can chain many)
else: the fallback, runs if nothing matched

Where you'll use this

Every access check ('is this user an admin?'), price rule ('free shipping over €50') and form validation in every app you use is an if/elif chain. Reading them fluently is reading business logic.

Common mistakes

Using = instead of == in a condition: Python catches this as a SyntaxError, unlike C.
Forgetting the colon at the end of if/elif/else lines.
Inconsistent indentation: mixing tabs and spaces breaks the block structure. Configure your editor to insert 4 spaces per Tab.
Chaining that never triggers: checking score >= 80 before score >= 90 means the 90 branch is unreachable.

Pro tip

Python chains comparisons naturally: 18 <= age < 65 works exactly like the math notation. Also, empty containers are falsy: 'if my_list:' is the Pythonic way to test non-emptiness.

Step 2

Watch how it runs line by line

Press play to watch Python execute this code, one line at a time.

Step 3

Try it yourself

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The lesson example is loaded and ready: press Run, then change something and run it again. Breaking it is part of learning. Want a clean slate? Tap “New blank”.

PYexample.py

⌘ + Enter to run

Output appears here…

Step 4

Pass the challenge +25 XP

Blank · autosaved

The starter defines temperature = 31. Print Hot if it is above 28, Mild if it is above 15 (but not hot), otherwise Cold.

Target output

Hot

PYchallenge.py

temperature = 31
if temperature > 28:
    print("Hot")
elif temperature > 15:
    print("Mild")
else:
    print("Cold")

Run your code to check it…

Step 5

Check your understanding

1. Which operator tests equality? = == === eq

2. When does an elif branch run? Always Only if its condition is True AND all previous conditions were False Only if the if was True Never

Last step

Your notes (saved on this device)

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Learn the idea

Where you'll use this

Common mistakes

Pro tip

Watch how it runs line by line

Try it yourself

Pass the challenge +25 XP

Check your understanding

Your notes (saved on this device)

Write Python that does something useful.