Making decisions with if

What you'll learn

How to compare values and choose between paths with if, elif and else.

Why it matters

Programs need to react: grant access, apply a discount, show a warning. Conditions are how they decide.

Explanation

A condition is a question with a True or False answer. Comparisons create them:

• == equal, != not equal
• > < >= <=

Combine them with and, or and not.

An if statement ends with a colon, and the code that belongs to it is indented (four spaces is the convention). Indentation is how Python knows what is inside.

Example

Step-by-step walkthrough

1. temperature is 28.
2. 28 > 30 is False, so the first block is skipped.
3. elif temperature >= 20 is True, so it prints Nice weather.
4. The else is skipped because an earlier branch already ran.
5. The last line prints False: age >= 18 is False and and needs both sides True.

Try it yourself

Change temperature to 35 and then 10. Which message appears each time?

if / elif / elsePython

Editable codetemperature = 28 if temperature > 30: print("It's hot. Drink water.") elif temperature >= 20: print("Nice weather.") else: print("Bring a jacket.")

Run ResetCopyCtrl+Enter to run

Common mistakes

• Using = instead of == in a condition. = assigns, == compares.
• Forgetting the colon after if, elif or else.
• Wrong indentation, so the code is not part of the block.
• Writing else if. Python's keyword is elif.

Quick quiz

Summary

• Conditions use ==, !=, >, < and and / or / not.
• if, elif, else choose a block; the colon and indentation matter.
• Only the first true branch runs.

Next: repeating work with loops.

if score >= 50:
    print("Pass")
else:
    print("Try again")

temperature = 28

if temperature > 30:
    print("It's hot. Drink water.")
elif temperature >= 20:
    print("Nice weather.")
else:
    print("Bring a jacket.")

age = 17
has_ticket = True
print(age >= 18 and has_ticket)