If Statements: Making Decisions

Python for Absolute Beginners · Lesson 9

What you’ll learn

• How if runs code only when a condition is true
• How else covers the “otherwise” case
• How elif chains several choices
• Why indentation is part of the code, not decoration
• The two most common if mistakes

The first decision

• Everything so far has run straight from top to bottom
• An if statement breaks that: it runs some code only when a condition is true, and skips it otherwise
• The condition is just a comparison, like the ones from Lesson 8
• The if line ends with a colon (:) — “here comes the code for this case”
• The lines underneath are indented (usually four spaces) — that is how Python knows they belong to the if

A first if statement

age = 12
if age >= 10:
    print("You are 10 or older.")
print("Done.")

Output:
You are 10 or older.
Done.

The final print is not indented, so it runs no matter what.

The same program, age 8

age = 8
if age >= 10:
    print("You are 10 or older.")
print("Done.")

Output:
Done.

The indented line is skipped entirely, because the condition is False.

Otherwise: else

• else covers the case where the condition is false
• else needs no condition of its own — it means “in every other case”
• Like if, it ends with a colon and its code is indented

Else in code

score = 45
if score >= 50:
    print("You passed!")
else:
    print("Keep practicing — you will get there.")

Output:
Keep practicing — you will get there.

The score is below 50, so the else block runs.

Several choices: elif

• elif (short for “else if”) chains more than two cases in order
• Python checks each condition from the top and runs the first block whose condition is true — and only that one
• Order matters: always put the strictest condition first, or a looser one above it will swallow every case below

Elif in code

score = 85
if score >= 90:
    print("Excellent!")
elif score >= 70:
    print("Good job!")
elif score >= 50:
    print("You passed.")
else:
    print("Keep practicing.")

Output:
Good job!

85 is not >= 90, so Python moves on; it is >= 70, so it prints “Good job!” and stops checking.

Indentation is not decoration

• In most languages, indentation is just tidiness. In Python, it is meaning
• Same lines, different indentation, different program
• With age 8, a program where both greeting lines are indented prints only “Goodbye.”
• With age 8, a program where only the first line is indented still prints “Welcome in!”
• Indent every line that belongs to the if, elif, or else above it — four spaces is the standard

The two most common if mistakes

• Forgetting the colon: an if line without : gives a syntax error — every if, elif, and else line ends with a colon
• Mixing indentation: some lines two spaces and others four, or tabs mixed with spaces, gives an indentation error
• Pick four spaces and use them everywhere — in IDLE, pressing Tab inserts spaces for you

Hands-on exercise

• Write a program that stores your age and prints “You are a teen” if it is 13 or older, and “You are not a teen yet” otherwise
• Extend it with elif: under 13 → “Child”, 13 to 19 → “Teen”, 20 or older → “Adult” — test it with three different ages
• Deliberately remove the colon from an if line, read the error, then put the colon back and confirm the program works

Key takeaways

• if runs its indented block only when its condition is true
• else covers every other case, with no condition of its own
• elif chains choices in order; the first true condition wins, and only it runs
• Every if / elif / else line ends with a colon; its block is indented four spaces
• Indentation is meaning in Python — same lines, different indent, different program

Take the quiz

Take the Lesson 9 quiz — 5 quick questions to check what you learned.

Next lesson

Loops: Repeating Without Retyping