if - elif - else — syntax, indentation rules: Foundations

Before you can read if score >= 90: you must already know six tiny things: what a value is, what a variable is, what True/False mean, what a comparison like >= does, what a : colon promises, and what indentation ties together. Then we meet the three keywords themselves — if, elif, else — and see exactly what each does. We build each one from nothing, in the order they stack.

1. A value — the thing your program holds

Why the topic needs it: an if decides based on what data you have. No values → nothing to decide about.

2. A variable — a labelled box for a value

Look at Figure s01: the box on the left has a name taped to it (marks), and inside sits the value 72. The = is the act of putting the card in the box — it is not a question.

Why the topic needs it: conditions like marks >= 40 read the value out of a box. Without variables there'd be nothing to compare.

3. Booleans — the two answer-cards `True` and `False`

Why this tool and not a number? Because a decision is binary — you either take the umbrella or you don't. A boolean is the smallest possible value that can express "yes" versus "no", which is precisely what a branch needs. See Booleans and comparison operators and Truthiness in Python for how other values (like 0 or "") get read as False.

marks = 72
if marks >= 40:
    print("Pass")
else:
    print("Fail")

marks = 72 — the = fills the box marks with the value 72.
marks >= 40 — the >= machine compares 72 and 40, produces the boolean True.
: promises a block; the indented print("Pass") is that block.
Because the boolean was True, the if block runs → Pass. The else block is skipped.

A multi-way trace (`if`–`elif`–`else`)

Worked example Three doors, one exit

score = 85
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"
print(grade)

Output: B

score >= 90 → 85 >= 90 → False. Door 1 says no; walk on.
elif score >= 80 → 85 >= 80 → True. Leave through this door: grade = "B".
Python stops — the >= 70 elif and the else are never even checked.

Forecast-then-verify: for score = 65, doors 1, 2, 3 all say False, so you leave through else → grade = "F".

Equipment checklist

Test yourself — cover the right side, answer, then reveal.

What does marks = 72 do, in plain words?

Puts the value 72 into the box named marks (assignment, not a question).

What are the only two boolean values?

True and False.

What does the operator >= hand back?

A boolean — True or False.

Read == aloud.

"is equal to?" — it asks a question and returns a boolean.

Which symbol assigns and which compares?

= assigns (fills a box); == compares (asks a question).

What does the colon : at the end of an if line promise?

That an indented block follows and belongs to that header.

How does Python know which lines are inside a block?

By their indentation (leading whitespace), not by braces.

Standard number of spaces per indent level?

4 spaces.

Why can't you mix tabs and spaces?

Python treats them as different characters → TabError.

What does the if keyword do when its condition is True?

Runs its indented block and skips the rest of the chain.

When does an elif block run?

Only when every condition above it was False and its own condition is True.

Can an elif appear without an if above it?

No — an elif may only follow an if or another elif, else SyntaxError.

When does the else block run, and how many are allowed?

When all preceding conditions were False; at most one else, at the end, and it's optional.

In a full if/elif/elif/else chain, how many blocks run?

Exactly one — the first True condition, or else if none are True.

Connections

Parent: if / elif / else
Booleans and comparison operators — where True/False and >= come from
Logical operators and, or, not — combining several conditions
Truthiness in Python — non-boolean values inside if
Indentation and code blocks — the grammar in full
while and for loops · match-case statement · Ternary conditional expression — reuse the same tools

Foundations — if - elif - else — syntax, indentation rules

1. A value — the thing your program holds

2. A variable — a labelled box for a value

3. Booleans — the two answer-cards `True` and `False`

4. Comparison operators — the machines that produce a boolean

5. The colon `:` — a promise that a block follows

6. Indentation — the grammar that groups lines

7. The three keywords themselves — `if`, `elif`, `else`

A one-look worked trace (`if`–`else`)

A multi-way trace (`if`–`elif`–`else`)

Equipment checklist

Connections