if - elif - else — syntax, indentation rules

HOW the syntax looks

if condition1:
    # block A — runs if condition1 is True
    statement_a
elif condition2:
    # block B — runs if condition1 False AND condition2 True
    statement_b
else:
    # block C — runs if both above were False
    statement_c

Formula The decision rule (logic, derived)

Let conditions be $c_1, c_2, \dots, c_n$ and blocks $B_1,\dots,B_n$ plus fallback $B_e$ . The chain executes exactly one block, the first whose condition is true: $\text{run } B_k \iff \big(\lnot c_1 \land \lnot c_2 \land \dots \land \lnot c_{k-1}\big) \land c_k$ and the else block: $\text{run } B_e \iff \lnot c_1 \land \lnot c_2 \land \dots \land \lnot c_n$ Why this form? Because Python checks tests in order and stops at the first True. The earlier conditions being false ( $\lnot c_i$ ) is built into reaching a later elif. This guarantees mutual exclusivity: never more than one branch runs.

Why must exactly one run (with else)? The conditions partition all possibilities: either some $c_k$ is the first True one, or none are → else. The set of cases is covered with no overlap. This is why a clean if/elif/else chain is exhaustive and exclusive.

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

Derivation: how Python reads indentation

Worked examples

Worked example 1 — Pass / Fail

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

Output: Pass

Why? marks >= 40 → 72 >= 40 → True, so the if block runs and else is skipped.

Worked example 2 — Grade bands (elif chain, order matters)

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

Output: B

Why this step? 90 test fails ( $85<90$ ). Move to elif score >= 80 → True → grade="B". Python stops — the 70 elif is never even checked.
Forecast-then-Verify: before reading the answer, predict the output for score = 70. (Answer: C, because >=70 is the first True.)

Worked example 3 — Nesting (block inside a block)

age = 20
citizen = True
if age >= 18:
    if citizen:
        print("Can vote")
    else:
        print("Not a citizen")
else:
    print("Too young")

Output: Can vote

Why? Outer if True → enter its block. Inside, citizen is True → inner if runs. The two indentation levels (8 spaces) tell Python the inner if belongs to the outer one.

Worked example 4 —

if without else

n = -3
if n < 0:
    print("negative")
print("done")

Output:

negative
done

Why? print("done") is at the same indentation as if, so it's outside the block and runs regardless. else is optional.

Common mistakes (Steel-man + fix)

Common mistake Many independent

ifs instead of elif Feels right: "More ifs = more thorough." Each looks like its own check. Reality:

if score >= 80: grade="B"
if score >= 70: grade="C"   # overwrites! score=85 -> C

Separate ifs are all evaluated; later ones can clobber earlier results. Fix: Use elif so the chain is mutually exclusive and stops at the first match.

Recall Feynman: explain to a 12-year-old

Imagine a hallway with doors. You walk and ask the first question on a door: "Is it true?" If yes, you go through that door and the hallway ends — you don't check the other doors. If no, you walk to the next door (elif) and ask again. If you reach the end and no door said yes, you go through the special last door marked else. The way you push your sentences a few spaces to the right is how you tie a set of actions to a particular door — everything pushed in together happens after you walk through that door.

Active recall

What punctuation must every if, elif, and else line end with?

A colon :

How does Python know which lines belong inside an if block?

By their indentation (leading whitespace), not braces.

How many elif blocks can one if chain have?

Zero or more.

How many else blocks can one if chain have?

At most one (and it's optional).

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

Exactly one — the first whose condition is True (or else if none are).

What error do you get from mixing tabs and spaces?

TabError: inconsistent use of tabs and spaces.

Difference between = and == in a condition?

= assigns a value; == tests equality. Conditions need ==.

Why prefer elif over multiple separate ifs?

elif makes branches mutually exclusive and stops at the first match; separate ifs all run and can overwrite results.

For score=70 in a chain checking >=90, >=80, >=70, else, what is selected?

The >=70 branch (first True one).

What is the standard number of spaces per indentation level in Python?

4 spaces.

Connections

Booleans and comparison operators — conditions evaluate to True/False
Logical operators and, or, not — building compound conditions
Truthiness in Python — non-bool values in if (0, "", [] are False)
Indentation and code blocks — the same rule governs loops and functions
while and for loops — also use : and indented blocks
match-case statement — alternative branching (Python 3.10+)
Ternary conditional expression — a if cond else b one-liner

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, normally Python code upar se neeche, line by line chalta hai. Lekin asli zindagi mein decisions condition pe depend karte hain — "agar baarish ho rahi hai to chhata le lo". Yahi kaam if / elif / else karta hai: program ko ek raasta choose karne ki power deta hai. if pehla sawaal poochta hai, agar True mila to uska block chalta hai aur baaki sab skip. Agar False hua to elif (else-if) wala agla sawaal check hota hai, aur agar koi bhi match nahi hua to else wala fallback chalta hai.

Sabse important baat: exactly ek hi branch chalta hai — jo pehle True mila wahi jeet gaya, baaki check hote hi nahi. Isliye grade waale example mein score=85 ke liye >=90 fail hua, >=80 True hua, "B" mil gaya, aur >=70 ko check hi nahi kiya. Yahi reason hai ki alag-alag if lagane se bachna chahiye — wahan saare if chalte hain aur baad waala result ko overwrite kar deta hai. elif use karo taaki branches mutually exclusive rahein.

Indentation Python ka grammar hai, decoration nahi. Jo lines ek branch ke andar hain unko 4 spaces andar push karte hain, aur har header line (if, elif, else) ke end mein colon : lagana zaroori hai — curly braces {} yahan nahi hote. Tab aur space mat mix karo, warna TabError aata hai. Aur dhyaan rakho: comparison ke liye == lagta hai, = to assignment hota hai. Yeh chhoti cheezein hi exams aur real coding dono mein sabse zyaada galtiyan karwati hain, isliye COIN mnemonic yaad rakho.

if - elif - else — syntax, indentation rules

WHY does this exist?

WHAT it is

HOW the syntax looks

Derivation: how Python reads indentation

Worked examples

Common mistakes (Steel-man + fix)

Active recall

Connections

Concept Map

Hinglish (regional understanding)

Go deeper — visual, from zero

Test yourself — Introduction to Programming (Python)

Connections