if / elif / else — Decisions

if / elif / else — Decisions

Programs become smart the moment they can choose. The if/elif/else structure is Python’s decision machine — and after the booleans lesson, you already know half of it.

The basic if

age = 20

if age >= 18:
    print("You can vote")
    print("Bring your ID")

print("This always runs")

The recipe: the if keyword, a condition (which produces True/False), a colon, then an indented block that runs only when the condition is True. The moment indentation returns, you’re outside the decision.

else — the other path

marks = 40

if marks >= 50:
    print("Passed!")
else:
    print("Failed — try again")

Exactly one of the two blocks runs. Never both, never neither. else needs no condition — it’s simply “everything else.”

elif — the ladder

When there are more than two outcomes, elif (short for “else if”) builds a ladder:

marks = 85

if marks >= 90:
    print("Grade: A")
elif marks >= 75:
    print("Grade: B")
elif marks >= 60:
    print("Grade: C")
elif marks >= 50:
    print("Grade: D")
else:
    print("Grade: F")

Output:

Grade: B

How the ladder executes (the key insight)

Python checks conditions top to bottom and runs only the first True block — then jumps past the entire ladder:

• 85 >= 90? False, skip
• 85 >= 75? True → run, and exit the ladder

This “first match wins” behavior is why the order matters. Reverse the ladder and 85 hits >= 50 first, printing D. Rule: check strictest conditions first.

You can also end a ladder with just an else and no final elif — the else catches everything no condition caught.

Conditions are just booleans

Everything from the booleans lesson plugs straight in:

temperature = 35
is_sunny = True

if temperature > 30 and is_sunny:
    print("Perfect for ice cream")

if not is_sunny or temperature > 40:
    print("Stay hydrated either way")

# Truthiness shortcut from the booleans lesson:
name = input("Name: ")
if name:                          # non-empty string = True
    print(f"Hello, {name}")
else:
    print("You didn't type a name!")

Nested ifs (and when to avoid them)

age = 20
has_id = True

if age >= 18:
    if has_id:
        print("Entry allowed")
    else:
        print("Bring your ID")

This works, but two levels deep is where nesting should stop. The same logic reads flatter as:

if age >= 18 and has_id:
    print("Entry allowed")
elif age >= 18:
    print("Bring your ID")

Flat beats nested — your future self will thank you.

Common Errors & Fixes

• IndentationError: expected an indented block — the block after : has no indentation. Indent 4 spaces.
• SyntaxError: invalid syntax pointing at the if line — missing colon, or = instead of ==.
• Wrong branch always runs — check your ladder order; strictest first.
• elif after else — illegal order: if → elifs → else, never else in the middle.

✅ Checkpoint

1. How many blocks of an if/elif/else ladder run? (Exactly one — the first True match)
2. What keyword continues the ladder? (elif)
3. Does else need a condition? (No — it’s the catch-all)
4. Why does ladder order matter? (First True wins; loose conditions early swallow the strict ones)

Next: for loops — making Python do the repeating.