if, elif, else

Big idea

if runs a block of code only when a condition is True. elif and else add other paths, and Python takes exactly one of them.

The idea

if condition:
    # runs only when condition is True

Two things matter: the colon at the end of the if line, and the indentation (4 spaces) of the lines that belong to it. That indented group is called a block. When the indentation stops, the block is over.

temperature = 31
if temperature > 30:
    print("It's hot!")
    print("Drink some water.")
print("Have a nice day.")

output
It's hot!
Drink some water.
Have a nice day.

The last print isn't indented, so it runs no matter what.

else: the other path

if age >= 18:
    print("You can vote.")
else:
    print("Not yet!")

elif: more than two paths

elif (short for "else if") checks another condition. Python tests each one top to bottom and runs the first that's True — then skips the rest.

score = 82
if score >= 90:
    print("A")
elif score >= 75:
    print("B")
elif score >= 50:
    print("C")
else:
    print("F")

B

82 is also >= 50, but that check never happens — the elif score >= 75 branch already ran.

Watch it

Order matters

score = 95
if score >= 50:
    print("Pass")
elif score >= 90:
    print("A")
else:
    print("Fail")

output
A

Put the most specific test first: score >= 90 before score >= 50.

Nesting and combining

A block can contain another if. Often and/or reads better than nesting:

if has_ticket:
    if age >= 18:
        print("Welcome")

if has_ticket and age >= 18:     # same thing, flatter
    print("Welcome")

A one-line choice

For picking between two values, there's a compact form called a conditional expression:

label = "even" if n % 2 == 0 else "odd"

Think of it like

A fork in a path with signposts. You read the signs in order and take the first one that matches you. else is the path for everyone else.

Common mistake

Forgetting the colon, or mixing indentation:

if x > 5          # ✗ SyntaxError: expected ':'
    print("big")

Key terms

• Condition — An expression that's True or False.
• Block — The indented lines that belong to an if, loop, or function.
• elif — "Otherwise, if…": another condition to check.