Making Decisions with if / elif / else

So far your programs have run every line, top to bottom, no matter what. The if statement is how you give a program a choice: run this block only when a condition is true. It’s the single most common way to make code react to its input.

x = 5
if x > 0:
    print('Positive')

Positive

Adding an else branch

x = -2
if x > 0:
    print('Positive')
else:
    print('Non-positive')

Non-positive

Chaining with elif

When you have more than two cases, chain them with elif (short for “else if”). Python checks each condition in order and runs the first block whose condition is true — then skips the rest. Both elif and else are optional.

x = 0
if x > 0:
    print('Positive')
elif x == 0:
    print('Zero')
else:
    print('Negative')

Zero

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

B

Flow of Execution

is_raining = True
if is_raining:
    print('Take an umbrella')
print('Go outside')

Take an umbrella
Go outside

Nested Conditionals

x = 10
y = 5
if x > 5:
    if y > 2:
        print('Both conditions met')
    else:
        print('Only first condition met')
else:
    print('First condition not met')

Both conditions met

Common Misconceptions

Myth: Python uses braces {} to define code blocks like C or Java. Reality: Python uses indentation (whitespace) to mark what’s inside a block. Line up your code consistently — four spaces is the convention.

Myth: Every elif whose condition is true will run. Reality: An if/elif chain stops at the first true condition, runs that one block, and skips all the rest.

Myth: The colon : at the end of an if line is optional. Reality: The colon is required — it marks the start of the indented block. Leaving it off raises a SyntaxError.

Under the Hood

Watch how execution jumps directly past unmatched branches:

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