If...Else

Run code only when a condition holds, and choose between branches.

if

A condition, a colon, and an indented block:

Python

temperature = 31

if temperature > 30:
    print("Hot")

print("Always runs")

Output

Hot
Always runs

The condition does not have to be a comparison. Any value works, judged by the truthiness rules: empty and zero are false, everything else is true.

Python

items = []

if items:
    print("we have items")

if not items:
    print("nothing here")

Output

nothing here

else

Python

age = 15

if age >= 18:
    print("Can vote")
else:
    print("Too young")

Output

Too young

elif

Use elif for the second and later conditions. Only the first true branch runs:

Python

score = 72

if score >= 90:
    grade = "A"
elif score >= 70:
    grade = "B"
elif score >= 50:
    grade = "C"
else:
    grade = "F"

print(grade)

Output

Order matters. Put the narrowest condition first, or a broader one will catch everything before it gets there:

Python

score = 95

if score >= 50:
    grade = "C"
elif score >= 90:
    grade = "A"

print(grade)

Output

The elif never gets a chance. That is a logic bug, and Python cannot warn you.

Combining conditions

Python

age = 25
member = True

if age >= 18 and member:
    print("Full access")

if age < 13 or age > 65:
    print("Discount")
else:
    print("Standard price")

Output

Full access
Standard price

Chained comparisons read better than two clauses joined by and:

Python

age = 25
print(18 <= age < 65)

Output

True

The conditional expression

When both branches only assign one value, this one-liner is clearer than four lines:

Python

age = 15
status = "adult" if age >= 18 else "minor"
print(status)

Output

minor

Keep it to one condition. Nested conditional expressions are hard to read and a normal if is right there.

Nesting

Python

logged_in = True
admin = False

if logged_in:
    if admin:
        print("Admin panel")
    else:
        print("User dashboard")
else:
    print("Please log in")

Output

User dashboard

Python

def describe(age):
    if age < 0:
        return "not a real age"
    if age < 18:
        return "minor"
    return "adult"

print(describe(-1), describe(10), describe(30))

Output

not a real age minor adult

An empty branch needs pass

Python

value = 5

if value > 3:
    pass
else:
    print("small")

print("done")

Output

done

Test yourself

2 questions

Why does this give C for a score of 95? if score >= 50: grade = "C" elif score >= 90: grade = "A"

Show the answer

The first condition is true, so the elif never runs — Only the first true branch runs, so the narrowest condition has to come first.

What does 'if items:' test for a list?

Show the answer

Whether it has anything in it — An empty list is falsy, which is why this is preferred over len(items) > 0.

Next chapter

Match

Structural pattern matching, and where it beats a chain of elif.