Conditionals — making decisions

Run different code depending on whether something is true. The pattern: if, optionally elif, optionally else.

The basic shape

age = 20

if age >= 18:
    print("You're an adult.")
else:
    print("You're a minor.")

Three things to notice:

• The line ends with a colon :
• The "what to do" is indented (4 spaces is the convention)
• The condition (age >= 18) is just any expression that's true or false

elif — more than two branches

score = 72

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

print(f"Grade: {grade}")

Python checks each condition top to bottom and runs the first one that's true.

Combining conditions: and, or, not

age = 22
has_ticket = True

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

if age < 18 or not has_ticket:
    print("Sorry, can't enter.")

Truthiness

Python is generous about what counts as "true" in a condition. These are all falsy:

• False
• None
• 0 (and 0.0)
• "" (an empty string)
• [], {} (empty list, empty dict)

Everything else is "truthy". So you can write nice short checks:

name = input("Name? ")

if name:                       # True if name is not empty
    print(f"Hi {name}")
else:
    print("You didn't enter a name.")

A small worked example

password = input("Pick a password: ")

if len(password) < 8:
    print("Too short — at least 8 characters please.")
elif password.lower() == password:
    print("Add at least one uppercase letter.")
else:
    print("Looks good!")