Making Decisions 🔀

if

grog = 8

if grog > 5:
    print("That is enough grog.")
    print("Genuinely, that is plenty.")

print("This line always runs, branch or no branch.")

That is enough grog.
Genuinely, that is plenty.
This line always runs, branch or no branch.

Two pieces of punctuation are doing all the work:

• The colon at the end of the if line. It means "a block follows". Forgetting it is the most common syntax error in Python, by a distance.
• The indentation of the next lines. Those four spaces are what puts those lines inside the if.

Indentation is not decoration. It is the syntax.

Most languages mark blocks with curly braces and treat layout as a matter of taste. Python has no braces: the layout is the structure. This is the single thing newcomers find strangest, and the thing they defend most fiercely after six months.

ready = True

if ready:
    print("inside the if")
    print("also inside")
print("outside again")

inside the if
also inside
outside again

if / else

password = "grog"

if password == "swordfish":
    print("Access granted.")
else:
    print("Access denied. You fight like a dairy farmer.")

Access denied. You fight like a dairy farmer.

if / elif / else: choosing from many

score = 87

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"Score {score} earns a {grade}.")

Score 87 earns a B.

elif is short for "else if". Python checks each condition in order and stops at the first true one. That is why score >= 80 works without also checking score < 90: if we reached that line at all, the first test must have failed.

Order matters enormously. This version is broken:

score = 95

if score >= 60:
    grade = "D"          # always wins, because it is checked first
elif score >= 90:
    grade = "A"          # unreachable
else:
    grade = "F"

print(f"Score {score} earns a {grade}. Which is wrong.")

Score 95 earns a D. Which is wrong.

Nesting

logged_in = True
is_admin = False

if logged_in:
    print("Welcome back.")
    if is_admin:
        print("Admin console available.")
    else:
        print("Standard account.")
else:
    print("Please sign in.")

Welcome back.
Standard account.

Each level of nesting is another four spaces. It works, and past two levels deep it becomes hard to read. When you find yourself at three or four, that is a signal to restructure, usually by pulling a piece out into a function (Lesson 17) or by returning early.

The one-line version

age = 20
status = "adult" if age >= 18 else "minor"
print(status)

crew = []
print(f"Crew: {len(crew) if crew else 'nobody yet'}")

adult
Crew: nobody yet

This is a conditional expression, and reads value-first: "adult, if age is 18 or more, otherwise minor". Lovely for short choices, unreadable for long ones. If it does not fit comfortably on one line, use a normal if.

match: the modern multi-way branch

Python 3.10 added match, which is tidier when you are comparing one value against many possibilities:

command = "north"

match command:
    case "north" | "n":
        print("You walk north. The jungle thickens.")
    case "south" | "s":
        print("You walk south. The beach is behind you.")
    case "look":
        print("Trees. So many trees.")
    case _:
        print(f"I do not know how to '{command}'.")

You walk north. The jungle thickens.

The _ case is the catch-all, like else. match can do far more than this (it can pull apart lists and objects), and Lesson 33 returns to it. For simple choices, either style is fine; match shines when there are many.

age = 19

if age < 18:
    print("Sorry, come back in a few years.")
elif age < 21:
    print("You are in, but soft drinks only.")
else:
    print("Enjoy your evening.")

You are in, but soft drinks only.

n = 15

if n % 3 == 0 and n % 5 == 0:
    print("FizzBuzz")
elif n % 3 == 0:
    print("Fizz")
elif n % 5 == 0:
    print("Buzz")
else:
    print(n)

FizzBuzz

score = 20

if score > 100:
    print("New high score!")
print("Congratulations!")

score = 20

if score > 100:
    print("New high score!")
    print("Congratulations!")
else:
    print("Not this time.")

Not this time.