Decisions with if, elif and else

With comparisons your program can now ask questions that Python answers with True or False. So far, though, it has only printed the answer. In this lesson it reacts to it and, like the bouncer from the basic ideas, takes one way or the other depending on the answer.

if: only when the condition is true

age = int(input("How old are you? "))

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

print("See you soon.")

The line with if consists of the word if, a condition and a colon. Everything indented below it is run by Python only when the condition is true; you already know this from functions, and here too four spaces are usual. The last line is not indented and therefore no longer belongs to it, so it runs in any case. Try it with 20 and with 15.

else: the other way

With 15, the program above only says "See you soon.", and the guest does not learn why they have to stay outside. With else you add the second way:

age = int(input("How old are you? "))

if age >= 18:
    print("Welcome!")
else:
    print("Sorry, only from 18.")

else stands at the same level as the if, has no condition of its own and also ends with a colon. Of the two blocks, Python always runs exactly one: the one under if when the condition is true, otherwise the one under else.

elif: more than two ways

A traffic light has three colours, and as in the basic ideas the program asks about one after the other. For this there is elif, short for else if:

light = "yellow"

if light == "red":
    print("Stop.")
elif light == "yellow":
    print("Slow down.")
else:
    print("Drive.")

Python checks the conditions from top to bottom and runs the block of the first one that is true. It skips all the others, and only if none holds does the block under else get its turn. Change light to "red" and to "green" and see which block runs each time.

Because the first true condition wins, the order matters. Here a temperature gets a word:

temperature = 25

if temperature >= 30:
    print("hot")
elif temperature >= 20:
    print("warm")
elif temperature >= 10:
    print("mild")
else:
    print("cold")

At 25 degrees the first condition is false and the second true, and the program prints "warm". That 25 is also at least 10 no longer plays a role, because the decision was made with the first match. That is why the second condition does not need to read temperature >= 20 and temperature < 30: anything that gets that far is already below 30. Turn the order around, and the program gets it wrong:

temperature = 35

if temperature >= 10:
    print("mild")
elif temperature >= 20:
    print("warm")
elif temperature >= 30:
    print("hot")
else:
    print("cold")

35 is at least 10, so the first condition wins straight away, and the checks for "warm" and "hot" never get a chance. With ranges you therefore start at one end, with the highest or the lowest value, and work your way to the other.

Combining instead of nesting

An if block may contain another if. The bouncer who checks age and guest list could be written like this:

age = 20
on_list = True

if age >= 18:
    if on_list:
        print("Come in!")

Because the two questions only lead to entry together, a condition with and is shorter and easier to read:

age = 20
on_list = True

if age >= 18 and on_list:
    print("Come in!")

You may also notice that if on_list: has no == True. It is not needed, because on_list already is a truth value. Nesting only pays off when each question needs an answer of its own, such as "Sorry, only from 18." for the younger ones and "You are not on the list." for the others.

Decisions in functions

A decision often sits inside a function that returns a different result depending on the case. Each way then gets its own return:

def temperature_word(temperature):
    if temperature >= 30:
        return "hot"
    elif temperature >= 20:
        return "warm"
    elif temperature >= 10:
        return "mild"
    else:
        return "cold"


print(temperature_word(25))
print(temperature_word(3))

Since return ends the function at once, after a match Python never reaches the lines below. That is why you often see the same decision written without elif and else, with the last case at the very bottom:

def describe_number(number):
    if number < 0:
        return "negative"
    if number == 0:
        return "zero"
    return "positive"


print(describe_number(-3))
print(describe_number(0))
print(describe_number(8))

Both ways of writing it are correct. What matters is that every way through the function arrives at a return, otherwise it gives None in that case.

Typical mistakes

The most common mistake is a single equals sign in the condition. = assigns and asks nothing, so Python rejects the line and suggests == itself:

answer = "yes"

if answer = "yes":
    print("Great.")

If the colon at the end of the if line is missing, Python reports SyntaxError: expected ':' and points with an arrow to the place where it belongs.

A mistake that gives no message at all is more treacherous. Anyone who wants to accept "yes" or just "y" as agreement easily writes this:

answer = "no"

if answer == "yes" or "y":
    print("You agreed.")
else:
    print("You declined.")

Although the answer is "no", the program agrees. Python reads the condition as (answer == "yes") or "y", and on the right there is no comparison, only the text "y". A text that is not empty counts as true in a condition, and so the whole condition is always true. The right way is to write each side as a comparison of its own, or to use in with a list straight away:

answer = "no"

if answer == "yes" or answer == "y":
    print("You agreed.")
else:
    print("You declined.")

if answer in ["yes", "y"]:
    print("Here too: agreed.")
else:
    print("Here too: declined.")

What is still missing

Your program can now take different ways depending on the input, but every decision is made exactly once. If someone types something quite different at a yes-or-no question, you want to ask again until a valid answer comes, and with a list of temperatures you want to print a word for each of them. For that, a part of the program has to run repeatedly, which is the job of loops, starting in part 3 of this path with Loops with for.

Takeaways

• if condition: runs the indented block below it only when the condition is true; else: gives the way for all other cases.
• With elif there can be as many ways as you like, and Python takes the first whose condition is true; the order matters.
• Two questions that only count together are combined with and instead of nesting two if statements.
• In a function each way can have its own return, and each way should arrive at one.
• The condition uses ==, not =, the line ends with a colon, and answer == "yes" or "y" is always true.