Conditional Statements

So far, every program ran the same lines in the same order. Conditional statements let a program make decisions: do this if something is true, otherwise do that.

if

temperature = 31
 
if temperature > 30:
    print("It's hot outside.")

The pieces:

• if is followed by a condition, an expression that is True or False.
• A colon : ends the line.
• The indented lines underneath only run when the condition is true.

else

else runs when the condition is false:

age = 16
 
if age >= 18:
    print("You can vote.")
else:
    print("Too young to vote.")

elif

For more than two outcomes, add elif ("else if"). Python checks from top to bottom and runs the first branch that is true, then skips the rest:

score = 85
 
if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
else:
    print("Keep practicing")

Output:

B

Notice that score >= 80 doesn't need to say score < 90. If the score were 95, the first branch would already have run.

Combining conditions

Use and, or, and not from the operators lesson:

age = 30
has_id = True
 
if age >= 18 and has_id:
    print("Welcome in.")

Nested conditions

You can put an if inside another if:

age = 20
has_ticket = True
 
if age >= 18:
    if has_ticket:
        print("Enter the show.")
    else:
        print("Buy a ticket first.")
else:
    print("Adults only.")

Nesting works, but deep nesting gets hard to read. When two conditions must both be true, and is often cleaner than nesting.

Truthiness

Python doesn't need a bool in an if. Every value counts as either truthy or falsy:

This makes some checks shorter:

name = input()
 
if name:
    print(f"Hello, {name}!")
else:
    print("You didn't type anything.")

Use bool(value) to see which way a value goes: bool("") is False, bool("0") is True.

Now try it yourself. The next step is three puzzles where your code has to make decisions.