Making decisions

Comparisons

Comparison operators produce a boolean, True or False:

5 == 5    # True  (equal)
5 != 3    # True  (not equal)
5 < 3     # False
5 >= 5    # True
"a" < "b" # True  (alphabetical order)

Watch out: = assigns a value, == compares two values.

if

temperature = 31

if temperature > 30:
    print("It is hot.")
    print("Drink some water.")

print("This always runs.")

The line ending in a colon starts a block. Everything indented underneath (use four spaces) belongs to that block and only runs when the condition is true. Indentation is not decoration in Python: it is how Python knows where the block ends.

else and elif

score = 72

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

print(grade)  # B

Python checks each condition from the top and runs the first block whose condition is true, then skips the rest. else catches everything left over.

and, or, not

age = 20
has_ticket = True

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

if age < 13 or age > 65:
    print("Discount applies.")

if not has_ticket:
    print("Buy a ticket first.")

Python also lets you chain comparisons the way you would in maths: if 0 <= x < 10:.

Truthiness

Any value can be used as a condition. Zero, empty strings, empty lists and None count as false; almost everything else counts as true.

name = input("Name: ")
if name:
    print(f"Hi {name}")
else:
    print("You did not type anything.")

Exercises

n = int(input("Number: "))
if n % 2 == 0:
    print("even")
else:
    print("odd")

year = int(input("Year: "))
leap = (year % 4 == 0 and year % 100 != 0) \
    or year % 400 == 0
print(leap)

if score >= 60:
    print("C")
elif score >= 90:
    print("A")