1.4 Selection: if, elif, else

Big ideas

• if condition: runs a block when the condition is true; elif tests another condition only if the earlier ones failed; else catches everything else. Only one branch of a chain runs.
• Comparisons: == != < <= > >=. Combine with and, or, not; and binds tighter than or, so use parentheses when mixing.
• Chained comparisons read like math: 0 <= x < 10.
• Every value has a truth value: 0, 0.0, "", [] and None are false; everything else is true.
• Nested ifs work, but flat elif chains are easier to read. Put the most specific test first when ranges overlap.

Code you should know

score = int(input("Score? "))
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"
print(grade)

B

age = 20
has_id = False
print(age >= 18 and has_id)
print(age >= 18 or has_id)
print(not has_id)
print(15 <= age < 65)
print(bool(0), bool(""), bool("no"), bool([]))

False
True
True
True
False False True False

for n in range(1, 16):
    if n % 15 == 0:
        print("FizzBuzz")
    elif n % 3 == 0:
        print("Fizz")
    elif n % 5 == 0:
        print("Buzz")
    else:
        print(n)

1 2 Fizz 4 Buzz Fizz 7 8 Fizz Buzz 11 Fizz 13 14 FizzBuzz (one per line)

Worked example

if x > 5:
    print("big")
if x > 6:
    print("bigger")
else:
    print("small")

Common mistakes

• Testing score >= 70 before score >= 90 in a chain, so an A gets a C.
• Writing if x == 1 or 2:, which is always true. You need x == 1 or x == 2.
• Using = in a condition.
• Forgetting that separate ifs all run while an if/elif chain runs one branch.