Bugs that hide

Think about special values

Some bugs only appear on special inputs: perfect squares, years divisible by 100, numbers at exactly the limit. Random tests can miss them for a long time.

Ask: which values does this code treat as special? Then test exactly those.

Break it: Break the prime check

This should print YES if n is prime and NO if not. It's right for most numbers. Find one where it's wrong.

n = int(input())
prime = n >= 2
for d in range(2, int(n ** 0.5)):
    if n % d == 0:
        prime = False
print("YES" if prime else "NO")

Input rules: one whole number from 1 to 1,000,000.

Break it: Break the leap year

This should print Leap or Not leap. A year is a leap year if it divides by 4, except years divisible by 100, unless they're also divisible by 400.

y = int(input())
print("Leap" if y % 4 == 0 and y % 100 != 0 else "Not leap")

Input rules: one year from 1 to 9999.