Leap Year

Write a function that determines whether a year is a leap year.

is_leap_year(year)

The rule: a year is a leap year if it's divisible by 4, except century years (divisible by 100), which are only leap years if they're also divisible by 400.

Examples

is_leap_year(2024)  // → True    (divisible by 4, not a century year)
is_leap_year(2023)  // → False   (not divisible by 4)
is_leap_year(1900)  // → False   (century year, not divisible by 400)
is_leap_year(2000)  // → True    (century year, divisible by 400)

Walkthrough

Thinking it through

This is the classic example of a rule that looks simple ("divisible by 4") but has a documented exception, then an exception to the exception. Turning it into a single boolean expression is good practice at combining and/or/not.

Break the rule into pieces:

1. Base rule: divisible by 4 → year % 4 == 0
2. Century exception: century years (divisible by 100) are not leap years... unless:
3. Exception to the exception: divisible by 400, in which case they are.

Put together:

def is_leap_year(year):
    return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)

Read it as: "divisible by 4, and (either it's not a century year, or it's specifically divisible by 400)". The parentheses around (year % 100 != 0 or year % 400 == 0) matter — without them, Python's operator precedence would group and before or differently, changing the meaning entirely.

Check it against the tricky cases: 1900 is divisible by 4 and by 100, but not by 400 — year % 100 != 0 is False and year % 400 == 0 is also False, so the whole or is False, and the year isn't a leap year. 2000 is divisible by 4, by 100, and by 400 — the or clause is True because of the 400 check, so it is.

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