Leap Year Program in Python

Problem statement

Decide whether a given year is a leap year. The rule has three parts: divisible by 4 is a leap, unless it is divisible by 100, unless it is also divisible by 400.

Approach 1: simple loop

Write the rule as nested if statements, one layer per clause. This mirrors the calendar rule exactly, which makes it easy to check line by line.

year = 2024

if year % 4 == 0:
    if year % 100 == 0:
        if year % 400 == 0:
            print("Leap year")
        else:
            print("Not a leap year")
    else:
        print("Leap year")
else:
    print("Not a leap year")
# Output: Leap year

Three questions in order: does 4 divide it? does 100 divide it? does 400 divide it? Each answer sends you one level deeper or straight to the output.

Approach 2: reusable function

The same rule compresses into one boolean expression. Putting it in a function keeps the formula testable — and it can be checked against known years in one line each.

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

for y in [1900, 2000, 2024, 2023]:
    print(y, is_leap(y))
# Output:
# 1900 False
# 2000 True
# 2024 True
# 2023 False

Read the expression as the spoken rule: "divisible by 4, and either not divisible by 100 or divisible by 400." The parentheses are not decoration — they group the exception and its exception.

Approach 3: Pythonic alternative

The standard library already contains the rule — calendar.isleap() — and using it means the calendar authority, not your typing, is on the line.

import calendar

print(calendar.isleap(2024))   # True
print(calendar.isleap(1900))   # False

Know the manual version for exams and interviews; use this one in real projects.

Dry run

Trace 1900, the famous trap year. Divisible by 4? 1900 % 4 == 0 → yes, continue. Divisible by 100? 1900 % 100 == 0 → yes, continue deeper. Divisible by 400? 1900 % 400 is 300, not 0 → Not a leap year. Now trace 2000: the first two answers are the same, but 2000 % 400 == 0 → leap. The 4-100-400 ladder explains both results.

Common errors

Stopping at the divisible-by-4 test — that alone calls 1900 a leap year, which it is not.
Checking 400 before 100 in the nested version and scrambling the exception logic.
Using = instead of == in the conditions.
Forgetting the parentheses in the boolean form, which changes the grouping and the answer.
Testing with only modern years — the bug lives in century years like 1700, 1800, 1900, 2100.

Practice on PyDebug

Turn the idea into a debugging habit with these free browser exercises: