A leap year is a year that is divisible by 4. The exception is the years that are divisible by 100 but not by 400. **This comprehensive guide teaches you how to check if a year is leap year in Python**. You will also learn how to find the next leap year given the current year.

## The Quick Answer

To check if a year is a leap year in Python, you can use this function:

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

Here are some example calls to the function:

print(is_leap(2000)) # --> True print(is_leap(1900)) # --> False print(is_leap(2016)) # --> True

Checking if a year is a leap year in Python is a common task in programming courses. **Thus, you should implement the logic yourself as done above.** But if you don’t need to implement the leap year functionality, use the built-in `calendar`

module’s `isleap`

function.

Here is an example:

from calendar import isleap print(isleap(2000)) print(isleap(1900)) print(isleap(2016))

This is a quick answer to the problem. Next, let’s take a look at a more detailed answer.

## What Is a Leap Year?

**You might know that a year is not exactly 365 days long**. It’s actually around 365.24 days in true length. So to compensate for this, we need to add an extra day every four years to the calendar. A year with 366 days is called a leap year.

**Definition**: A leap year is a year that is (evenly) divisible by 4. The exception is the years that are divisible by 100 but not by 400.

Leap year examples:

**2012****2016****1986**

Examples of non-leap years:

**2011****2003****1997****2009**

Exceptions:

- 1900 is
**not**a leap year because it is divisible by 4 and 100,**but not by 400**. - 2000
**is**a leap year because it is divisible by 4, 100, and 400.

Now that you understand how leap years work and why they are needed in the first place, let’s take a look at a common technique for finding a leap year.

## Modulo—The Remainder in Division

Modulo returns the remainder of an integer division. In other words, it gives you the leftovers of a “fair division”.

For example, if you have 7 slices of pizza and 4 hungry guests, you can share one slice per person, but 3 slices are leftover. You could figure out the number of leftover pieces by calculating `7 modulo 4`

, which gives `3`

.

### How to Calculate Modulo in Python?

In Python, the modulo operator is the percent sign `%`

.

For example, you can calculate `7 modulo 4`

in Python by:

7 % 4

This returns a result of 3. Try it yourself!

## How to Find Leap Year in Python Using Modulo?

To know whether a year is a leap year or not, you can use the modulo to calculate the remainder in the division.

**By definition, a leap year is evenly divisible by 4. But the exception is the years that are not divisible by 100 but are divisible by 400.**

### 1. Use If-Else Statements

Here is a way you can turn this definition into Python code:

def is_leap(year): if year % 4 == 0: if year % 100 == 0: if year % 400 == 0: return True else: return False else: return True else: return False

- Here the outer if-else checks if the year is divisible by 4. If it’s not, the year cannot be a leap year.
- The next if-else tests if the year is divisible by 100. If it is, the only way it can be a leap year is if it’s also divisible by 400. If it’s not, the year is not a leap year. And if the year is not divisible by 100, it must be a leap year (because it’s divisible by 4).

The above code certainly works, but you can simplify it by flattening down the nested if-else expressions by returning the result earlier.

### 2. Simplify the Code

Let’s reformulate the definition of a leap year into three checks in plain English:

- If a year is divisible by 400, it must be a leap year.
- If a year is not divisible by 400 but is divisible by 100, it’s not a leap year.
- If a year is not divisible by 400 or 100 but is divisible by 4, it’s a leap year.

With these in mind, you can simplify the if-else mess you saw earlier to a much simpler function that only uses if-statements:

def is_leap(year): if year % 400 == 0: return True if year % 100 == 0: return False if year % 4 == 0: return True return False

Even though this code looks much better, you can simplify it even further. As a matter of fact, you don’t even need to use if-statements at all. Instead, you can chain the remainder checks with logical operators like this:

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

Now you know how to check if the current year is a leap year in Python. Next, let’s take a look at how you can figure out the next leap year.

## Next Leap Year in Python

To calculate the next leap year in Python:

**Specify**the current year.**Get**the previous leap year.**Add**4 to the previous leap year.**Check**if the resulting year is a leap year.**If it’s not, add**4 years to the year until it is a leap year.

Here is how it looks in code:

def is_leap(year): return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0) def next_leap(year): next_possible = year - (year % 4) + 4 if is_leap(next_possible): return next_possible else: while is_leap(next_possible) == False: next_possible += 4 return next_possible

Example run:

year = int(input("Year: ")) print(f"The next leap year is {next_leap(year)}")

Output:

Year: 2021 The next leap year is 2024

## Conclusion

A leap year is a year with 366 days. It occurs every four years on years divisible by 4 (but not on years divisible by 100 **and** not 400).

You can use the modulo to figure out leap years in Python.

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

Thanks for reading. I hope you enjoyed it.

Happy coding!

## Further Reading

#### About the Author

- I'm an entrepreneur and a blogger from Finland. My goal is to make coding and tech easier for you with comprehensive guides and reviews.

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