Ternary operator

Leap
Leap in Python 
def leap_year(year):
    return not year % 400 if not year % 100 else not year % 4

A ternary operator uses a maximum of two checks to determine if a year is a leap year.

It starts by testing the outlier condition of the year being evenly divisible by 100. It does this by using the modulo operator. Also, by using the falsiness of 0, the not operator can be used instead of comparing equality to 0.

  • If the year is evenly divisible by 100, then the expression is True, and the ternary operator returns if the year is evenly divisible by 400.
  • If the year is not evenly divisible by 100, then the expression is False, and the ternary operator returns if the year is evenly divisible by 4.
year year % 100 == 0 year % 400 == 0 year % 4 == 0 is leap year
2020 False not evaluated True True
2019 False not evaluated False False
2000 True True not evaluated True
1900 True False not evaluated False

Although it uses a maximum of only two checks, the ternary operator tests an outlier condition first, making it less efficient than another approach that would first test if the year is evenly divisible by 4, which is more likely than the year being evenly divisible by 100. The ternary operator was fastest in benchmarking when the year was a leap year or was evenly divisible by 100, but those are the least likely conditions.

18th Dec 2024 · Found it useful?

Other Approaches to Leap in Python

Other ways our community solved this exercise
def leap_year(year):
    return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)
Boolean chain

Use a chain of boolean expressions. 

import datetime


def leap_year(year):
    return (datetime.datetime(year, 2, 28)
            + datetime.timedelta(days=1)).day == 29
datetime addition

Use datetime addition. 

from calendar import isleap

def leap_year(year):
    return isleap(year)
calendar.isleap() function

Use the calendar module.