无法弄清楚为什么此代码未通过单个测试

Question

I'm trying to learn python more by solving the hacker rank puzzles and I can't understand why it is failing

This code is supposed to determine whether or not a year that is or above 1900 is, in fact, a leap year, the website says it passes 5 of 6 tests. For the input 2100 it's returning true instead of false.

def is_leap(year):
    leap = False

    if year%4==0:
        return True
    elif year%100==0:
        return False
    elif year%400==0:
        return True


    return leap

year = int(input())
print(is_leap(year))

I expect it to return false if the year is not divisible by 100 and return true if it is divisible by 4 or 400.

Answer 1

Let's see what happens if the year is, eg 1900, a year that was not a leap year:

Since year % 4 is 0 , we return True immediately, which is of course wrong. In fact, any number that is divisible by 100 or 400 must also be divisible by 4, meaning that we never even get to the bottom two conditions.

So the solution is to reorder your conditions from most to least specific:

if year % 400 == 0:
    return True
elif year % 100 == 0:
    return False
elif year % 4 == 0:
    return True

Your second misunderstanding has to do with return : A return statement doesn't somehow "mark" a value to be the return value at the end of the function, it immediately exits the function, returning that value. For better consistency, I would remove the leap variable, and just return False at the end of the function. Alternatively, you could assign to leap inside the conditions instead of return ing there.

Answer 2

Your if statement is not changing the variable leap .

Try changing the variable leap like below.

def is_leap(year):                                                                                                                                                          
    leap = False                                                                                                                                                            

    if year%4==0:                                                                                                                                                           
        leap = True                                                                                                                                                         
    elif year%100==0:                                                                                                                                                       
        leap = False                                                                                                                                                        
    elif year%400==0:                                                                                                                                                       
        leap = True                                                                                                                                                         


    return leap                                                                                                                                                             

year = int(input())                                                                                                                                                            
print(is_leap(year))   

Answer 3

def foo(year: int) -> bool:
    "find a year is leapyear or not"
    if year % 4 == 0 and year % 100 != 0:
        return True
    return year % 4 == 0 and year % 100 == 0 and year % 400 == 0

The function takes an integer as argument for the year parameter and returns True if the year is a leap year and False for the opposite. A year needs to be divisible by 4 and not by 100 to be a confirmed leap year. So, the conditional statement checks for that. If the condition's met it returns True else that function does not return anything and continues downward. Then, if the number is divisible by 4 and 100 and 400 as this case also means the year is a confirmed leap year, it returns True. We have done all checks and still if that's not True then certainly the years not a leap year.