Whenever solving a problem like this, it's always good to build it up one step at a time. You'll be tempted to jump right to the solution but it's much easier to take very small steps. It might seem extreme but the first step in a solution should do almost nothing. I tend to start by simply outputting the input of my program to make sure that at least the input is being read correctly, before trying to do any logic with it:
year = int(input("Enter a year: "))
print(year)
python leap_year.py
Now we know that the year is being read in correctly (note that since we're reading in a number, we explicitly convert it with int()).
The first step is to implement the simplest of the logical checks: is the year divisible by $4$? If it is, print out an appropriate message:
year = int(input("Enter a year: "))
if year % 4 == 0:
print(year, "is a leap year")
and check that it works for a know leap year.
python leap_year.py
For a year that we know is not a leap year, what do we get:
python leap_year.py
We get no output, so let's fix that next, by adding an else:
year = int(input("Enter a year: "))
if year % 4 == 0:
print(year, "is a leap year")
else:
print(year, "is not a leap year")
python leap_year.py
python leap_year.py
So far it's looking like it's doing the right thing. We have a working leap year calculator which covers most cases correctly.
Let's check the next corner case: what happens if the year is divisible by 100. The rules say that this should not be a leap year:
python leap_year.py
So it's currently getting this wrong. We need to add in a check to see if the year is divisible by 100 and set it to be labelled as not a leap year.
Here we need to be careful as any number which is divisible by $100$ is also divisible by $4$ but we want a special thing to happen in the former case. The divisible-by-$100$ check is more specific than the divisible-by$4$ check, so it must come first in the if/elif/else chain:
year = int(input("Enter a year: "))
if year % 100 == 0:
print(year, "is not a leap year")
elif year % 4 == 0:
print(year, "is a leap year")
else:
print(year, "is not a leap year")
python leap_year.py
python leap_year.py
python leap_year.py
The code is working for all those cases correctly, however, the last condition to think about is what happens for the year 2000:
python leap_year.py
This year should be a leap year as it is divisible by $400$. Again this is more specific than the previous two so must be added before them in the chain:
year = int(input("Enter a year: "))
if year % 400 == 0:
print(year, "is a leap year")
elif year % 100 == 0:
print(year, "is not a leap year")
elif year % 4 == 0:
print(year, "is a leap year")
else:
print(year, "is not a leap year")
python leap_year.py
python leap_year.py
python leap_year.py
python leap_year.py