The Gregorian calendar

In the Gregorian calendar a year is a leap year if it is divisible by 4 with the exceptions that years divisible by 100 are not leap years unless they are also divisible by 400. The following Python program determines if year is a leap year.

year = 1900

if not year % 400:
    is_leap_year = True
elif not year % 100:
    is_leap_year = False
elif not year % 4:
    is_leap_year = True
    is_leap_year = False

s_ly = 'is a' if is_leap_year else 'is not a'
print('{:4d} {:s} leap year'.format(year, s_ly))

Hence the output:

1900 is not a leap year