Gregorian Easter

gregorian_easter_gauss.py

#!/usr/bin/env python3
"""
Sunday 05 April, 2015 - Buenos Aires, Argentina
Python 3.4
Computes the date for the Gregorian Easter by using
Gauss's formula.
"""
from datetime import date


def gauss_easter_algorithm(year):
    """Given a year (int), return the date for easter on that year.
    :return: datetime.date with the (year, month, day) for easter."""
    MARCH = 3
    APRIL = 4
    a = year % 19
    b = year % 4
    c = year % 7
    k = year // 100
    p = (13 + (8 * k)) // 25
    q = k // 4
    M = (15 - p + k - q) % 30
    N = (4 + k - q) % 7
    d = ((19 * a) + M) % 30
    e = ((2 * b) + (4 * c) + (6 * d) + N) % 7
    if (d + e) < 10:
        easter = (d + e + 22, MARCH)
    else:
        easter = (d + e - 9, APRIL)
    # exceptions
    if easter == (26, APRIL):
        easter = (19, APRIL)
    if easter == (25, APRIL) and d == 28 and e == 6 and a > 10:
        easter = (18, APRIL)
    return date(year, *easter[::-1])


def interactive():
    ready = False
    while not ready:
        year = input("Enter the year to calculate easter's date (YYYY): ")
        try:
            if not year.isdigit() or len(year) != 4:
                raise ValueError("Invalid year or format: {}".format(year))
            year = int(year)
        except ValueError as ex:
            print(str(ex))
        else:
            result = gauss_easter_algorithm(year)
            easter = result.strftime("%d, %B")
            print("For {year} Easter is on: {easter}".format(year=year,
                                                             easter=easter))
            retry = input("Try again? (y/n): ")
            if retry.lower() != 'y':
                ready = True


if __name__ == '__main__':
    interactive()