PythonEaster.py

#!/usr/bin/env python
iMIN_VERSION = 3    #Minimum version of the Python for this script.

import os
import sys
import datetime

##Globals for the Easter functions
iEDM_JULIAN  = 1
iEDM_ORTHODOX  = 2
iEDM_WESTERN  = 3
iFIRST_EASTER_YEAR  = 326
iFIRST_VALID_GREGORIAN_YEAR  = 1583
iLAST_VALID_GREGORIAN_YEAR  = 4099


def xRunningCorrectPythonVersion(sMinimumVersion='3'):
    '''Assumes a version number returned from the system in a format similar to: '3.2.1' .
       We can check against (major), (major and minor), or (major, minor, and micro) release levels.
       The default minimum is a major release level of '3' .
       Input of type Integer or Long is converted to type String.
       
       Return value: True or False, whether the version is sufficient.
    '''
    
    import platform
    
    #Find length of version requested.
    #   The length can be between 1 and 5, e.g. between a version like '3' and '3.2.1' .
    #Cast input as string, in case an integer has been passed.
    sMinVer = str(sMinimumVersion)
    iVerLen = len(sMinVer)     
    if (iVerLen < 1) or (iVerLen > 5):
        raise ValueError('Release levels of 1 - 5 characters in length, only, are valid: e.g. \'3\' to \'3.2.1\' .')
    
    #Find the version of Python interpreting this script
    sPythonVersion = platform.python_version()
    
    #Check major release level
    if int(sPythonVersion[:1]) < int(sMinVer[:1]):
        return False
        
    #Check minor release level
    if len(sMinVer) > 2:
        if int(sPythonVersion[2:3]) < int(sMinVer[2:3]):
            return False
    
    #Check micro release level
    if len(sMinVer) > 4:
        if int(sPythonVersion[4:5]) < int(sMinVer[4:5]):
            return False
    
    #Having passed all checks, we are OK.
    return True


def pF15_CalcDateOfEaster(iYearToFind, iDatingMethod):
    # default values for invalid arguments
    imDay = 0
    imMonth = 0
    # intermediate results (all integers)
    iFirstDig = 0
    iRemain19 = 0
    iTempNum = 0
    # tables A to E results (all integers)
    iTableA = 0
    iTableB = 0
    iTableC = 0
    iTableD = 0
    iTableE = 0
    
    #  Calculate Easter Sunday date
    # first 2 digits of year (integer division)
    iFirstDig = iYearToFind // 100
    # remainder of year / 19
    iRemain19 = iYearToFind % 19

    if (iDatingMethod == iEDM_JULIAN) or (iDatingMethod == iEDM_ORTHODOX):
        #  calculate PFM date
        iTableA = ((225 - 11 * iRemain19) % 30) + 21

        #  find the next Sunday
        iTableB = (iTableA - 19) % 7
        iTableC = (40 - iFirstDig) % 7

        iTempNum = iYearToFind % 100 
        iTableD = (iTempNum + (iTempNum // 4)) % 7

        iTableE = ((20 - iTableB - iTableC - iTableD) % 7) + 1
        imDay = iTableA + iTableE
        
        # convert Julian to Gregorian date
        if iDatingMethod == iEDM_ORTHODOX:
           # 10 days were # skipped#  in the Gregorian calendar from 5-14 Oct 1582
            iTempNum  = 10
            # Only 1 in every 4 century years are leap years in the Gregorian
            # calendar (every century is a leap year in the Julian calendar)
            if iYearToFind > 1600 :
                iTempNum = iTempNum + iFirstDig - 16 - ((iFirstDig - 16) // 4)
            
            imDay = imDay + iTempNum
    else:
        #That is iDatingMethod == iEDM_WESTERN
        #  calculate PFM date
        iTempNum = ((iFirstDig - 15) // 2) + 202 - 11 * iRemain19
        lFirstList = [21, 24, 25, 27, 28, 29, 30, 31, 32, 34, 35, 38]
        lSecondList = [33, 36, 37, 39, 40]
        if iFirstDig in lFirstList:
            iTempNum = iTempNum - 1
        elif iFirstDig in lSecondList:
            iTempNum = iTempNum - 2
            
        iTempNum = iTempNum % 30

        iTableA  = iTempNum + 21
        if iTempNum == 29 :
            iTableA = iTableA - 1
        if ((iTempNum == 28) and (iRemain19 > 10)):
            iTableA = iTableA - 1

        #  find the next Sunday
        iTableB = (iTableA - 19) % 7

        iTableC = (40 - iFirstDig) % 4
        if iTableC == 3 :
            iTableC = iTableC + 1
        if iTableC > 1 :
            iTableC = iTableC + 1

        iTempNum = iYearToFind % 100
        iTableD = (iTempNum + iTempNum // 4) % 7

        iTableE = ((20 - iTableB - iTableC - iTableD) % 7) + 1
        imDay = iTableA + iTableE

    #  return the date
    if imDay > 61 :
        imDay = imDay - 61
        imMonth = 5
        # for imMethod 2, Easter Sunday can occur in May
    elif imDay > 31 :
        imDay = imDay - 31
        imMonth = 4
    else:
        imMonth = 3

    return datetime.date(iYearToFind, imMonth, imDay)


def main(argv): 

    #Check two arguments: year and dating method, the second of which is optional
    if (len(argv) > 3) or (len(argv) < 1): 
        sys.stderr.write("USAGE: %s <year, in which the date of Easter is sought> ?<dating method; Western/Gregorian is the default>? \n" % (argv[0])) 
        return None
    elif len(argv) == 1:
        sYearToFind = datetime.date.today().strftime('%Y')
        sDatingMethod = str(iEDM_WESTERN)
    elif len(argv) == 3:
        sYearToFind = argv[1]
        sDatingMethod = argv[2]
    else:
        #That is: len(argv) == 2:
        sYearToFind = argv[1]
        sDatingMethod = None
    if sDatingMethod is None:
        sDatingMethod = str(iEDM_WESTERN)
    
    #Confirm that the arguments are valid
    try:
        iYearToFind = int(sYearToFind)
    except Exception:
        sys.stderr.write("The first argument should be the year, for which the data of Easter is sought.")
        return None
    try:
        iDatingMethod = int(sDatingMethod)
    except Exception:
        sys.stderr.write("The second argument is the dating method: 1, 2, or 3.")
        return None
    if (iDatingMethod < iEDM_JULIAN) or (iDatingMethod > iEDM_WESTERN):
        sys.stderr.write("The dating method must be 1, 2, or 3. You used: %s .\n" % sDatingMethod)
        return None
    if (iYearToFind < iFIRST_EASTER_YEAR) or (iYearToFind > iLAST_VALID_GREGORIAN_YEAR):
        sys.stderr.write("The year requested is not valid. You used: %s .\n" % sYearToFind)
        return None
    
    #Find the date of Easter
    dEasterDate = pF15_CalcDateOfEaster(iYearToFind, iDatingMethod)
    print(dEasterDate.strftime("%Y-%m-%d  -OR-  %d %B %Y")) 
    

if __name__ == "__main__":
    #Initially, check that we are running the correct version of the Python interpreter.
    if not xRunningCorrectPythonVersion(iMIN_VERSION):
        sys.exit('The minimum version of Python required is: ' + str(iMIN_VERSION) + ' .')
    #The main procedure accesses the web pages or files, based on the arguments supplied.
    sys.exit(main(sys.argv))