class - how to properly overload the __add__ method in python -

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i required write class involving dates. supposed overload + operator allow days being added dates. explain how works: date object represented (2016,4,15) in format year,month, date. adding integer 10 should yield (2016,4,25). date class has values self.year,self.month,self.day

my problem code supposed work in form (date+10) (10+date). date - 1. should work in sense of adding negative number of days. date(2016,4,25) - 1 returns date(2016,4,24).

my code works in form of (date+10) not in form (10+d) or (d-1).

def __add__(self,value):     if type(self) != int , type(self) != date or (type(value) != int , type(value) != date):         raise typeerror     if type(self) == date:         day = self.day         month = self.month         year = self.year         value = value     if type(value) != int:         raise typeerror     days_to_add = value     while days_to_add > 0:         day+=1         if day == date.days_in(year,month):             month+=1             if month > 12:                 day = 0                 month = 1                 year+=1             day = 0         days_to_add -=1     return(date(year,month,day))

these errors get

typeerror: unsupported operand type(s) +: 'int' , 'date'

typeerror: unsupported operand type(s) -: 'date' , 'int'

__radd__ handles right side addition need implement well.

i seeing flaws in implementation recommend using datetime module (especially datetime.timedelta class) @ least handle basic date arithmetic correctly:

import datetime  class date(object):     def __init__(self, year, month, day):         self.year = year         self.month = month         self.day = day      def as_date(self):         return datetime.date(self.year, self.month, self.day)      def __add__(self, other):         if isinstance(other, int):             date = self.as_date() + datetime.timedelta(days=other)             return date(date.year, date.month, date.day)         else:             raise valueerror("int value required")      def __radd__(self, other):         return self.__add__(other)      def __sub__(self, other):         return self.__add__(-other)      def __rsub__(self, other):         raise runtimeerror("doesn't make sense.")      def __repr__(self):         return str(self.as_date())

demo:

>>> date = date(2015, 10, 23) >>> print date + 10 # __add__ called 2015-11-02  >>> print 20 + date # __radd__ called 2015-11-12  >>> print date - 25 # __sub__ called 2015-09-28  >>> print 25 - date # __rsub__ called  runtimeerror: doesn't make sense

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