Return a new instance from a class method-CodePudding

I've implemented a date class, which calculates the next day's date and the previous day's date. eg. if today is 3/26/2022 (MM/DD/YYYY) then my method nextday gives 3/27/2022. However, instead of returning a string, I am trying to return an instance but it's not working properly. It's calculating the day correctly but changes the original instance. This is my class:

class Date:
    """
    Assigning class arguments
    """
    min_year = 1800
    dow_jan1 = "Wednesday"
    def __init__(self,month=1,day=1,year=min_year):
        """
        Assigning instance arguments and checking the validity of dates.
        If Not valid then an Exception is raised.
        """
        self.c_month = month
        self.c_day = day
        self.c_year = year
        
        if self.c_year < self.min_year:
            raise Exception("Invalid Year")
        elif self.c_month <1 or self.c_month > 12:
            raise Exception("Invalid Month")
        elif self.c_day<1 or self.c_day > 31:
            raise Exception("Invalid Day")
        else:
            if self.c_month == 2:
                if self.year_is_leap(self.c_year):
                    if self.c_day<1 or self.c_day > 29:
                        raise Exception("Invalid Day")
                else:
                    if self.c_day<1 or self.c_day > 28:
                        raise Exception("Invalid Day")
            else:
                months_31_days = [1,3,5,7,8,10,12]
                if self.c_month in months_31_days:
                    if self.c_day<1 or self.c_day > 31:
                        raise Exception("Invalid Day")
                else:
                    if self.c_day<1 or self.c_day > 30:
                        raise Exception("Invalid Day")


    def year_is_leap(self,year=None):
        """
        Finds if a year is Leap or not
        Parameters:
            year : takes a year which is to be checked
                    however if a year is not provided then the instance argument year (self.c_year)
                    is set as default value
        """
        if year is None:
            year = self.c_year
            
        if (year % 4) == 0:
            if (year % 100) == 0:
                if (year % 400) == 0:
                    return True
                else:
                    return False
            else:
                 return True
        else:
            return False

    
    def __str__(self):
        """
        returns the date in suitable format
        eg. 2/14/1900 => February 14, 1900
        """
        months = {1:"January",2:"February",3:"March",4:"April",
                  5:"May",6:"June",7:"July",8:"August",9:"September",
                  10:"October",11:"November",12:"December"}
        return "{} {}, {}".format(months[self.c_month],self.c_day,self.c_year)
        

    def nextday(self):
        """
        Returns next date in date object
        """
        leap_year = self.year_is_leap(self.c_year)
        #print(leap_year)

        if self.c_month in (1, 3, 5, 7, 8, 10, 12):
            month_length = 31
        elif self.c_month == 2:
            if leap_year:
                month_length = 29
            else:
                month_length = 28
        else:
            month_length = 30


        if self.c_day < month_length:
            self.c_day  = 1
        else:
            self.c_day = 1
            if self.c_month == 12:
                self.c_month = 1
                self.c_year  = 1
            else:
                self.c_month  = 1
        print("The next date is [mm-dd-yyyy] %d-%d-%d." % (self.c_month, self.c_day,self.c_year))
        return self #Date(self.c_month,self.c_day,self.c_year) #Date #self.__class__()

    ```

I've tried to return `self`, `Date()`, `Date(self.c_month,self.c_day,self.c_year)` and `self.__class__()`, however none of it worked.
When I run:

firstdate = Date(1,1,Date.min_year) print(firstdate) print("The date after ", firstdate, "is", firstdate.nextday())


I am getting the output:

January 1, 1800 The next date is [mm-dd-yyyy] 1-2-1800. The date after January 2, 1800 is January 2, 1800

CodePudding user response：

The most straightforward solution will be creating a new instance of Date with same values as self (basically copy it) and work with it, not changing self:

class Date:
    # other methods here

    def nextday(self):
        """
        Returns next date in date object
        """
        new_date = Date(self.c_month, self.c_day, self.c_year)
        is_leap_year = new_date.year_is_leap(new_date.c_year)

        if new_date.c_month in {1, 3, 5, 7, 8, 10, 12}:
            month_length = 31
        elif new_date.c_month == 2:
            month_length = 28   is_leap_year
        else:
            month_length = 30


        if new_date.c_day < month_length:
            new_date.c_day  = 1
        else:
            new_date.c_day = 1
            if new_date.c_month == 12:
                new_date.c_month = 1
                new_date.c_year  = 1
            else:
                new_date.c_month  = 1
        print("The next date is [mm-dd-yyyy] %d-%d-%d." % (new_date.c_month, new_date.c_day,new_date.c_year))
        return new_date

You could also implement some convenience methods (e.g. .copy to copy the date or __format__ to print the date, but that's out of question scope)

CodePudding user response：

The problem is that you change the object attributes. For example on nextday method, you write self.c_day = 1, you are changing the object itself. Instead, if you want to return a new Date, you need to write c_day = 1 then return Date(c_month, c_day, c_year)