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Python Object-Oriented Python Advanced Objects Math

Begana Choi
Begana Choi
Courses Plus Student 13,122 Points

Still confused with self and self.value

I'm still confused with self and self.value concept.

I can understand that self.value is just an attribute of self

and self and Numstring itself.

but why don't we use self.value for add method and other methods for add?

class Numstring:
    def __init__(self, value):
        self.value = str(value)

    def __str__(self):
        return self.value

    def __int__(self):
        return int(self.value)

    def __float__(self):
        return float(self.value)

    #from here, why don't we use self.value ??
    def __add__(self, other):
        if '.' in self.value:
            return float(self) + other
        return int(self) + other

    def __radd__(self, other):
        return self + other

    def __iadd__(self, other):
        self.value = self + other
        return self.value

2 Answers

By this point in the code, we have already defined the __float__() method, so we can just convert the whole object (self) to a float. It's handier to have separate values for str versions and int/float versions rather than keep converting self.value. self is the whole object, and self.value is the value of self as a str datatype. self.value is an attribute.

Also, sorry, I didn't quite read the code when I answered earlier, I should have said that we can't use self.value because it's a str but we can't use self because it's a Numstring. We have to convert them.

Also you need to be careful when assigning values to self. So for example in the __iadd__ method, you will notice he uses self.value instead of self for assignment.

If he used self for assignment, the object would no longer be Numstring class type, but would be int or float instead!

However, if you try it though, it will still be replaced because there are still a few more errors in the __iadd__ method:

  1. the value should be converted to string before assigned to self.value or else the print function will break after you try something like +=. Alternatively, change the __str__ method to always return value after casting into string (return str(self.value))
  2. the __iadd__ method should return the object, not the object value.

The correct implementation should be more like this:

    def __iadd__(self, other):
        self.value = str( self + other)
        return self

If you look higher up in the code, you see that self.value is actually an str datatype.

def __init__(self, value):
        self.value = str(value)

If we tried to use self.value for the arithmetic methods (__add__, __mul__, __radd__ etc.), we would get an error, because it would be adding an str to an int or float. So, we convert self.value to a number datatype (int or float) so it is a valid datatype to do maths with. I hope this helps!

Begana Choi
Begana Choi
Courses Plus Student 13,122 Points

but then we need to convert them like float(self.value) not flaot(self) I'm asking difference between self and self.value :)