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Python Object-Oriented Python Advanced Objects Math

Chad Goldsworthy
Chad Goldsworthy
4,209 Points

__iadd__ changes the data type?

I saw someone else ask this, but the question was still a bit unresolved. When including the iadd method in the class, it changes the data type. For example, with the class show in this video:

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)

    def __add__(self, other):
        if "." in self.value:
            return float(self.value) + other
        return int(self.value) + other

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

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

Now, for example, if I were to use the regular add operator:

age = NumString(5)
age + 5 
print(age.__class__.__name__) # this would return "NumString"
age.value # this would return "5"

But if I were to use the in place add operator:

age += 1
print(age.__class__.__name__) # this would return "int"
age.value # this returns an AttributeError

So using the dunder iadd method changes the variables data type, is this expected? What if you didn't want it to change the data type?

2 Answers

Chris Freeman
Chris Freeman
Treehouse Moderator 68,404 Points

Good question. The behavior of __iadd__ is up to the designer. In this case, __iadd__ method, self + other is evaluated which triggers a call to __add__. The method __add__ returns an int or a float type object. This int or float is assigned to self.value, but then the self.value is returned. This is what is assigned to the left-side of the statement and where the new type comes from.

If you wanted to keep the object type as NumString then __iadd__ should return self instead of self.value:

class Numstring:
    def __iadd__(self, other):
        self.value = str(self + other)  # use str() to keep value correct
        return self

>>> age = NumString(5)
>>> age.value
>>> age += 6
>>> age
<__main__.NumString object at 0x7f630f78a860>
>>> age.value

>>> age = NumString(5)
>>> age.value
>>> age + 7
>>> age.value
>>> age += 7
>>> age.value
>>> type(age)
<class '__main__.NumString'>

Post back if you have more questions. Good luck!

Chad Goldsworthy
Chad Goldsworthy
4,209 Points

A ha ! I get it now, thank you Chris.