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Python Object-Oriented Python Advanced Objects Math

Damar Wicaksono
Damar Wicaksono
5,562 Points

Is __iadd__ method supposed to change the data type of NumString?

Following the example in the video, adding an integer in place to an object of NumString class seems to change its data type.

Is this an expected behavior (perhaps it stems from the fact that adding the object with a number returns either float or int thus overwrite the original object)? Should we simply return self instead of self.value in the __iadd__ method? Or did I miss something?

2 Answers

I had the same question on this and I believe that is what's happening. The code given in the video is:

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) + other
        else:
            return int(self) + other

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

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

Then the example usage is:

>>> age = NumString(25)
>>> age += 1
>>> age
26

If age was still a NumString object, typing only ">>> age" into the console should have output something like "<numstring.NumString object at 0x7f4e251826d8>"

Looking at the Python documentation on iadd, using age += 1 is equivalent to age = age.iadd(1), so the return value of iadd is being assigned back to age, in this case the integer 26.

I tried a different implementation by returning self, rather than self.value. Additionally, I also converted the result of the add operation to a str, since the NumString object is expecting a string for its value attribute.

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

This seems to do what's expected:

>>> age = NumString(25)
>>> age += 1
>>> age
<numstring.NumString object at 0x7f5fe6cdb668>
>>> print(age)
26
YURI PAPINIAN
YURI PAPINIAN
11,838 Points

Great point, Jon. I face the same problem. Yes, Jon's solution works for iadd. However, it doesn't work for imul. How come?