How would I be able to use a /- operator on a class object when there is no actual value that I am adding or subtracting from? I would assume that the actual value is 0 from a sample code:

x = 2.5
print( x)
print(-x)
print(0-x)

>>> 2.5
>>> -2.5
>>> -2.5

However, when doing this on my class Interval on its __add__ and __radd__ method and trying to account for a nonexistent or 0? value, I get the TypeError: bad operand type for unary : 'Interval'

class Interval:
    def __init__(self, mini, maxi):
        self.mini = mini
        self.maxi = maxi
    def __add__(self, other):
        if isinstance(other,(int,float)):
            mini_sum = self.mini   other
            maxi_sum = self.maxi   other
            return Interval(mini_sum, maxi_sum)
        elif isinstance(other, Interval):
            mini_sum = self.mini   other.mini
            maxi_sum = self.maxi   other.maxi
            return Interval(mini_sum, maxi_sum)
        elif other == 0 or None:
            return Interval(self.mini, self.maxi)
        else:
            raise TypeError('Value to add must be an int, float, or Interval class object')


if __name__ == '__main__':
    x = Interval(2.5,3.0)
    print( x)


>>> TypeError: bad operand type for unary  : 'Interval'

My __add__ and __radd__ have the same code, so I've only included __add__ to keep the post shorter.

Am I handling this with the wrong method? I assumed x would use the __add__ method, but perhaps I am wrong here?

CodePudding user response：

The unary and - operators use the __pos__() and __neg__() special methods.

(I wanted to give a better example than this but I'm being called away. This should illustrate the idea. Just like using __add__(), et al, you'd want to create and return a new object with the correct value)

As an example:

class Foo:
    def __neg__(self):
        return "I'm a negative Foo!"
    def __pos__(self):
        return "I'm a positive Foo!"

>>> f = Foo()
>>>  f
I'm a positive Foo!"
>>> -f
I'm a negative Foo!"