Why does a^=b throw an error when a=a^b does not?

Question

My class’ xor function is working as intended but when doing a^=b, I get ‘TypeError: unsupported operand type(s) for ^=: ‘NoneType’ and ‘Multiset.” Relevant code below:

def __xor__(self,o):
    d=Multiset() #subclass of dict
    for x in self|o: #for each item in both multisets
        m=abs((self[x] if x in self else 0) - (o[x] if x in o else 0)) #determines absolute difference of multiplicities of each element
        if m>0:
            d.add(x,m) #adds item x with multiplicity m to d
    return d

def __ixor__(self,o):
    self=self^o

I have confirmed that a^b returns a Multiset and that setting a=a^b does not throw an error. I have tried commenting out the ixor definition as well as rewriting it as follows:

def __ixor__(self,o):
    buffer=self^o
    self=buffer

At this point, I’m just about out of ideas of what the problem could be.

Asked By: Mckay Holmes

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Source

Answer 1

The implementation of __ixor__ method is wrong the assignment self = buffer cannot be correctly assigned as it is not passed by reference.

In Python you can try perform an assignment like self = buffer and so you’re actually rebinding the local variable self to a new object (in this case, the buffer object). However, this doesn’t modify the instance itself, and that’s why you’re not seeing the expected changes outside of the method.

So try to modify the attributes of the instance itself to reflect the changes made by the XOR operation. Here’s the corrected implementation:

def __ixor__(self, o):
    result = self ^ o  # Calculate the XOR result using __xor__
    self.clear()  # Clear the current instance
    self.update(result)  # Update the instance with the new result
    return self  # Return the modified instance

In this implementation, we first calculate the XOR result using the __xor__ method. Then, we clear the current instance using the clear method (assuming Multiset has this method) to remove all existing items. Finally, we update the instance with the new result using the update method.

By returning self at the end, we ensure that the method conforms to the expected behavior of an in-place operation and that the instance is properly modified.

Answered By: Franz Kurt

Why does a^=b throw an error when a=a^b does not?

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