How to properly overwrite __add__ method and create new object in Python?

Question

So I wrote a stupid example class:

class Pair:

    def __init__(self, x, y):
        self._x = x
        self._y = y

    # add two objects of type Paar
    def __add__(self, other):
        new_x = self._x + other._x
        new_y = self._y + other._y
        
        # better this?
        self._x = new_x
        self._y = new_y
        return self

        # or this?
        # return Paar(new_x, new_y)

Now I want to add two instances of this class and I'm just a little stuck in my head. Which of the two options would be the preferred one to use?

Answer 1

The first solution is the second(commented) one. In the second one, you return a new instance of the class while in the first, you modify inplace the first instance, which is not the expected behaviour. To illustrate the issue:

class Pair_v1:

    def __init__(self, x, y):
        self._x = x
        self._y = y

    # add two objects of type Pair
    def __add__(self, other):
        new_x = self._x + other._x
        new_y = self._y + other._y
        return Pair_v1(new_x, new_y)


# Create two instances of Pair
pair1 = Pair_v1(2.0, 5.0)
pair2 = Pair_v1(7.0, 2.7)

# Add pair1 to pair2
pair1 += pair2

print('x = {}'.format(pair1._x))
print('y = {}'.format(pair1._y))

# Create a third instances as the sum of the two previous
pair3 = pair1 + pair2

print('In this case, pair1 IS NOT modified when creating pair3 (expected behavior).')
print('x = {}'.format(pair1._x))
print('y = {}'.format(pair1._y))
print('\n')

class Pair_v2:

    def __init__(self, x, y):
        self._x = x
        self._y = y

    # add two objects of type Pair
    def __add__(self, other):
        self._x += other._x
        self._y += other._y
        
        return self
    
# Create two instances of Pair
pair1 = Pair_v2(2.0, 5.0)
pair2 = Pair_v2(7.0, 2.7)

# Add pair1 to pair2
pair1 += pair2

print('x = {}'.format(pair1._x))
print('y = {}'.format(pair1._y))

# Create a third instances as the sum of the two previous
pair3 = pair1 + pair2

print('In this case, pair1 IS modified when creating pair3 (unexpected behavior)')
print('x = {}'.format(pair1._x))
print('y = {}'.format(pair1._y))