如何正确覆盖 __add__ 方法并在 Python 中创建新对象？

Question

所以我写了一个愚蠢的示例类：

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)

现在我想添加这个类的两个实例，我只是有点卡在我的脑海里。这两个选项中的哪一个更适合使用？

Answer 1

第一个解决方案是第二个（已评论）。在第二个中，您返回该类的一个新实例，而在第一个中，您就地修改第一个实例，这不是预期的行为。 为了说明问题：

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))