Implementation of class in Python

Question

The problem is to implement scalar and inner product in the vector class in Python. Here is the code:

class Point(object):
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def __repr__(self):
        return 'Point(%s, %s)' % (self.x, self.y)
    def __mul__(self,other):
        x, y = self.x*other.x, self.y*other.y
        return self.__class__(x,y)
    def __rmul__(self,other):
        x,y = other*self.x,other*self.y
        return self.__class__(x,y)
    def __add__(self,other):
        x,y = self.x + other.x, self.y + other.y
        return self.__class__(x, y)
    def __sub__(self,other):
        x,y = self.x - other.x, self.y - other.y
        return self.__class__(x, y)

With inner product it works great, but with scalar multiplication(like if I call Point(3,2)*2) it gives the following error: AttributeError: 'int' object has no attribute 'x'.

How do I fix this?

What kind of vector multiplication are you doing? A dot product between vectors is a number, not a vector. — decorator-factory
– decorator-factory, Commented Nov 30, 2019 at 9:49

decorator-factory · Accepted Answer · 2019-11-30 10:05:35Z

1

Look carefully at your __mul__ method:

    def __mul__(self,other):
        x, y = self.x*other.x, self.y*other.y
        return self.__class__(x,y)

When you're trying to multiply a Point and an int, you're passing the int as the second argument (to the other parameter) to Point.__mul__. Then your method will try to access other.x and other.y, and an int doesn't have these attributes:

You can manually check for the type of other and decide whether you should be doing scalar or vector product:

    def __mul__(self,other):
        if isinstance(other, Point):
            x, y = self.x*other.x, self.y*other.y
            return self.__class__(x,y)
        elif isinstance(other, (int, float, complex)):
            x, y = other * self.x, other * self.y
            return self.__class__(x, y)
        else:
            raise TypeError

    def __rmul__(self, other):
        return self * other

Also, the way you're doing multiplication is strange. A dot product of two vectors should be a number, not a vector. https://en.wikipedia.org/wiki/Dot_product

edited Nov 30, 2019 at 10:05

answered Nov 30, 2019 at 9:40

decorator-factory

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1 Comment

Farid Hasanov Over a year ago

Thank you, the answer is very helpful. Yes, I messed up with definition a little bit, it should be like x1*x2 +y1*y2. Thanks again :)

azro · Accepted Answer · 2019-11-30 09:59:19Z

1

You need to add a check for the type of other, it can be a Point instance or other

Here the code for __mul__

def __mul__(self, other):
    if isinstance(other, Point):
        return Point(self.x * other.x, self.y * other.y)  # Point * Point
    return Point(self.x * other, self.y * other)          # Point * othertype

def __rmul__(self, other):
    return Point(self.x * other, self.y * other)          # othertype * Point

edited Nov 30, 2019 at 9:59

answered Nov 30, 2019 at 9:40

azro

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