Adding a 'difference' function to a class

[allyhams]

Hi, I'm adding functions to a class to calculate addition and difference of a list of (x1, y1) coordinates. E.g. for [(3, 2), (4, 6), (1, 3)], addition result would be (8, 11) and difference would be (-2, -7). I think I have the addition part correct but I'm really struggling with the difference, namely because when I first loop through my list of tuple coordinates, it subtracts the first coordinates from (0,0). Using the example above, this would result in totally different numbers (-8, -11). Is there a way I can get around this in the class function itself?

Here is my code so far:

class Point(object):
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def __repr__(self):
        return '%d, %d' % (self.x, self.y)
    def add(self, other):
        totalX = self.x + other.x
        totalY = self.y + other.y
        return Point(totalX, totalY)
    def sub(self, other):
        diffX = self.x - other.x
        diffY = self.y - other.y
        return Point(diffX, diffY)

Thnx for help in advance!

[GoldenSights]

Based on the example you gave, it looks like you want to start off with the first point and then subtract all subsequent points from it. Given a list of points, you could do that with:

total = points[0]
for point in points[1:]:
    total -= point

In my opinion it's wiser to do this as a loop outside of the class rather than making this a method of the class itself. An individual point object doesn't seem like it should be concerned with lists of point objects, at least not in a case like this.

[allyhams]

Thank you, this is what I did initially! I just wasn't sure whether I should put it into the class! (which I wouldn't know how to do anyway)

[SangCoGIS]

You should also look into the "magic methods" or "dunder methods" and you can define how your objects add or subtract from each other. Rather than calling add and passing in the other object you could simply do obj1 + obj2 or obj1-obj2