Question

I’m trying to create a class that will allow me to add/multiply/divide objects of the same class together or add/multiply numeric arguments to each member of the class

So my class is for coordinates (I am aware there are great packages out there that do everything I want better than I could ever hope to on my own, but now I’m just curious).

class GpsPoint(object):
    """A class for representing gps coordinates"""
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z
    def __add__(self, other):
        return GpsPoint(self.x + other.x, self.y + other.y, self.z + other.z)
    def __radd__(self, other):
        return GpsPoint(self.x + other, self.y + other, self.z + other)
    def __str__(self):
        return "%d, %d, %d" % (self.x, self.y, self.z)

This was my original attempt. I found it worked, but only if I used a numeric argument first

>>foo = GpsPoint(1,2,3)
>>print 5 + foo
6, 7, 8
>>print foo + 5
AttributeError: 'int' object has no attribute 'x'

So, what is the pythonic way to do this, is there a pythonic way, is this just silly? I see what the philosophical problem is with using isinstance() and I know I could toss in a try except block I’m just curious how I should go about this.

Answer 1

The “Pythonic” way is to “ask forgiveness rather than permission” – that is, instead of checking the type beforehand, try to add and, if it fails, catch the exception and deal with it, like so:

class GpsPoint(object):
    """A class for representing gps coordinates"""
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z
    def __add__(self, other):
        try:
            return GpsPoint(self.x + other.x, self.y + other.y, self.z + other.z)
        except AttributeError:
            return GpsPoint(self.x + other, self.y + other, self.z + other)
    def __radd__(self, other):
        try:
            return GpsPoint(self.x + other.x, self.y + other.y, self.z + other.z)
        except AttributeError:
            return GpsPoint(self.x + other, self.y + other, self.z + other)
    def __str__(self):
        return "%d, %d, %d" % (self.x, self.y, self.z)