Implementing sum() for custom datatype

Question

As I understand it, for it to be possible to cast sum() on an object, it must be an iterable, and must be "addable", i.e. it must implement methods __iter__ and __add__. However, when I do this for my class Point (just an example), this doesn't work.

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __add__(self, other):
        return Point(self.x + other.x, self.y + other.y)

    def __iter__(self):
        return self

    def __str__(self):
        return str((self.x, self.y))


print(Point(2, 2) + Point(1, 1))
>>> (3, 3)    # As expected; good!

points = [Point(0, 0), Point(2, 0), Point(0, 2), Point(2, 2)]
print(sum(points))    # Expect (4, 4)
>>> TypeError: unsupported operand type(s) for +: 'int' and 'Point'

---

If I implement __radd__ the same as __add__ I then get an attribute error when I try sum():

AttributeError: 'int' object has no attribute 'x'

Based on the errors my Points are somewhere being separated to just ints but I'm not sure where. Thanks for any help.

Answer 1

This happens because sum starts with a default value of int and when doing sum(points) what really is happening is sum is first trying to add 0 + Point(0, 0) and hence the error. If you check the help on sum, this will become pretty evident,

Help on built-in function sum in module builtins:

sum(iterable, start=0, /) Return the sum of a 'start' value (default: 0) plus an iterable of numbers

When the iterable is empty, return the start value.
This function is intended specifically for use with numeric values and may
reject non-numeric types.

Change the line from,

>>> print(sum(points))

to

>>> print(sum(points, Point(0, 0)))