Python sum function... Does it add a hidden zero? [duplicate]

I have a class with a custom __add__ method.

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

    def __add__(self, example):
        return Example(
          x=self.x + example.x,
          y=self.y + example.y
               )

Now let's say I create two objects and add them.

example_1, example_2 = Example(4, 5), Example(2, 3)
example_summed = example_1 + example_2
print(example_summed.x)

# Outputs:

-> 6

Okay, works as expected. However in practise I have to sum a large, variable number of objects. So I want to sum them with sum function. When I do:

example_summed = sum([example_1, example_2])

I get the error:

TypeError: unsupported operand type(s) for +: 'int' and 'Example'

It's almost as if the sum function is trying to add an int into the sum. Presumably, if I'm right, it's adding a zero (addition identity operator).

So, why is this failing? Is there a hidden zero?