python - TypeError after overriding the __add__ method

Question

I am trying to understand how __add__ works:

class MyNum:
    def __init__(self,num):
        self.num=num
    def __add__(self,other):
        return MyNum(self.num+other.num)
    def __str__(self):
        return str(self.num)

If I put them in a list

d=[MyNum(i) for i in range(10)]

this works

t=MyNum(0)
for n in d:
    t=t+n
print t

But this does not:

print sum(d)
TypeError: unsupported operand type(s) for +: 'int' and 'instance'

What am I doing wrong? How can I get the sum() to work?

My problem is how to use the sum on a list of objects that support the __add__, need to keep it as generic as possible.

Answer 1

You need to define __radd__ as well to get this to work.

__radd__ is reverse add. When Python tries to evaluate x + y it first attempts to call x.__add__(y). If this fails then it falls back to y.__radd__(x).

This allows you to override addition by only touching one class. Consider for example how Python would have to evaluate 0 + x. A call to 0.__add__(x) is attempted but int knows nothing about your class. You can't very well change the __add__ method in int, hence the need for __radd__. I suppose it is a form of dependency inversion.

As Steven pointed out, sum operates in place, but starts from 0. So the very first addition is the only one that would need to use __radd__. As a nice exercise you could check that this was the case!