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):
                 


        

Answer 1

>>> help(sum)
Help on built-in function sum in module __builtin__:

sum(...)
    sum(sequence[, start]) -> value

    Returns the sum of a sequence of numbers (NOT strings) plus the value
    of parameter 'start' (which defaults to 0).  When the sequence is
    empty, returns start.

In other words, provide a start value:

sum(d, MyNum(0))

Edit pasted from my below comment:

sum works with a default start value of the integer zero. Your MyNum class as written does not know how to add itself to integers. To solve this you have two options. Either you can provide a start value to sum that has the same type as you class, or you can implement __radd__, which Python calls when adding values of differing types (such as when the first value in d is added to the default start value of zero).