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

I oppose relaying on sum() with a start point, the loop hole exposed below,

In [51]: x = sum(d, MyNum(2))

In [52]: x.num
Out[52]: 47

Wondering why you got 47 while you are expecting like …start from 2nd of MyNum() while leaving first and add them till end, so the expected result = 44 (sum(range(2,10))

The truth here is that 2 is not kept as start object/position but instead treated as an addition to the result

sum(range(10)) + 2

oops, link broken !!!!!!

Use radd

Here below the correct code. Also note the below

Python calls __radd__ only when the object on the right side of the + is your class instance eg: 2 + obj1

#!/usr/bin/env python

class MyNum:
    def __init__(self,num):
        self.num=num

    def __add__(self,other):
        return MyNum(self.num+other.num)

    def __radd__(self,other):
        return MyNum(self.num+other)

    def __str__(self):
        return str(self.num)

d=[MyNum(i) for i in range(10)]
print sum(d)    ## Prints 45
d=[MyNum(i) for i in range(2, 10)]
print sum(d)    ## Prints 44
print sum(d,MyNum(2))   ## Prints 46 - adding 2 to the last value (44+2)