Operator Overloading Basics

[Fredrik Lundh]

Mohit Bhatt wrote:
> Hello,
> 
> I just started out with python( couple of weeks).
> 
> I have a query regarding Operator Overloading
> 
> class c1:
>             def __init__(self,value):
>                         self.data = value
>             def __add__ (self,operand2):
>                         self.data += operand2
> 
> obj1 = c1(1)
> obj1 + 10 # this works just fine

it modifies obj1, so I'm not sure I agree that it works fine.  but sure, 
it doesn't raise an exception.

> 10 + obj1 # throws exception
> 
> Exception Details
> 
> Traceback (most recent call last):
>   File "<pyshell#38>", line 1, in -toplevel-
>     10+ obj1
> 
> TypeError: unsupported operand type(s) for +: 'int' and 'instance'
> 
> Q. What do I have to do to make the following line work?
> 
>   10 + obj1

define "work".

if you want it to modify obj1 (and thus confuse the heck out of anyone 
trying to use your class), implement __radd__.

if you want to create a sane accumulator, I recommend implementing += 
assignment instead, by overloading __iadd__ instead of __add__.

</F>