Q:

Access instance variable from one instance to another

I am learning python. Suppose I have my Class named Dog, and two instances named Fido and Molly. I want to change an attribute of second instance by overloading + operator with __add__ so when I type Fido+Molly
the Molly.touched attribute will be set. How is accessing from one instance to another instance's attributes deployed in python?

A:

I think you are interested in how the __add__ special method works.  In essence it works as x.__add__(y) where x and y can be instance objects.  See an example of it's implementation here.
You would need to overload __add__() in Dog to return something that updates the y.touched attribute.  Example:
class Dog(object):

    def __init__(self, touched):
        self.touched = touched

    def __add__(self, other):
        other.touched = other.touched + self.touched 
        #return None                                    # optional, explicit

fido = Dog(1)
molly = Dog(2)

fido + molly
molly.touched
# 3

