Python OOP Programming

Question

I am new to python OOP programming. I was doing this tutorial on overloading operators from here(Scroll down to operator Overloading). I couldn't quite understand this piece of code. I hope somebody will explain this in detail. To be precise I didn't understand the how are 2 objects being added here and what are the lines

def __str__(self):
 return 'Vector (%d, %d)' % (self.a, self.b) 
def __add__(self,other):
 return Vector(self.a + other.a, self.b + other.b) 

doing here?

#!/usr/bin/python
class Vector:
 def __init__(self, a, b):
 self.a = a
 self.b = b
 def __str__(self):
 return 'Vector (%d, %d)' % (self.a, self.b)
 def __add__(self,other):
 return Vector(self.a + other.a, self.b + other.b)
v1 = Vector(2,10)
v2 = Vector(5,-2)
print v1 + v2

This generates an output Vector(7,8). How are the objects v1 and v2 being added here?

• wwii: Special Method Names
• M Leonard: In situations like these, it's always good to step through code, line by line, in the debugger. It brings to light so much more information than you thought. It basically will walk you through what each operation is doing.

Answer 1

v1 + v2 is treated as a call to v1.__add__(v2), with self == v1 and other == v2.

Answer 2

This is the Python data model and your question is answered here

Basically when v1 + v2 is performed python internally performs v1.__add__(v2)

Answer 3

This code is performing vector addition, the same way you would add two vectors on paper, it combines the corresponding components using scalar addition. You are overriding the __add__ method to tell the interpreter how addition should be performed for your class.

The code:

self.a + other.a

combines the a component of your vector class. The code:

self.b + other.b

combines the b component of your vector class using the appropriate addition function for the type of b.

Those new component values are passed to the constructor of the Vector class to return a new Vector.

The + operator will invoke the __add__ method on your class to perform the addition.