[Tutor] special class methods

[Sean 'Shaleh' Perry]

On 05-Jun-2002 Cameron Stoner wrote:
> Hi all,
> 
> Why would you want to use the __add__ special class method in a class?  I
> know what it is soposed to do, but can't you just add two objects together. 
> I read somewhere that in Python everything is an object.  So having a special
> class method used to add objects seem to be redundant to me.  Is this true?
> 

when you define __add__ (or its brothers) the interpreter calls them when you
use the respective symbols.

so:

>>> class Foo:
...   def __init__(self, v = 5):
...     self.value = v
...   def __add__(self, other):
...     return self.value * other.value 
... 
>>> a = Foo(3)
>>> b = Foo(7)
>>> a + b
21 # not 10

yes, this is a silly example.  But you define how the class acts with the math
ops by defining the equivalent function.

A more useful example would be something like:

class Rational:
  def __init__(self, top, bottom):
    self.numerator = top
    self.denominator = bottom

  def __add__(self, other):
    # do common denominator math then add the two numbers and reduce

then you could do:

a = Rational(1,2) # 1/2
b = Rational(2,3) # 2/3

c = a + b # 3/6 + 4/6 => 7/6

or

a = Rational(1,3)
b = Rational(1,6)

c = a + b # 2/6 + 1/6 => 3/6 => 1/2