some magic with python

Here I am trying to discuss about some special method attributes in python.

we define a class like

>>> class myClass:
...     def __init__(self,value):
...             self.value=value
...             print 'object initialized'

And an object instantiation is done by

>>> a=myClass(10)
object initialized
>>> b=myClass(20)
object initialized

This seems usual but the magic is in evaluating the function myClass(value). an expression in the formmyClass(args) causes a call to __init__(self,args) defined inside class myClass.

let us make a new object c with its value attribute as sum of those of objects a and b.

>>> c=myClass(a.value+b.value)
object initialized
>>> c.value
30

It will be awesome if we can perform

>>> c=a+b
object initialized
>>> c.value
30

 We can make this possible by including a magic function __add__ to the class definition of myClass.

>>> class myClass:
...     def __init__(self,value):
...             self.value=value
...             print 'object initialized'
...     def __add__(self,obj):
...             c=myClass(self.value+obj.value)
...             return c

 Here c=a+b is evaluated as c=a.__add__(b)
There are more such methods enables operator overloading for arithmetic operators

Operator	Special method
-	__sub__(self, other)
//	__floordiv__(self, other)
/	__div__(self, other)
%	__mod_(self, other)
**	__pow__
<<	__lshift__(self, other)
&	__and__(self, other)
|	__or__(self, other)
^	__xor__(self, other)

There are corresponding reflected arithmetic operators too. If we use __radd__ instead of __add__ in above example, c=a+b will be evaluated as c=b.__radd__(a). We can define custom behavior for a lot of operations other than these arithmetic operators, like attribute assignments, comparisons, unary arithmetic operations like '+', '-' etc

Now let us look on some awesome special methods which enables creation of sequence types.

We have a class mySequence

>>> class mysequance:
...     def __init__(self,list):
...             self.value=list
...     def __iter__(self):
...             return iter(self.value)
...     def __getitem__(self,index):
...             return self.value[index]
...     def __len__(self):
...             return len(self.value)
... 
>>> s=mysequance([1,2,3,4,5,6])
 

Now we can perform some  list operations as follows

>>> len(s)
6
>>> s[1]
2
>>> it=iter(s)
>>> for i in it:
...     print i
... 
1
2
3
4
5
6