python class special methods or magic methods

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What happens when we create an object in python class ?

class Address(object):
    def __init__(self, city, pin):
        self.city = city 
        self.pin = pin
creating instance of the object
a = Address("hyderabad", "500082")

before creating the instance of the class "__new__" method will be called. This method takes  parameter "class", "args", "kwargs" and  It will bind the data type to given class. After it will call the "__init__" method with arguments and keyword arguments.

>>> a = Address.__new__(Address)
>>> type(a)
__main__.Address
>>> a.city
AttributeError: 'Address' object has no attribute 'city'
# object created but not initialised that's the reason we get error
>>> a.__init__("hyderabad", "500082")
# now we can access the attributes
>>> a.city
'hyderabad'

we can cosider bilt-in methods of an object as magic methods. We can also override the built-in methods functionality.

list of magic methods:

Binary Operators

Operator           Method
+                       object.__add__(self, other)
-                        object.__sub__(self, other)
*                        object.__mul__(self, other)
//                       object.__floordiv__(self, other)
/                        object.__div__(self, other)
%                      object.__mod__(self, other)
**                      object.__pow__(self, other[, modulo])
<<                     object.__lshift__(self, other)
>>                     object.__rshift__(self, other)
&                       object.__and__(self, other)
^                       object.__xor__(self, other)
|                        object.__or__(self, other)

Assignment Operators:

Operator          Method
+=                     object.__iadd__(self, other)
-=                      object.__isub__(self, other)
*=                      object.__imul__(self, other)
/=                      object.__idiv__(self, other)
//=                     object.__ifloordiv__(self, other)
%=                    object.__imod__(self, other)
**=                     object.__ipow__(self, other[, modulo])
<<=                   object.__ilshift__(self, other)
>>=                   object.__irshift__(self, other)
&=                     object.__iand__(self, other)
^=                      object.__ixor__(self, other)
|=                      object.__ior__(self, other)

Unary Operators:

Operator          Method
-                       object.__neg__(self)
+                      object.__pos__(self)
abs()                object.__abs__(self)
~                      object.__invert__(self)
complex()        object.__complex__(self)
int()                  object.__int__(self)
long()               object.__long__(self)
float()               object.__float__(self)
oct()                object.__oct__(self)
hex()               object.__hex__(self)

Comparison Operators

Operator          Method
<                      object.__lt__(self, other)
<=                    object.__le__(self, other)
==                    object.__eq__(self, other)
!=                     object.__ne__(self, other)
>=                    object.__ge__(self, other)
>                      object.__gt__(self, other)

Let's take an example to override the functionality "+" [__add__] operator

class Vector(object):
    def __init__(self, *args):
        """ Create a vector, example: v = Vector(1,2) """
        if len(args) == 0:
             self.values = (0,0)
        else:
             self.values = args

    def __add__(self, other):
        """ Returns the vector addition of self and other """
        added = tuple(a + b for a, b in zip(self.values, other.values) )
        return Vector(*added)
now use the "+" operator with two vectors
>>> v1 = Vector(1, 2)
>>> v2 = Vector(10, 13)
>>> v3 = v1 + v2
>>> v3.values
(11, 15)

When statement "v3 = v1 + v2 " executes "__add__"  is called and it returns a new Vector object.

for more information please visit the python docs

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