python: new-style and old-style class

A new-style class inherits from object (or type) where as old-style class does not. In python 3.0 old-style class is removed.

In this post I am going to explore differences between the two.

1: types

Here we can seen that type of obj is not of class but of built-in type instance.  Attribute __class__ of obj gives information about class to which it belongs.
So type and __class__ attributes differ for old-style class.

Lets see what happens with New-style class (python 3.6): 

type and __class__ attributes does not differ for new-style class.


2: MRO (Method Resolution Order)

In addition to above differences, old-style class does not support mro.

whereas new-style class does have that:

Also one can see that new-style class inherits from a object.


3: super
super() does not works with old-style classes.

Works with new-style

the reason being is that super() expect type object for class In case of old-style type(ABC) is classobj not type But in case of new-style type(NewABC) is type.

4: descriptor protocol
New-style classes have implemented descriptor protocol whereas old-style does not.


5: raising an exception
In new style objects cannot be raised as exception unless derived from Exception.

Thanks for reading.