ex42_SD.txt

An offical explaination in the Python 2 documentation can be found here:
https://docs.python.org/2/reference/datamodel.html#new-style-and-classic-classes

Summary:
Effectively it unified class and type, this was in Python 2.2. When Python 3 
was made, this old type of class was removed. When a class is defineed in 
Python 3 without specifying the inherited from class, object is used by 
default. NOTE: it is bad form to not be explict that object is being used.

Other:
I would also recommend running the script files like:
python3 -i ex42.py

Doing this runs the script and then puts Python into interactive mode. 
Interactive mode will mean that your Python environment will still have all 
the classes and variables defined. For example, you will be able to access 
the variable `rover` and call methods like `type(rover) is Dog`.