Using class methods and attributes

Generally, we expect objects to be stateful and classes to be stateless. While typical, a stateless class is not a requirement. We can create class objects which have attributes as well as methods. A class can also have mutable attributes, in the rare cases that this is necessary.

One use for class variables is to create parameters that apply to all instances of the class. When a name is not resolved by the object instance, the class is searched next. Here is a small hierarchy of classes that rely on a class-level attribute:

class Units(float):
    units= None
    def __repr__(self):
        text = super().__repr__()
        return "{0} {1}".format(text, self.units)

class Height(Units):
    units= "inches"

The Units class definition extends the float class. It introduces a class-level attribute named units. It overrides the __repr__() special method of float. This method uses the superclass __repr__() method to get the essential text...