OOP

Abstract Class

There is a another way to make a class effectively an abstract class: put a method in it that only raises a NotImplemented or NotImplementedError exception. If any class inherits from this class, and does *not* override the method definition, then when an object is instantiated from this class and invokes the method, the NotImplemented exception will be raised. On the other hand, if a class inherits from this class, and *does* override the method, then the overriding method will be invoked and it will (presumably) yield a correct answer.

ABCMeta class

The abc module contains a metaclass called ABCMeta. Setting a class's metaclass to ABCMeta and making one of its methods virtual makes it an ABC. A virtual method is one that the ABC says must exist in child classes, but doesn't necessarily actually implement. For example, the Vehicle class may be defined as follows:

from abc import ABCMeta, abstractmethod

class Vehicle(object):
    """A vehicle for sale by Jeffco Car Dealership.


    Attributes:
        wheels: An integer representing the number of wheels the vehicle has.
        miles: The integral number of miles driven on the vehicle.
        make: The make of the vehicle as a string.
        model: The model of the vehicle as a string.
        year: The integral year the vehicle was built.
        sold_on: The date the vehicle was sold.
    """

    __metaclass__ = ABCMeta

    base_sale_price = 0

    def sale_price(self):
        """Return the sale price for this vehicle as a float amount."""
        if self.sold_on is not None:
            return 0.0  # Already sold
        return 5000.0 * self.wheels

    def purchase_price(self):
        """Return the price for which we would pay to purchase the vehicle."""
        if self.sold_on is None:
            return 0.0  # Not yet sold
        return self.base_sale_price - (.10 * self.miles)

    @abstractmethod
    def vehicle_type():
        """"Return a string representing the type of vehicle this is."""
        pass