Exercise - Inheritance

Inheritance, also known as subclassing, is one of the fundamental principles of OOP. You can use inheritance to define new classes that inherit the attributes and methods of other classes. OOP programmers often speak of "is-a" relationships. You might have an animal class that defines the basic characteristics and behavior of an animal. But cats and dogs are different, so you could use inheritance to define new classes named cat and dog. A cat is an animal, and a dog is an animal. In the cat and dog classes, you are free to add attributes and methods that differentiate a cat from a dog. The cat class could have a meow() method, for example, and the dog class could have a bark() method. Both cat and dog would share attributes and methods inherited from animal.

Inheritance promotes code reuse. If you want to write a class to represent missing persons, you don't have to copy and paste the code for a class that represents persons. You simply inherit from the person class and add attributes and methods that are unique to missing persons. If you make a change to the person class, also known as the base class or super class, those changes automatically propagate down to the class for missing persons.

In this unit, you will see inheritance at work by writing a new class named MissingPerson that inherits from Person, but it adds attributes and methods of its own. You also learn how to invoke methods in the base class, and how to remove inherited attributes that don't make sense in the subclass.

Create a class to represent missing persons

One characteristic that differentiates a missing person from another person is the date that the person went missing. Let's write a missing person class that includes a missing_since attribute and a get_years_missing() method that computes the number of years the person has been missing.

  1. Return to your MissingPersons.py file and enter the following class definition under the definition of Person. Feel free to remove the print statements and definition of aPerson at the bottom of the file.

    class MissingPerson(Person):
        def __init__(self, name, photo, date_of_birth, date_missing):
            # Construct the base object
            Person.__init__(self, name, photo, date_of_birth)
            # Add a missing_since attribute
            self.missing_since = date_missing
        # Add a get_years_missing() method
        def get_years_missing(self):
            return int((datetime.datetime.now() - self.missing_since).days / 365.25)

    You defined a new class (subclass) named MissingPerson that inherits from Person. When created, a MissingPerson object creates a Person object by calling the latter's __init__() method. Then, it defines an instance attribute of its own (missing_since) and a method that subtracts the date the person went missing from today's date to compute how long that person has been missing.

  2. Add the following code at the bottom of the file to create a MissingPerson object to show how long the person has been missing:

    aPerson = MissingPerson("Adam", faces.images[0], datetime.datetime(1990, 9, 16), datetime.datetime(2016, 1, 1))
    print(aPerson.name + ' has been missing for ' + str(aPerson.get_years_missing()) + ' years')

    Observe that the MissingPerson object contains the name attribute inherited from Person in addition to the get_years_missing() method it added itself. When you create a MissingPerson object, you must provide the date that the person went missing, as well as a name, a photo, and a date of birth. That data comes from the MissingPerson object's __init__() method, which serves the same purpose in an inherited class as it does in a base class.

  3. Save the file, and then run the code:

    python3 MissingPersons.py

If you want, you can use a print(dir(aPerson)) statement to list the object's methods and attributes and confirm that the list includes members defined in Person and members defined in MissingPerson.

Override an inherited method

Occasionally, it's useful to override a method that's inherited from the base class to modify the way the method works in the subclass. You have already seen how to override methods: implement a method of the same name in the subclass. But what if you need to call the base class's version of the method from the inherited class?

In South Korea, babies are considered one year old when they are born. Consequently, they turn age two on their first birthday, age three on their second birthday, and so on. Suppose you wanted to create a special version of the MissingPerson class named MissingSKPerson that adds 1 to the integer returned by the get_age() method. That's what Python's super() method is for.

  1. Add the following class definition under your other class definitions. Delete the lines that define aPerson and printing.

    class MissingSKPerson(MissingPerson):
        def __init__(self, name, photo, date_of_birth, date_missing):
            MissingPerson.__init__(self, name, photo, date_of_birth, date_missing)
        # Override the get_age() method
        def get_age(self):
            return super().get_age() + 1

    Notice that get_age() is overridden in the inherited class, but rather than duplicate the code for the base class's get_age() method and add 1 to the result, it invokes the base class's get_age() method and adds 1.

  2. Now, test your code by adding the following statements to the bottom of the file:

    date_birth = datetime.datetime(1990, 9, 16)
    date_missing = datetime.datetime(2016, 1, 1)
    face = faces.images[0]
    name = "Adam"
    aPerson = Person(name, face, date_birth)
    aPerson = MissingPerson(name, face, date_birth, date_missing)
    aPerson = MissingSKPerson(name, face, date_birth, date_missing)

    Can you predict what the output from the three print() statements will be before you run the code?

  3. Save your file, and then run the code to see if you were correct:

    python3 MissingPersons.py

Remove an inherited attribute

Suppose you defined a new class named AnonymousPerson that inherits from Person. In that case, you might want to remove the name attribute so that an anonymous person remains just that—anonymous.

You can include calls to Python's delattr() function in a class definition to remove attributes that are inherited from the base class. Here's how an AnonymousPerson class might look:

class AnonymousPerson(Person):
    def __init__(self, photo, date_of_birth):
        Person.__init__(self, '', photo, date_of_birth)
        delattr(self, 'name')

Here's how you would create an instance:

aPerson = AnonymousPerson(faces.images[0], datetime.datetime(1990, 9, 16))

The name attribute doesn't exist in the subclass. An attempt to access it on an AnonymousPerson object would generate a runtime error.

In case you wondered, you can't delete methods that are inherited from a base class. However, you can override them and change the way they work (or simply have them raise an error or do nothing).