Python Inheritance

In an object-oriented language, inheritance is a mechanism for deriving part of a class definition from another existing class definition. This allows the programmer to “share” code between classes, reducing the amount of code that must be written.

Consider the Student class we created earlier:

class Student:

    def __init__(self, first: str, last: str) -> None:
        self.__first: str = first
        self.__last: str = last
        self.__credit_points: int = 0
        self.__credit_hours: int = 0
        
    # properties for first and last omitted
    
    @property
    def gpa(self) -> float:
        """Gets the student's grade point average.
        """
        return self.__credit_points / self.__credit_hours
    
    def add_course_grade(self, grade: Grade, hours: int) -> None:
        """Records a final grade for a course taken by this student.
        
        Args
           grade: the grade earned by the student
           hours: the number of credit hours in the course
        """
        self.__credit_hours += hours
        if grade == Grade.A:
            self.__credit_points += 4 * hours
        elif grade == Grade.B:
            self.__credit_points += 3 * hours
        elif grade == Grade.C:
            self.__credit_points += 2 * hours
        elif grade == Grade.D:
            self.__credit_points += 1 * hours
        elif grade == Grade.F:
            self.__credit_points += 0 * hours

This would work well for representing a student. But what if we are representing multiple kinds of students, like undergraduate and graduate students? We’d need separate classes for each, but both would still have names and calculate their GPA the same way. So, it would be handy if we could say “an undergraduate is a student, and has all the properties and methods a student has” and “a graduate student is a student, and has all the properties and methods a student has.” This is exactly what inheritance does for us, and we often describe it as an is-a relationship. We distinguish this from the interface mechanism we looked at earlier by saying it is a strong is-a relationship, as an Undergraduate student is, for all purposes, also a Student.

Let’s define an undergraduate student class:

class UndergraduateStudent(Student):

    def __init__(self, first: str, last: str) -> None:
        super().__init__(first, last)

In Python, we list the classes that a new class is inheriting from in parentheses at the end of the class definition. So, class UndergraduateStudent(Student): indicates that UndergraduateStudent inherits from (is a) Student. Thus, it has the attributes first and last that are inherited from Student, as well as the gpa property. Similarly, it inherits the add_course_grade() method.

In fact, the only method we need to define in our UndergraduateStudent class is the constructor - and we only need to define this because the base class has a defined constructor taking two parameters, first and last names. This Student constructor must be invoked by the UndergraduateStudent constructor - that’s what the super().__init__(first, last) line does - it invokes the Student constructor with the first and last parameters passed into the UndergraduateStudent constructor. In Python, the super() method call is usually the first line in the child class’s constructor, but it doesn’t have to be. It can be omitted if the parent class includes a default (parameter-less) constructor.

Inheritance, State, and Behavior

Let’s define a GraduateStudent class as well. This will look much like an UndergraduateStudent, but all graduates have a bachelor’s degree:

class GraduateStudent(Student):
    
    def __init__(self, first: str, last: str, degree: str) -> None:
        super().__init__(first, last)
        self.__bachelor_degree = degree
        
    @property
    def bachelor_degree(self) -> str:
        return self.__bachelor_degree

Here we added a property for bachelor_degree. Since the attribute itself is meant to be a private attribute (the name begins with two underscores __), it should only be written to by the class, as is done in the constructor. To the outside world, it is treated as read-only through the getter method. Of course, in Python, nothing is truly private, so a determined developer can always access these attributes if desired.

Thus, the GraduateStudent has all the state and behavior encapsulated in Student, plus the additional state of the bachelor’s degree title.

Protected Attributes

What you might not expect is that any fields that are private in the base class are inaccessible in the derived class. This is due to the way that Python performs name mangling of names that begin with two underscores __. Thus, the private fields credit_points and credit_hours cannot be used in a method defined in GraduateStudent. This is again part of the encapsulation and data hiding ideals - we’ve encapsulated and hid those variables within the base class, and any code outside that assembly, even in a derived class, is not allowed to mess with it.

However, we often will want to allow access to such variables in a derived class. In Python, we can use a single underscore _ in front of a variable or method name to indicate that it should be treated like a protected attribute, which is only accessed by the class that defines it and any classes that inherit from that class. However, as with anything else in Python, this attribute will still be accessible to any code within our program, so it is up to developers to respect the naming scheme and not try to access those directly.

In UML, protected attributes are denoted by a hash symbol # as the visibility of the attribute.

Inheritance and Memory

What happens when we construct an instance of GraduateStudent? First, we invoke the constructor of the GraduateStudent class:

grad_student: GraduateStudent = GraduateStudent("Willie", "Wildcat", "Computer Science")

This constructor then invokes the constructor of the base class, Student, with the arguments "Willie" and "Wildcat". Thus, we allocate space to hold the state of a student, and populate it with the values set by the constructor. Finally, execution returns to the super class of GraduateStudent, which allocates the additional memory for the reference to the bachelor_degree property. Thus, the memory space of the GraduateStudent contains an instance of the Student, somewhat like nesting dolls.

Because of this, we can treat a GraduateStudent object as a Student object. For example, we can store it in a list that contains Student instances, along with UndergraduateStudent objects:

students: List[Student] = list()
students.append(grad_student)
students.append(UndergraduateStudent("Dorothy", "Gale"))

Because of their relationship through inheritance, both GraduateStudent class instances and UndergraduateStudent class instances are considered to be of type Student, as well as their supertypes.

Nested Inheritance

We can go as deep as we like with inheritance - each base type can be a superclass of another base type, and has all the state and behavior of all the inherited base classes.

This said, having too many levels of inheritance can make it difficult to reason about an object. In practice, a good guideline is to limit nested inheritance to two or three levels of depth.

Abstract Classes

If we have a base class that only exists to be inherited from (like our Student class in the example), we can mark it as abstract by inheriting from the ABC class. ABC is short for abstract base class. An abstract class cannot be instantiated (that is, we cannot create an instance of it by calling its constructor) unless all of its abstract methods have been overridden. It can still define fields and methods, but you can’t construct it. If we were to re-write our Student class as an abstract class:

from abc import ABC


class Student(ABC):
    
    def __init__(self, first: str, last: str) -> None:
        self.__first: str = first
        self.__last: str = last
        self.__credit_points: int = 0
        self.__credit_hours: int = 0
        
    # properties for first and last omitted
    
    @property
    def gpa(self) -> float:
        """Gets the student's grade point average.
        """
        return self.__credit_points / self.__credit_hours
    
    def add_course_grade(self, grade: Grade, hours: int) -> None:
        """Records a final grade for a course taken by this student.
        
        Args
           grade: the grade earned by the student
           hours: the number of credit hours in the course
        """
        self.__credit_hours += hours
        if grade == Grade.A:
            self.__credit_points += 4 * hours
        elif grade == Grade.B:
            self.__credit_points += 3 * hours
        elif grade == Grade.C:
            self.__credit_points += 2 * hours
        elif grade == Grade.D:
            self.__credit_points += 1 * hours
        elif grade == Grade.F:
            self.__credit_points += 0 * hours

Now with Student as an abstract class, attempting to create a Student instance:

the_wiz: Student = Student("Wizard", "Oz")

would still be allowed since our Student class does not define any abstract methods. However, we can add an abstract method, such as the student_type method shown below.

    @abstractmethod
    def student_type(self) -> str:
        raise NotImplementedError

If that method is placed within our Student class, we could no longer directly instantiate the class since it contains an abstract method. However, we can still create instances of the derived classes GraduateStudent and UndergraduateStudent, and treat them as Student instances, provided that they override the abstract method student_type in their code. It is best practice to make any class that serves only as a base class for derived classes and will never be created directly an abstract class.

Interfaces and Inheritance

A class can use both inheritance and interfaces. In Python, a class can inherit multiple base classes, either as interfaces or as true parent classes. They work the same way - how the class is handled really depends on the code in the class that is being inherited.

class UndergraduateStudent(Student, ITeachable, IEmailable):

For more on multiple inheritance in Python, check out the Multiple Inheritance in Python article from Real Python.