Understanding Class Inheritance in Python 3
Object-oriented programming creates reusable patterns of code to curtail redundancy in development projects. One way that object-oriented programming achieves recyclable code is through inheritance, when one subclass can leverage code from another base class.
This tutorial will go through some of the major aspects of inheritance in Python, including how parent classes and child classes work, how to override methods and attributes, how to use the
super() function, and how to make use of multiple inheritance.
What Is Inheritance?
Inheritance is when a class uses code constructed within another class. If we think of inheritance in terms of biology, we can think of a child inheriting certain traits from their parent. That is, a child can inherit a parent’s height or eye color. Children also may share the same last name with their parents.
Classes called child classes or subclasses inherit methods and variables from parent classes or base classes.
We can think of a parent class called
Parent that has class variables for
eye_color that the child class
Child will inherit from the
Child subclass is inheriting from the
Parent base class, the
Child class can reuse the code of
Parent, allowing the programmer to use fewer lines of code and decrease redundancy.
Parent or base classes create a pattern out of which child or subclasses can be based on. Parent classes allow us to create child classes through inheritance without having to write the same code over again each time. Any class can be made into a parent class, so they are each fully functional classes in their own right, rather than just templates.
Let’s say we have a general
Bank_account parent class that has
Business_account child classes. Many of the methods between personal and business accounts will be similar, such as methods to withdraw and deposit money, so those can belong to the parent class of
Business_account subclass would have methods specific to it, including perhaps a way to collect business records and forms, as well as an
Animal class may have
sleeping() methods, and a
Snake subclass may include its own specific
Let’s create a
Fish parent class that we will later use to construct types of fish as its subclasses. Each of these fish will have first names and last names in addition to characteristics.
We’ll create a new file called
fish.py and start with the
__init__() constructor method, which we’ll populate with
last_name class variables for each
Fish object or subclass.
class Fish: def __init__(self, first_name, last_name="Fish"): self.first_name = first_name self.last_name = last_name
We have initialized our
last_name variable with the string
"Fish" because we know that most fish will have this as their last name.
Let’s also add some other methods:
class Fish: def __init__(self, first_name, last_name="Fish"): self.first_name = first_name self.last_name = last_name def swim(self): print("The fish is swimming.") def swim_backwards(self): print("The fish can swim backwards.")
We have added the methods
swim_backwards() to the
Fish class, so that every subclass will also be able to make use of these methods.
Since most of the fish we’ll be creating are considered to be bony fish (as in they have a skeleton made out of bone) rather than cartilaginous fish (as in they have a skeleton made out of cartilage), we can add a few more attributes to the
class Fish: def __init__(self, first_name, last_name="Fish", skeleton="bone", eyelids=False): self.first_name = first_name self.last_name = last_name self.skeleton = skeleton self.eyelids = eyelids def swim(self): print("The fish is swimming.") def swim_backwards(self): print("The fish can swim backwards.")
Building a parent class follows the same methodology as building any other class, except we are thinking about what methods the child classes will be able to make use of once we create those.
Child or subclasses are classes that will inherit from the parent class. That means that each child class will be able to make use of the methods and variables of the parent class.
For example, a
Goldfish child class that subclasses the
Fish class will be able to make use of the
swim() method declared in
Fish without needing to declare it.
We can think of each child class as being a class of the parent class. That is, if we have a child class called
Rhombus and a parent class called
Parallelogram, we can say that a
Rhombus is a
Parallelogram, just as a
Goldfish is a
The first line of a child class looks a little different than non-child classes as you must pass the parent class into the child class as a parameter:
Trout class is a child of the
Fish class. We know this because of the inclusion of the word
Fish in parentheses.
With child classes, we can choose to add more methods, override existing parent methods, or simply accept the default parent methods with the
pass keyword, which we’ll do in this case:
... class Trout(Fish): pass
We can now create a
Trout object without having to define any additional methods.
... class Trout(Fish): pass terry = Trout("Terry") print(terry.first_name + " " + terry.last_name) print(terry.skeleton) print(terry.eyelids) terry.swim() terry.swim_backwards()
We have created a
terry that makes use of each of the methods of the
Fish class even though we did not define those methods in the
Trout child class. We only needed to pass the value of
"Terry" to the
first_name variable because all of the other variables were initialized.
When we run the program, we’ll receive the following output:
OutputTerry Fish bone False The fish is swimming. The fish can swim backwards.
Next, let’s create another child class that includes its own method. We’ll call this class
Clownfish, and its special method will permit it to live with sea anemone:
... class Clownfish(Fish): def live_with_anemone(self): print("The clownfish is coexisting with sea anemone.")
Next, let’s create a
Clownfish object to see how this works:
... casey = Clownfish("Casey") print(casey.first_name + " " + casey.last_name) casey.swim() casey.live_with_anemone()
When we run the program, we’ll receive the following output:
OutputCasey Fish The fish is swimming. The clownfish is coexisting with sea anemone.
The output shows that the
casey is able to use the
swim() as well as its child class method of
If we try to use the
live_with_anemone() method in a
Trout object, we’ll receive an error:
Outputterry.live_with_anemone() AttributeError: 'Trout' object has no attribute 'live_with_anemone'
This is because the method
live_with_anemone() belongs only to the
Clownfish child class, and not the
Fish parent class.
Child classes inherit the methods of the parent class it belongs to, so each child class can make use of those methods within programs.
Overriding Parent Methods
So far, we have looked at the child class
Trout that made use of the
pass keyword to inherit all of the parent class
Fish behaviors, and another child class
Clownfish that inherited all of the parent class behaviors and also created its own unique method that is specific to the child class. Sometimes, however, we will want to make use of some of the parent class behaviors but not all of them. When we change parent class methods we override them.
When constructing parent and child classes, it is important to keep program design in mind so that overriding does not produce unnecessary or redundant code.
We’ll create a
Shark child class of the
Fish parent class. Because we created the
Fish class with the idea that we would be creating primarily bony fish, we’ll have to make adjustments for the
Shark class that is instead a cartilaginous fish. In terms of program design, if we had more than one non-bony fish, we would most likely want to make separate classes for each of these two types of fish.
Sharks, unlike bony fish, have skeletons made of cartilage instead of bone. They also have eyelids and are unable to swim backwards. Sharks can, however, maneuver themselves backwards by sinking.
In light of this, we’ll be overriding the
__init__() constructor method and the
swim_backwards() method. We don’t need to modify the
swim() method since sharks are fish that can swim. Let’s take a look at this child class:
... class Shark(Fish): def __init__(self, first_name, last_name="Shark", skeleton="cartilage", eyelids=True): self.first_name = first_name self.last_name = last_name self.skeleton = skeleton self.eyelids = eyelids def swim_backwards(self): print("The shark cannot swim backwards, but can sink backwards.")
We have overridden the initialized parameters in the
__init__() method, so that the
last_name variable is now set equal to the string
skeleton variable is now set equal to the string
"cartilage", and the
eyelids variable is now set to the Boolean value
True. Each instance of the class can also override these parameters.
swim_backwards() now prints a different string than the one in the
Fish parent class because sharks are not able to swim backwards in the way that bony fish can.
We can now create an instance of the
Shark child class, which will still make use of the
swim() method of the
Fish parent class:
... sammy = Shark("Sammy") print(sammy.first_name + " " + sammy.last_name) sammy.swim() sammy.swim_backwards() print(sammy.eyelids) print(sammy.skeleton)
When we run this code, we’ll receive the following output:
OutputSammy Shark The fish is swimming. The shark cannot swim backwards, but can sink backwards. True cartilage
Shark child class successfully overrode the
swim_backwards() methods of the
Fish parent class, while also inheriting the
swim() method of the parent class.
When there will be a limited number of child classes that are more unique than others, overriding parent class methods can prove to be useful.
super() function, you can gain access to inherited methods that have been overwritten in a class object.
When we use the
super() function, we are calling a parent method into a child method to make use of it. For example, we may want to override one aspect of the parent method with certain functionality, but then call the rest of the original parent method to finish the method.
In a program that grades students, we may want to have a child class for
Weighted_grade that inherits from the
Grade parent class. In the child class
Weighted_grade, we may want to override a
calculate_grade() method of the parent class in order to include functionality to calculate a weighted grade, but still keep the rest of the functionality of the original class. By invoking the
super() function we would be able to achieve this.
super() function is most commonly used within the
__init__() method because that is where you will most likely need to add some uniqueness to the child class and then complete initialization from the parent.
To see how this works, let’s modify our
Trout child class. Since trout are typically freshwater fish, let’s add a
water variable to the
__init__() method and set it equal to the string
"freshwater", but then maintain the rest of the parent class’s variables and parameters:
... class Trout(Fish): def __init__(self, water = "freshwater"): self.water = water super().__init__(self) ...
We have overridden the
__init__() method in the
Trout child class, providing a different implementation of the
__init__() that is already defined by its parent class
Fish. Within the
__init__() method of our
Trout class we have explicitly invoked the
__init__() method of the
Because we have overridden the method, we no longer need to pass
first_name in as a parameter to
Trout, and if we did pass in a parameter, we would reset
freshwater instead. We will therefore initialize the
first_name by calling the variable in our object instance.
Now we can invoke the initialized variables of the parent class and also make use of the unique child variable. Let’s use this in an instance of
... terry = Trout() # Initialize first name terry.first_name = "Terry" # Use parent __init__() through super() print(terry.first_name + " " + terry.last_name) print(terry.eyelids) # Use child __init__() override print(terry.water) # Use parent swim() method terry.swim()
OutputTerry Fish False freshwater The fish is swimming.
The output shows that the object
terry of the
Trout child class is able to make use of both the child-specific
water while also being able to call the
__init__() variables of
The built-in Python function
super() allows us to utilize parent class methods even when overriding certain aspects of those methods in our child classes.
Multiple inheritance is when a class can inherit attributes and methods from more than one parent class. This can allow programs to reduce redundancy, but it can also introduce a certain amount of complexity as well as ambiguity, so it should be done with thought to overall program design.
To show how multiple inheritance works, let’s create a
Coral_reef child class than inherits from a
Coral class and a
Sea_anemone class. We can create a method in each and then use the
pass keyword in the
Coral_reef child class:
class Coral: def community(self): print("Coral lives in a community.") class Anemone: def protect_clownfish(self): print("The anemone is protecting the clownfish.") class CoralReef(Coral, Anemone): pass
Coral class has a method called
community() that prints one line, and the
Anemone class has a method called
protect_clownfish() that prints another line. Then we call both classes into the inheritance tuple. This means that
Coral is inheriting from two parent classes.
Let’s now instantiate a
... great_barrier = CoralReef() great_barrier.community() great_barrier.protect_clownfish()
great_barrier is set as a
CoralReef object, and can use the methods in both parent classes. When we run the program, we’ll see the following output:
OutputCoral lives in a community. The anemone is protecting the clownfish.
The output shows that methods from both parent classes were effectively used in the child class.
Multiple inheritance allows us to use the code from more than one parent class in a child class. If the same method is defined in multiple parent methods, the child class will use the method of the first parent declared in its tuple list.
Though it can be used effectively, multiple inheritance should be done with care so that our programs do not become ambiguous and difficult for other programmers to understand.
This tutorial went through constructing parent classes and child classes, overriding parent methods and attributes within child classes, using the
super() function, and allowing for child classes to inherit from multiple parent classes.
Inheritance in object-oriented coding can allow for adherence to the DRY (don’t repeat yourself) principle of software development, allowing for more to be done with less code and repetition. Inheritance also compels programmers to think about how they are designing the programs they are creating to ensure that code is effective and clear.