Polymorphism

https://www.digitalocean.com/community/tutorials/how-to-apply-polymorphism-to-classes-in-python-3

Introduction

For object-oriented programming in Python, it means that a particular object belonging to a particular class can be used in the same way as if it were a different object belonging to a different class

What Is Polymorphism?

Polymorphism is an important feature of class definition in Python that is utilized when you have commonly named methods across classes or subclasses. This allows functions to use objects of any of these polymorphic classes without needing to be aware of distinctions across the classes.

Polymorphism can be carried out through inheritance, with subclasses making use of base class methods or overriding them.

Duck Typing

Python’s duck typing, a special case of dynamic typing, uses techniques characteristic of polymorphism. The term “duck typing” is derived from a quote of writer James Whitcomb Riley: “When I see a bird that walks like a duck and swims like a duck and quacks like a duck, I call that bird a duck.”

When several classes or subclasses have the same method names, but different implementations for these same methods, the classes are polymorphic because they are using a single interface to use with entities of different types. A function will be able to evaluate these polymorphic methods without knowing which classes are invoked.

Creating Polymorphic Classes

class Shark():
    def swim(self):
        print("The shark is swimming.")

    def swim_backwards(self):
        print("The shark cannot swim backwards, but can sink backwards.")

    def skeleton(self):
        print("The shark's skeleton is made of cartilage.")


class Clownfish():
    def swim(self):
        print("The clownfish is swimming.")

    def swim_backwards(self):
        print("The clownfish can swim backwards.")

    def skeleton(self):
        print("The clownfish's skeleton is made of bone.")

In the code above, both the Shark and Clownfish class have three methods with the same name in common. However, each of the functionalities of these methods differ for each class.

Let’s instantiate these classes into two objects:

sammy = Shark()
sammy.skeleton()

casey = Clownfish()
casey.skeleton()

When we run the program with the python polymorphic_fish.py command, we can see that each object behaves as expected:

The shark's skeleton is made of cartilage.
The clownfish's skeleton is made of bone.

Now that we have two objects that make use of a common interface, we can use the two objects in the same way regardless of their individual types.

Polymorphism with Class Methods

To show how Python can use each of these different class types in the same way, we can first create a for loop that iterates through a tuple of objects. Then we can call the methods without being concerned about which class type each object is. We will only assume that these methods actually exist in each class.

sammy = Shark()

casey = Clownfish()

for fish in (sammy, casey):
    fish.swim()
    fish.swim_backwards()
    fish.skeleton()

The output is as follows:

The shark is swimming.
The shark cannot swim backwards, but can sink backwards.
The shark's skeleton is made of cartilage.
The clownfish is swimming.
The clownfish can swim backwards.
The clownfish's skeleton is made of bone.

The for loop iterated first through the sammy instantiation of the Shark class, then the casey object of the Clownfish class, so we see the methods related to the Shark class first, then the Clownfish class.

This shows that Python is using these methods in a way without knowing or caring exactly what class type each of these objects is. That is, using these methods in a polymorph.

Polymorphism with a Function

Let’s create a function called in_the_pacific() which takes in an object we can call fish. Though we are using the name fish, any instantiated object will be able to be called into this function:

def in_the_pacific(fish):
    fish.swim()

sammy = Shark()

casey = Clownfish()

in_the_pacific(sammy)
in_the_pacific(casey)

Conclusion

By allowing different objects to leverage functions and methods in similar ways through polymorphism, making use of this Python feature provides greater flexibility and extendability of your object-oriented code.