Python Polymorphism

The word "polymorphism" means "many forms", and in programming it refers to methods/functions/operators with the same name that can be executed on many objects or classes.

Function Polymorphism

An example of a Python function that can be used on different objects is the len() function.

String

For strings len() returns the number of characters:

x = "Hello World!"

print(len(x))

Tuple

For tuples len() returns the number of items in the tuple:

mytuple = ("apple", "banana", "cherry")

print(len(mytuple))

Dictionary

For dictionaries len() returns the number of key/value pairs in the dictionary:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}

print(len(thisdict))

Class Polymorphism

Polymorphism is often used in Class methods, where we can have multiple classes with the same method name.

For example, say we have three classes: Car, Boat, and Plane, and they all have a method called move():

class Car:
  def __init__(self, brand, model):
    self.brand = brand
    self.model = model

  def move(self):
    print("Drive!")

class Boat:
  def __init__(self, brand, model):
    self.brand = brand
    self.model = model

  def move(self):
    print("Sail!")

class Plane:
  def __init__(self, brand, model):
    self.brand = brand
    self.model = model

  def move(self):
    print("Fly!")

car1 = Car("Ford", "Mustang")       #Create a Car class
boat1 = Boat("Ibiza", "Touring 20") #Create a Boat class
plane1 = Plane("Boeing", "747")     #Create a Plane class

for x in (car1, boat1, plane1):
  x.move()

Look at the for loop at the end. Because of polymorphism we can execute the same method for all three classes.

Inheritance Class Polymorphism

What about classes with child classes with the same name? Can we use polymorphism there?

Yes. If we use the example above and make a parent class called Vehicle, and make Car, Boat, Plane child classes of Vehicle, the child classes inherits the Vehicle methods, but can override them:

class Vehicle:
  def __init__(self, brand, model):
    self.brand = brand
    self.model = model

  def move(self):
    print("Move!")

class Car(Vehicle):
  pass

class Boat(Vehicle):
  def move(self):
    print("Sail!")

class Plane(Vehicle):
  def move(self):
    print("Fly!")

car1 = Car("Ford", "Mustang") #Create a Car object
boat1 = Boat("Ibiza", "Touring 20") #Create a Boat object
plane1 = Plane("Boeing", "747") #Create a Plane object

for x in (car1, boat1, plane1):
  print(x.brand)
  print(x.model)
  x.move()

Child classes inherits the properties and methods from the parent class.

In the example above you can see that the Car class is empty, but it inherits brand, model, and move() from Vehicle.

The Boat and Plane classes also inherit brand, model, and move() from Vehicle, but they both override the move() method.

Because of polymorphism we can execute the same method for all classes.