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:
Tuple
For tuples len() returns the number of items in the tuple:
Dictionary
For dictionaries len() returns the number of key/value pairs in the dictionary:
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():
例子
Different classes with the same method:
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()
亲自尝试 »
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:
例子
Create a class called Vehicle and make Car, Boat, Plane child classes of Vehicle:
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()
亲自尝试 »
Child classes inherit 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.
这 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.
