Python inheritance and polymorphism
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Inheritance allows programmer to create a general class first then later extend it to more specialized class. It also allows programmer to write better code.
Using inheritance you can inherit all access data fields and methods, plus you can add your own methods and fields, thus inheritance provide a way to organize code, rather than rewriting it from scratch.
In object-oriented terminology when class X extend class Y , then Y is called super class or base class
Syntax to create a subclass is:
| 123 | classSubClass(SuperClass):# data fields# instance methods |
Let take an example to illustrate the point.
| 12345678910111213141516171819202122232425262728293031 | classVehicle: def__init__(self,name,color): self.__name=name # __name is private to Vehicle classself.__color=color defgetColor(self): # getColor() function is accessible to class Car returnself.__color defsetColor(self,color): # setColor is accessible outside the class self.__color=color defgetName(self):# getName() is accessible outside the class returnself.__nameclassCar(Vehicle): def__init__(self,name,color,model):# call parent constructor to set name and color super().__init__(name,color) self.__model=model defgetDescription(self): returnself.getName()+self.__model+" in "+self.getColor()+" color"# in method getDescrition we are able to call getName(), getColor() because they are # accessible to child class through inheritancec=Car("Ford Mustang","red","GT350")print(c.getDescription())print(c.getName())# car has no method getName() but it is accessible through class Vehicle |
Expected Output:
| 12 | Ford MustangGT350 inred colorFord Mustang |
here we have created base class Vehicle and it’s subclass Car . Notice that we have not defined getName() in Car class but we are still able to access it, because class Car inherits it from Vehicle class. In the above code super() method is used to call method of the base class. Here is the how super() works
Suppose you need to call a method called get_information() in the base class from child class , you can do so using the following code.
| 1 | super().get_information() |
similarly you can call base class constructor from child class constructor using the following code.
| 1 | super().__init__() |
Multiple inheritance
Unlike languages like Java and C#, python allows multiple inheritance i.e you can inherit from multiple classes at the same time like this,
| 123 | classSubclass(SuperClass1,SuperClass2,...): # initializer # methods |
Let’s take an example:
| 123456789101112131415161718 | classMySuperClass1(): defmethod_super1(self): print("method_super1 method called")classMySuperClass2(): defmethod_super2(self): print("method_super2 method called")classChildClass(MySuperClass1,MySuperClass2): defchild_method(self): print("child method")c=ChildClass()c.method_super1()c.method_super2() |
Expected Output:
| 12 | method_super1 method calledmethod_super2 method called |
As you can see becuase ChildClass inherited MySuperClass1 , MySuperClass2 , object of ChildClass is now able to access method_super1() and method_super2() .
Overriding methods
To override a method in the base class, sub class needs to define a method of same signature. (i.e same method name and same number of parameters as method in base class).
| 12345678910111213141516171819 | classA(): def__init__(self): self.__x=1 defm1(self): print("m1 from A")classB(A): def__init__(self): self.__y=1 defm1(self): print("m1 from B")c=B()c.m1() |
Expected Output:
| 1 | m1 fromB |
Here we are overriding m1() method from the base class. Try commenting m1() method in B class and now m1() method from Base class i.e class A will run.
Expected Output:
| 1 | m1 fromA |
isinstance() function
isinstance() function is used to determine whether the object is an instance of the class or not.
Syntax: isinstance(object,class_type)
| 12345678 | >>>isinstance(1,int)True>>>isinstance(1.2,int)False>>>isinstance([1,2,3,4],list)True |
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