Multiple Inheritance in Python - GeeksforGeeks

22 Feb, 2022
Inheritance is the mechanism to achieve the re-usability of code as one class(child class) can derive the properties of another class(parent class). It also provides transitivity ie. if class C inherits from P then all the sub-classes of C would also inherit from P. 
Multiple Inheritance When a class is derived from more than one base class it is called multiple Inheritance. The derived class inherits all the features of the base case. 
 
Syntax:

Class Base1:
       Body of the class

Class Base2:
     Body of the class

Class Derived(Base1, Base2):
     Body of the class
In the coming section, we will see the problem faced during multiple inheritance and how to tackle it with the help of examples. 
The Diamond Problem 
It refers to an ambiguity that arises when two classes Class2 and Class3 inherit from a superclass Class1 and class Class4 inherits from both Class2 and Class3. If there is a method “m” which is an overridden method in one of Class2 and Class3 or both then the ambiguity arises which of the method “m” Class4 should inherit. 
When the method is overridden in both classes 
Python3
# Python Program to depict multiple inheritance# when method is overridden in both classes class Class1:    def m(self):        print("In Class1")       class Class2(Class1):    def m(self):        print("In Class2") class Class3(Class1):    def m(self):        print("In Class3")         class Class4(Class2, Class3):    pass       obj = Class4()obj.m()
Output:
In Class2
Note: When you call obj.m() (m on the instance of Class4) the output is In Class2. If Class4 is declared as Class4(Class3, Class2) then the output of obj.m() will be In Class3. 
When the method is overridden in one of the classes 
Python3
# Python Program to depict multiple inheritance# when method is overridden in one of the classes class Class1:    def m(self):        print("In Class1")       class Class2(Class1):    pass class Class3(Class1):    def m(self):        print("In Class3")         class Class4(Class2, Class3):    pass       obj = Class4()obj.m()
Output:
In Class3
When every class defines the same method
Python3
# Python Program to depict multiple inheritance# when every class defines the same method class Class1:    def m(self):        print("In Class1")       class Class2(Class1):    def m(self):        print("In Class2") class Class3(Class1):    def m(self):         print("In Class3")         class Class4(Class2, Class3):    def m(self):        print("In Class4")   obj = Class4()obj.m() Class2.m(obj)Class3.m(obj)Class1.m(obj)
Output:
In Class4
In Class2
In Class3
In Class1
The output of the method obj.m() in the above code is In Class4. The method “m” of Class4 is executed. To execute the method “m” of the other classes it can be done using the class names.Now, to call the method m for Class1, Class2, Class3 directly from the method “m” of the Class4 see the below example 
Python3
# Python Program to depict multiple inheritance# when we try to call the method m for Class1,# Class2, Class3 from the method m of Class4  class Class1:    def m(self):        print("In Class1")       class Class2(Class1):    def m(self):        print("In Class2") class Class3(Class1):    def m(self):        print("In Class3")         class Class4(Class2, Class3):    def m(self):        print("In Class4")          Class2.m(self)        Class3.m(self)        Class1.m(self) obj = Class4()obj.m()
Output:
In Class4
In Class2
In Class3
In Class1
To call “m” of Class1 from both “m” of Class2 and “m” of Class3 instead of Class4 is shown below:
Python3
# Python Program to depict multiple inheritance# when we try to call m of Class1 from both m of# Class2 and m of Class3 class Class1:    def m(self):        print("In Class1")       class Class2(Class1):    def m(self):        print("In Class2")        Class1.m(self) class Class3(Class1):    def m(self):        print("In Class3")        Class1.m(self)        class Class4(Class2, Class3):    def m(self):        print("In Class4")          Class2.m(self)        Class3.m(self)      obj = Class4()obj.m()
Output:
In Class4
In Class2
In Class1
In Class3
In Class1
The output of the above code has one problem associated with it, the method m of Class1 is called twice. Python provides a solution to the above problem with the help of the super() function. Let’s see how it works. 
The super Function 
Python3
# Python program to demonstrate# super() class Class1:    def m(self):        print("In Class1") class Class2(Class1):    def m(self):        print("In Class2")        super().m() class Class3(Class1):    def m(self):        print("In Class3")        super().m() class Class4(Class2, Class3):    def m(self):        print("In Class4")          super().m()      obj = Class4()obj.m()
Output:
In Class4
In Class2
In Class3
In Class1
Super() is generally used with the __init__ function when the instances are initialized. The super function comes to a conclusion, on which method to call with the help of the method resolution order (MRO). 
In Python, every class whether built-in or user-defined is derived from the object class and all the objects are instances of the class object. Hence, the object class is the base class for all the other classes.In the case of multiple inheritance, a given attribute is first searched in the current class if it’s not found then it’s searched in the parent classes. The parent classes are searched in a left-right fashion and each class is searched once.If we see the above example then the order of search for the attributes will be Derived, Base1, Base2, object. The order that is followed is known as a linearization of the class Derived and this order is found out using a set of rules called Method Resolution Order (MRO).To view the MRO of a class:  
Use the mro() method, it returns a list Eg. Class4.mro()
Use the _mro_ attribute, it returns a tuple Eg. Class4.__mro__  
Example: 
Python3
# Python program to demonstrate# super() class Class1:    def m(self):        print("In Class1") class Class2(Class1):    def m(self):        print("In Class2")        super().m() class Class3(Class1):    def m(self):        print("In Class3")        super().m() class Class4(Class2, Class3):    def m(self):        print("In Class4")          super().m()      print(Class4.mro())         #This will print listprint(Class4.__mro__)        #This will print tuple
Output:
[<class ‘__main__.Class4’>, <class ‘__main__.Class2’>, <class ‘__main__.Class3’>, <class ‘__main__.Class1’>, <class ‘object’>] (<class ‘__main__.Class4’>, <class ‘__main__.Class2’>, <class ‘__main__.Class3’>, <class ‘__main__.Class1’>, <class ‘object’>)  
anushkalaharia
trdeshmukh2012
python-inheritance
Python-OOP
Python
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