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.
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
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 Class2Note:
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
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 Class3When every class defines the same method
Python3
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 Class1The 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
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 Class1To call “m” of Class1 from both “m” of Class2 and “m” of Class3 instead of Class4 is shown below:
Python3
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 Class1The 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
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 Class1Super() 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).
Method resolution order:
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
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())
print(Class4.__mro__)
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’>)