Inheritance in Python
Inheritance is the capability of one class to derive or inherit the properties from another class. The benefits of inheritance are:
- It represents real-world relationships well.
- It provides reusability of a code. We don’t have to write the same code again and again. Also, it allows us to add more features to a class without modifying it.
- It is transitive in nature, which means that if class B inherits from another class A, then all the subclasses of B would automatically inherit from class A.
Below is a simple example of inheritance in Python
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What is object class?
Like Java Object class, in Python (from version 3.x), object is root of all classes.
In Python 3.x, “class Test(object)” and “class Test” are same.
In Python 2.x, “class Test(object)” creates a class with object as parent (called new style class) and “class Test” creates old style class (without object parent). Refer this for more details.
Subclassing (Calling constructor of parent class)
A child class needs to identify which class is its parent class. This can be done by mentioning the parent class name in the definition of the child class.
Eg: class subclass_name (superclass_name):
_ _ _
_ _ _
‘a’ is the instance created for the class Person. It invokes the __init__() of the referred class. You can see ‘object’ written in the declaration of the class Person. In Python, every class inherits from a built-in basic class called ‘object’. The constructor i.e. the ‘__init__’ function of a class is invoked when we create an object variable or an instance of the class.
The variables defined within __init__() are called as the instance variables or objects. Hence, ‘name’ and ‘idnumber’ are the objects of the class Person. Similarly, ‘salary’ and ‘post’ are the objects of the class Employee. Since the class Employee inherits from class Person, ‘name’ and ‘idnumber’ are also the objects of class Employee.
If you forget to invoke the __init__() of the parent class then its instance variables would not be available to the child class.
The following code produces an error for the same reason.
Traceback (most recent call last): File "/home/de4570cca20263ac2c4149f435dba22c.py", line 12, in print (object.name) AttributeError: 'B' object has no attribute 'name'
Different forms of Inheritance:
1. Single inheritance: When a child class inherits from only one parent class, it is called single inheritance. We saw an example above.
2. Multiple inheritance: When a child class inherits from multiple parent classes, it is called multiple inheritance.
Unlike Java and like C++, Python supports multiple inheritance. We specify all parent classes as a comma-separated list in the bracket.
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3. Multilevel inheritance: When we have a child and grandchild relationship.
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4. Hierarchical inheritance More than one derived classes are created from a single base.
5. Hybrid inheritance: This form combines more than one form of inheritance. Basically, it is a blend of more than one type of inheritance.
Private members of parent class
We don’t always want the instance variables of the parent class to be inherited by the child class i.e. we can make some of the instance variables of the parent class private, which won’t be available to the child class.
We can make an instance variable by adding double underscores before its name. For example,
File "/home/993bb61c3e76cda5bb67bd9ea05956a1.py", line 16, in print (object1.d) AttributeError: type object 'D' has no attribute 'd'
Since ‘d’ is made private by those underscores, it is not available to the child class ‘D’ and hence the error.