Python provides support for inheritance like others programming language
Inheritance is a
concept by which a class can extend the facilities of another class or multiple other
classes.
Syntax:
class BaseClassName:
Base class statements
class DerivedClassName(BaseClassName):
Derived class statements
DerivedClassName is a derived class (or sub class) inherited from BaseClassName
which is known as base class (or super class)
The name
BaseClassName must be defined in a scope containing the derived class.
The derived class will have all the attributes of its base class
Example:
class Person:
def printme(self):
print("I am in Person:printme()")
class Employee(Person): # Inherits/Extends the Person class
pass
p = Employee() # Create instance of derived class or sub class
p.printme() # prints "I am in Person:printme"
Employee class inherits Person in class
Employee(Person).
TheEmployee will inherits all of its attributes , currently
method printme
Therefore, with "p" which is an instance of derived class
can also invoke printme
A derived class overrides the base class method
If a method/attribute is defined in the
derived class and in the base class, the member in the derived class will override the one in
the base class.
Example:
class Person:
def printme(self):
print("I am in Person:printme()")
class Employee(Person):
def printme(self):
print("I am in Employee:printme()")
p = Employee() # Create instance of derived class or sub class
p.printme() # prints "I am in Employee:printme"
Method printme is defined both in derived class and base class.
When we call printme
using the derived class instance, it will completes replace (override) the method of base
class
Therefore it will print "I am in Employee:printme"
We call base class method from derived class
A derived class can extend or improve the base
class method by calling it in side another method
We can call/invoke a base class method in
many ways
Example 1
# One way of calling base class method and imprive its features
class Person:
def printme(self):
print("I am in Person:printme()")
class Employee(Person):
def printme(self):
print("I am in Employee:printme()")
Person.printme(self)
print("Ending Employee:printme()")
e = Person() # Create instance of base class
e.printme() # Call base class printme()
print("-" * 30) # Separator
p = Employee() # Create instance of derived class or sub class
p.printme() # Call derived class printme()
e.printme() class the base class printme and print me one line i.e."I am in
Person:printme()"
In derived class printme(), Person.printme(self) calls
base class printme(). Here we have added 2 more lines to print before and after the call.
So
we have improved the base class printme method somehow from 1 line to 2 line.
Like wise we
improve the base class method features in python
We can call a base class method using super() built-in function
Let us change
the previous example to use super
class Person:
def printme(self):
print("I am in Person:printme()")
class Employee(Person):
def printme(self):
print("I am in Employee:printme()")
super(Person, self) # Using super() calling base class printme()
print("Ending Employee:printme()")
e = Person() # Create instance of base class
e.printme() # Call base class printme()
print("-" * 30) # Separator
p = Employee() # Create instance of derived class or sub class
p.printme() # Call derived class printme()
Here super(Person, self) calls the printme() method in base class Person
A python class can be derived from multiple base classes
The derived class will have all the
features of the base classes that it inherits
Example:
class BaseClass1:
pass
class BaseClass2:
pass
class DerivedClass(BaseClass1, BaseClass2):
pass
Here DerivedClass class is derived from both BaseClass1 and BaseClass2
The
base classes of a derived class can be found using a special attribute of a class namely
__bases__
If we want to see the base classes of DerivedClass :
Example:
print(DerivedClass.__bases__)
__bases__ provides a tuple containing base classes of a derived class
Multilevel inheritance refers to a mechanism in Object Oriented programing where once derived
class is derived from base class and the same derived class is again derived into a new
class
Example:
class BaseClass1:
pass
class BaseClass2(BaseClass1):
pass
class DerivedClass(BaseClass2):
pass
print(DerivedClass.__bases__) # (<class '__main__.BaseClass2'>,), DerivedClass's base class is BaseClass2
print(BaseClass2.__bases__) # (<class '__main__.BaseClass2'>,), BaseClass2's base class is BaseClass1
Here DerivedClass class is derived from BaseClass1 and BaseClass1
is derived fromBaseClass2
The attributes when we trying to fetch, is first searched in the current Class.
If python does
not find the attribute, it tries to search in all its base classes in depth first and left to
right order until it finds it first occurrence.
This order is known as Method Resolution
Order in Python
Example:
class BaseClass2:
y = 20
class BaseClass1:
x = 10
class DerivedClass(BaseClass1, BaseClass2):
z = 30
d = DerivedClass() # Create a derived class object
print(d.z) # prints 30
print(d.y) # prints 20
print(d.x) # prints 10
Here DerivedClass is derived from BaseClass1 and BaseClass2print(d.z)
- When we try to fetch "z" - as per MRO python checks it in DerivedClass first and it
finds "Z" in DerivedClass, so it stops searching and prints
it.print(d.y) - "y" is not found in DerivedClass , the python
searches "y" in all its base classes. As per MRO left base class is BaseClass1, so
python searches "y" in it and stops and prints as "y" is foundprint(d.z)
- For "z" python again goes as per MRO - first DerivedClass, then BaseClass1 - not
found , then tries BaseClass2 and it is found it stops search and prints value of
"z"
If python does not find the attribute in any of its base classes it raises
AttributeError