One of the major benefits of object oriented programming is reuse of code and one of the ways this is achieved is through the inheritance mechanism. Inheritance can be best imagined as implementing a type and subtype relationship between classes.

Suppose you want to write a program which has to keep track of the teachers and students in a college. They have some common characteristics such as name, age and address. They also have specific characteristics such as salary, courses and leaves for teachers and, marks and fees for students.

You can create two independent classes for each type and process them but adding a new common characteristic would mean adding to both of these independent classes. This quickly becomes unwieldy.

A better way would be to create a common class called SchoolMember and then have the teacher and student classes inherit from this class i.e. they will become sub-types of this type (class) and then we can add specific characteristics to these sub-types.

There are many advantages to this approach. If we add/change any functionality in SchoolMember, this is automatically reflected in the subtypes as well. For example, you can add a new ID card field for both teachers and students by simply adding it to the SchoolMember class. However, changes in the subtypes do not affect other subtypes. Another advantage is that if you can refer to a teacher or student object as a SchoolMember object which could be useful in some situations such as counting of the number of school members. This is called polymorphism where a sub-type can be substituted in any situation where a parent type is expected i.e. the object can be treated as an instance of the parent class.

Also observe that we reuse the code of the parent class and we do not need to repeat it in the different classes as we would have had to in case we had used independent classes.

The SchoolMember class in this situation is known as the base class or the superclass. The Teacher and Student classes are called the derived classes or subclasses.

We will now see this example as a program.

				
#!/usr/bin/python
# Filename: inherit.py

class SchoolMember:
	'''Represents any school member.'''
	def __init__(self, name, age):
		self.name = name
		self.age = age
		print '(Initialized SchoolMember: %s)' % self.name
	
	def tell(self):
		'''Tell my details.'''
		print 'Name:"%s" Age:"%s"' % (self.name, self.age),

class Teacher(SchoolMember):
	'''Represents a teacher.'''
	def __init__(self, name, age, salary):
		SchoolMember.__init__(self, name, age)
		self.salary = salary
		print '(Initialized Teacher: %s)' % self.name

	def tell(self):
		SchoolMember.tell(self)
		print 'Salary: "%d"' % self.salary

class Student(SchoolMember):
	'''Represents a student.'''
	def __init__(self, name, age, marks):
		SchoolMember.__init__(self, name, age)
		self.marks = marks
		print '(Initialized Student: %s)' % self.name
	
	def tell(self):
		SchoolMember.tell(self)
		print 'Marks: "%d"' % self.marks

t = Teacher('Mrs. Shrividya', 40, 30000)
s = Student('Swaroop', 22, 75)

print # prints a blank line

members = [t, s]
for member in members:
	member.tell() # works for both Teachers and Students
				
				

				
$ python inherit.py
(Initialized SchoolMember: Mrs. Shrividya)
(Initialized Teacher: Mrs. Shrividya)
(Initialized SchoolMember: Swaroop)
(Initialized Student: Swaroop)

Name:"Mrs. Shrividya" Age:"40" Salary: "30000"
Name:"Swaroop" Age:"22" Marks: "75"