Class inheritance 13 session - SHAN

Class Inheritance
Overriding Methods
Data Hiding

Class Inheritance
✔ Create a class by deriving it from a preexisting class by listing the
parent class in parentheses after the new class name.
✔ The child class inherits the attributes of its parent class, and you can
use those attributes as if they were defined in the child class. A child
class can also override data members and methods from the parent.
SYNTAX
class SubClassName (ParentClass1[, ParentClass2, ...]):
'Optional class documentation string'
class_suite

class Parent: # define parent class
parentAttr = 100
def __init__(self):
print "Calling parent constructor"
def parentMethod(self):
print 'Calling parent method'
def setAttr(self, attr):
Parent.parentAttr = attr
def getAttr(self):
print "Parent attribute :", Parent.parentAttr
class Child(Parent): # define child class

def childMethod(self):
print 'Calling child method'
c = Child() # instance of child
c.childMethod() # child calls its method
c.parentMethod() # calls parent's method
c.setAttr(200) # again call parent's method
c.getAttr() # again call parent's method
OUTPUT:
Calling child constructor
Calling child method
Calling parent method

class A: # define your class A
.....
class B: # define your calss B
.....
class C(A, B): # subclass of A and B
.....
✔ The issubclass(sub, sup) boolean function returns true if the given
subclass sub is indeed a subclass of the superclass sup.
✔ The isinstance(obj, Class) boolean function returns true if obj is an
instance of class Class or is an instance of a subclass of Class

Overriding Methods
✔ One reason for overriding parent's methods is because you may want
special or different functionality in your subclass.
class Parent: # define parent class
def myMethod(self):
print 'Calling parent method'
class Child(Parent): # define child class
def myMethod(self):
print 'Calling child method'
c = Child() # instance of child
c.myMethod() # child calls overridden method

Data Hiding
An object's attributes may or may not be visible outside the class
definition. For these cases, you can name attributes with a double
underscore prefix, and those attributes will not be directly visible to
outsiders
class JustCounter:
__secretCount = 0
def count(self):
self.__secretCount += 1
print self.__secretCount
counter = JustCounter()
counter.count()
counter.count()
print counter.__secretCount

OUTPUT:
1
2
Traceback (most recent call last):
File "test.py", line 12, in <module>
print counter.__secretCount
AttributeError: JustCounter instance has no attribute '__secretCount'

Python protects those members by internally changing the name to
include the class name. You can access such attributes as
object._className__attrName.
print counter._JustCounter__secretCount
OUTPUT:
1
2
2

Class inheritance 13 session - SHAN

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Class inheritance 13 session - SHAN