PYTHON-Chapter 3-Classes and Object-oriented Programming: MAULIK BORSANIYA

Overview of OOP Terminology
Class − A user-defined prototype for an object that defines a set of
attributes that characterize any object of the class. The attributes
are data members (class variables and instance variables) and
methods, accessed via dot notation.
Class variable − A variable that is shared by all instances of a class.
Class variables are defined within a class but outside any of the
class's methods. Class variables are not used as frequently as
instance variables are.
Data member − A class variable or instance variable that holds data
associated with a class and its objects.
Function overloading − The assignment of more than one behavior
to a particular function. The operation performed varies by the
types of objects or arguments involved.

• Instance variable − A variable that is defined inside a method
and belongs only to the current instance of a class.
• Inheritance − The transfer of the characteristics of a class to other
classes that are derived from it.
• Instance − An individual object of a certain class. An object obj that
belongs to a class Circle, for example, is an instance of the class
Circle.
• Method − A special kind of function that is defined in a class
definition.
• Object − A unique instance of a data structure that's defined by its
class. An object comprises both data members (class variables and
instance variables) and methods.
• Operator overloading − The assignment of more than one function
to a particular operator.

#Class Constructor Function
class Employee:
#Common base class for all employees
empCount = 0
def __init__(self, name, salary):
self.name = name
self.salary = salary
Employee.empCount += 1
def displayCount(self):
print "Total Employee %d" ,Employee.empCount
def displayEmployee(self):
print "Name : ", self.name, ", Salary: ", self.salary
#Creating Instance Object
emp1 = Employee("Zara", 2000)
emp2 = Employee("Manni", 5000)
#Accessing Object
emp1.displayEmployee()
print "Total Employee %d" % Employee.empCount
The first method __init__() is a
special method, which is called
class constructor or initialization
method that Python calls when
you create a new instance of this
class.
To create instances of a class, you
call the class using class name and
pass in whatever arguments
its __init__ method accepts.
You access the object's attributes
using the dot operator with object.
Class variable would be accessed
using class name as follows −

class Employee:
'Common base class for all employees'
empCount = 0
self.name = name
print ("Total Employee %d" % Employee.empCount)
print ("Name : ", self.name, ", Salary: ", self.salary)
"This would create first object of Employee class"
emp1 = Employee("Zara", 2000)
"This would create second object of Employee class"
emp2 = Employee("Manni", 5000)

Calc Via Class
class cal():
def __init__(self,a,b):
self.a=a
self.b=b
def add(self):
return self.a+self.b
def mul(self):
return self.a*self.b
def div(self):
return self.a/self.b
def sub(self):
return self.a-self.b
a=int(input("Enter first number: "))
b=int(input("Enter second number: "))
obj=cal(a,b)
choice=1
while choice!=0:
print("0. Exit")
print("1. Add")
print("2. Subtraction")
print("3. Multiplication")
print("4. Division")
choice=int(input("Enter choice: "))
if choice==1:
print("Result: ",obj.add())
elif choice==2:
print("Result: ",obj.sub())
elif choice==3:
print("Result: ",obj.mul())
elif choice==4:
print("Result: ",round(obj.div(),2))
elif choice==0:
print("Exiting!")
else:
print("Invalid choice!!")
print()

Built-In Class Attributes
Every Python class keeps following built-in attributes and they can be
accessed using dot operator like any other attribute −
• __dict__ − Dictionary containing the class's namespace.
• __doc__ − Class documentation string or none, if undefined.
• __name__ − Class name.
• __module__ − Module name in which the class is defined. This
attribute is "__main__" in interactive mode.
• __bases__ − A possibly empty tuple containing the base classes, in
the order of their occurrence in the base class list.

class Employee:
'Common base class for all employees'
empCount = 0
self.name = name
print ("Name : ", self.name, ", Salary: ", self.salary)
print ("Employee.__doc__:", Employee.__doc__)
print ("Employee.__name__:", Employee.__name__)
print ("Employee.__module__:", Employee.__module__)
print ("Employee.__bases__:", Employee.__bases__)
print ("Employee.__dict__:", Employee.__dict__)

Destroying Objects (Garbage Collection) / Destruction
• Python deletes unneeded objects (built-in types or class
instances) automatically to free the memory space. The
process by which Python periodically reclaims blocks of
memory that no longer are in use is termed Garbage
Collection.
• Python's garbage collector runs during program execution
and is triggered when an object's reference count reaches
zero. An object's reference count changes as the number of
aliases that point to it changes.
• You normally will not notice when the garbage collector
destroys an orphaned instance and reclaims its space. But a
class can implement the special method __del__(), called a
destructor, that is invoked when the instance is about to be
destroyed. This method might be used to clean up any non
memory resources used by an instance.

class Point:
def __init__( self, x=0, y=0):
self.x = x
self.y = y
def __del__(self):
class_name = self.__class__.__name__
print (class_name, "destroyed")
pt1 = Point()
pt2 = pt1
pt3 = pt1
print (id(pt1), id(pt2), id(pt3)) # prints the ids of the obejcts
del pt1
del pt2
del pt3

Class Inheritance
• Instead of starting from scratch, you can 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
• Derived classes are declared much like their parent class; however,
a list of base classes to inherit from is given after the class name −
class SubClassName (ParentClass1[, ParentClass2, ...]):
'Optional class documentation string‘

Get And SET Attribute
Get and set attribute of objects.
The syntax of setattr() method is:
setattr(object, name, value)
setattr() Parameters
The setattr() method takes three parameters
• object - object whose attribute has to be set
• name - string which contains the name of the
attribute to be set
• value - value of the attribute to be set

class Person:
name = 'Adam'
p = Person()
print('Before modification:', p.name)
# setting name to 'John‘
setattr(p, 'name', 'John')
print('After modification:', p.name)
#Mainly GET and SET used for ….
class Person:
name = 'Adam'
p = Person()
# setting attribute name to None
setattr(p, 'name', None)
print('Name is:', p.name)
# setting an attribute not present
# in Person
setattr(p, 'age', 23)
print('Age is:', p.age)

Get Attribute
The getattr() method returns the value of the named attribute of an object. If not
found, it returns the default value provided to the function.
• The syntax of getattr() method is:
getattr(object, name[, default])
getattr() Parameters
The getattr() method takes multiple parameters:
• object - object whose named attribute's value is to be returned
• name - string that contains the attribute's name
• default (Optional) - value that is returned when the named attribute is not
found
#E.G.
class Person:
age = 23
name = "Adam“
person = Person()
# when default value is provided
print('The gender is:', getattr(person, 'gender', 'Male'))
# when no default value is provided
print('The gender is:', getattr(person, 'gender'))

#Inheritance Example
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 __init__(self):
print ("Calling child constructor")
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

Overriding Methods
- You can always override your parent class methods. One reason for
overriding parent's methods is because you may want special or
different functionality in your subclass.
• Example
class Parent: # define parent class
def myMethod(self):
print ('Calling parent method vadil')
class Child(Parent): # define child class
def myMethod(self):
print ('Calling child method 6okrav')
c = Child() # instance of child
c.myMethod() # child calls overridden method

What is MRO ?
• A class can be derived from more than one base classes in Python.
This is called multiple inheritance.
• In multiple inheritance, the features of all the base classes are
inherited into the derived class. The syntax for multiple inheritance
is similar to single inheritance.
E.g.. class Super1:
pass
class Super2:
pass
class MultiDerived(Super1, Super2):
pass
- In the multiple inheritance scenario, any specified attribute is searched
first in the current class. If not found, the search continues into parent
classes in depth-first, left-right fashion without searching same class
twice.
- MRO Stand for Method resolution Order.

• So, in the above example of MultiDerived class
the search order is [MultiDerived, Super1,
Super2, object]. This order is also called
linearization of MultiDerived class and the set
of rules used to find this order is called
Method Resolution Order (MRO).
• MRO ensures that a class always appears
before its parents and in case of multiple
parents, the order is same as tuple of base
classes.

Duck Typing
• Duck typing is a concept that says that the “type”
of the object is a matter of concern only at
runtime and you don’t need to to explicitly
mention the type of the object before you
perform nay kind of operation on that object.
• You don’t need to define the type of things like
int,float etc…its called duck typing.
• The following example can help in understanding
this concept -
• def calc(a,b):
• return a+b

Method Overloading
• Same name different arguments in method that
is called an method overloading.
• Python doesn’t support method overloading but
if we want to do then we can do as per below.
### Error
def product(a, b):
p = a * b
print(p)
def product(a, b, c):
p = a * b*c
print(p)
#product(4, 5)
product(4, 5, 5)

It’s Work If Different Arguments as a
Parameter
# Function to take multiple arguments
def add(datatype, *args):
# if datatype is int
# initialize answer as 0
if datatype =='int':
answer = 0
# if datatype is str
# initialize answer as ''
if datatype =='str':
answer =''
# Traverse through the arguments
for x in args:
# This will do addition if the
# arguments are int. Or concatenation
# if the arguments are str
answer = answer + x
print(answer)
# Integer
add('int', 5, 6)
# String
add('str', 'Hi ', 'Geeks')

Abstract Class
Abstract classes: Force a class to implement
methods.
Abstract classes can contain abstract methods:
methods without an implementation.
Objects cannot be created from an abstract class.
A subclass can implement an abstract class.
Meaning of @
This shows that the function/method/class
you're defining after a decorator is just basically
passed on as an argument to the
function/method immediately after the @ sign.

import abc
class AbstractAnimal(object):
__metaclass__ = abc.ABCMeta
@abc.abstractmethod
def walk(self):
''' data '''
@abc.abstractmethod
def talk(self):
''' data '''
class Duck(AbstractAnimal):
name = ''
def __init__(self, name):
print('duck created.')
self.name = name
def walk(self):
print('walks')
def talk(self):
print('quack')
obj = Duck('duck1')
obj.talk()
obj.walk()

Done Till Mid
Your Task
• Different between interface and abstract class ?
My Task
• Revision Of 3 Chapter
• Write the question answer of 3 chapter.
• IMP question of mid
• Example of library program

PYTHON-Chapter 3-Classes and Object-oriented Programming: MAULIK BORSANIYA

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