PYTHON-Classes and Object-oriented Programming
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.
OOP Terminology Class:
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 behaviour
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() emp2.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 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)
"This would create first object of Employee class" emp1 =
Employee("Zara", 2000) "This would create second object of
Employee class" emp2 = Employee("Manni", 5000)
emp1.displayEmployee() emp2.displayEmployee() print ("Total Employee
%d" % Employee.empCount)
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
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 It 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 pre-existing 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 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.
What
is __MetaClass__: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()
