Python programming : Classes objects

Classes And Objects Team Emertxe

Creation of Class General Format  Class is a model or plan to create the objects  Class contains,  Attributes: Represented by variables  Actions : Performed on methods  Syntax of defining the class, Syntax Example class Classname(object): """docstrings""" Attributes def __init__(self): def method1(): def method2(): class Student: """The below block defines attributes""" def __init__(self): self.name = "Ram" self.age = 21 self.marks = 89.75 """The below block defines a method""" def putdata(self): print("Name: ", self.name) print("Age: ", self.age) print("Marks: ", self.marks)

Creation of Class Program #To define the Student calss and create an Object to it. #Class Definition class Student: #Special method called constructor def __init__(self): self.name = "Ram" self.age = 21 self.marks = 75.90 #This is an instance method def putdata(self): print("Name: ", self.name) print("Age: ", self.age) print("Marks: ", self.marks) #Create an instance to the student class s = Student() #Call the method using an Object s.putdata()

The Self Variable  ‘Self’ is the default variable that contains the memory address of the instance of the current class s1 = Student()  s1 contains the memory address of the instance  This memory address is internally and by default passed to ‘self’ variable Usage-1: def __init__(self):  The ‘self’ variable is used as first parameter in the constructor Usage-2: def putdata(self):  The ‘self’ variable is used as first parameter in the instance methods

Constructor Constructor with NO parameter  Constructors are used to create and initialize the 'Instance Variables'  Constructor will be called only once i.e at the time of creating the objects  s = Student() Example def __init__(self): self.name = "Ram" self.marks = 99

Constructor Constructor with parameter Example def __init__(self, n = "", m = 0): self.name = n self.marks = m Instance-1 s = Student() Will initialize the instance variables with default parameters Instance-2 s = Student("Ram", 99) Will initialize the instance variables with parameters passed

Constructor Program #To create Student class with a constructor having more than one parameter class Student: #Constructor definition def __init__(self, n = "", m = 0): self.name = n self.marks = m #Instance method def putdata(self): print("Name: ", self.name) print("Marks: ", self.marks) #Constructor called without any parameters s = Student() s.putdata() #Constructor called with parameters s = Student("Ram", 99) s.putdata()

Types Of Variables  Instance variables  Class / Static variables

Types Of Variables Instance Variables  Variables whose separate copy is created for every instance/object  These are defined and init using the constructor with 'self' parameter  Accessing the instance variables from outside the class,  instancename.variable class Sample: def __init__(self): self.x = 10 def modify(self): self.x += 1 #Create an objects s1 = Sample() s2 = Sample() print("s1.x: ", s1.x) print("s2.x: ", s2.x) s1.modify() print("s1.x: ", s1.x) print("s2.x: ", s2.x)

Types Of Variables Class Variables  Single copy is created for all instances  Accessing class vars are possible only by 'class methods'  Accessing class vars from outside the class,  classname.variable class Sample: #Define class var here x = 10 @classmethod def modify(cls): cls.x += 1 #Create an objects s1 = Sample() s2 = Sample() print("s1.x: ", s1.x) print("s2.x: ", s2.x) s1.modify() print("s1.x: ", s1.x) print("s2.x: ", s2.x)

Namespaces Introduction  Namespace represents the memory block where names are mapped/linked to objects  Types:  Class namespace  - The names are mapped to class variables  Instance namespace  - The names are mapped to instance variables

Namespaces Class Namespace #To understand class namespace #Create the class class Student: #Create class var n = 10 #Access class var in class namespace print(Student.n) #Modify in class namespace Student.n += 1 #Access class var in class namespace print(Student.n) #Access class var in all instances s1 = Student() s2 = Student() #Access class var in instance namespace print("s1.n: ", s1.n) print("s2.n: ", s2.n) 10n Class Namespace 10n 10n Instance NamespaceInstance Namespace 11n Class Namespace 11n 11n Instance NamespaceInstance Namespace Before modifyng class variable ‘n’ After modifyng class variable ‘n’ If class vars are modified in class namespace, then it reflects to all instances

Namespaces Instance Namespace #To understand class namespace #Create the class class Student: #Create class var n = 10 s1 = Student() s2 = Student() #Modify the class var in instance namespace s1.n += 1 #Access class var in instance namespace print("s1.n: ", s1.n) print("s2.n: ", s2.n) 10n Class Namespace 10n 10n Instance NamespaceInstance Namespace 10n Class Namespace 11n 10n Instance NamespaceInstance Namespace Before modifyng class variable ‘n’ After modifyng class variable ‘n’ If class vars are modified in instance namespace, then it reflects only to that instance

Types of Methods  Types:  Instance Methods  - Accessor  - Mutator  Class Methods  Static Methods

Types of Methods Instance Methods ● Acts upon the instance variables of that class ● Invoked by instance_name.method_name() #To understanf the instance methods class Student: #Constructor definition def __init__(self, n = "", m = 0): self.name = n self.marks = m #Instance method def putdata(self): print("Name: ", self.name) print("Marks: ", self.marks) #Constructor called without any parameters s = Student() s.putdata() #Constructor called with parameters s = Student("Ram", 99) s.putdata()

Types of Methods Instance Methods: Accessor + Mutator #To understand accessor and mutator #Create the class class Student: #Define mutator def setName(self, name): self.name = name #Define accessor def getName(self): return self.name #Create an objects s = Student() #Set the name s.setName("Ram") #Print the name print("Name: ", s.getName()) Accessor Mutator ● Methods just reads the instance variables, will not modify it ● Generally written in the form: getXXXX() ● Also called getter methods ● Not only reads the data but also modifies it ● Generally wriiten in the form: setXXXX() ● Also called setter methods

Types of Methods Class Methods #To understand the class methods class Bird: #Define the class var here wings = 2 #Define the class method @classmethod def fly(cls, name): print("{} flies with {} wings" . format(name, cls.wings)) #Call Bird.fly("Sparrow") Bird.fly("Pigeon") ● This methods acts on class level ● Acts on class variables only ● Written using @classmethod decorator ● First param is 'cls', followed by any params ● Accessed by classname.method()

Types of Methods Static Methods #To Understand static method class Sample: #Define class vars n = 0 #Define the constructor def __init__(self): Sample.n = Sample.n + 1 #Define the static method @staticmethod def putdata(): print("No. of instances created: ", Sample.n) #Create 3 objects s1 = Sample() s2 = Sample() s3 = Sample() #Class static method Sample.putdata() ● Needed, when the processing is at the class level but we need not involve the class or instances ● Examples: - Setting the environmental variables - Counting the number of instances of the class ● Static methods are written using the decorator @staticmethod ● Static methods are called in the form classname.method()

Passing Members ● It is possible to pass the members(attributes / methods) of one class to another ● Example: e = Emp() ● After creating the instance, pass this to another class 'Myclass' ● Myclass.mymethod(e) - mymethod is static

Passing Members Example #To understand how members of one class can be passed to another #Define the class class Emp: def __init__(self, name, salary): self.name = name self.salary = salary def putdata(self): print("Name: ", self.name) print("Salary: ", self.salary) #Create Object e = Emp("Ram", 20000) #Call static method of Myclass and pass e Myclass.mymethod(e) #Define another class class Myclass: @staticmethod def mymethod(e): e.salary += 1000 e.putdata()

Passing Members Exercise 1. To calculate the power value of a number with the help of a static method

Inner Class Introduction ● Creating class B inside Class A is called nested class or Inner class ● Example: Person's Data like, - Name: Single value - Age: Single Value - DoB: Multiple values, hence separate class is needed

Inner Class Program: Version-1 #To understand inner class class Person: def __init__(self): self.name = "Ram" self.db = self.Dob() def display(self): print("Name: ", self.name) #Define an inner class class Dob: def __init__(self): self.dd = 10 self.mm = 2 self.yy = 2002 def display(self): print("DoB: {}/{}/{}" . format(self.dd, self.mm, self.yy)) #Creating Object p = Person() p.display() #Create inner class object i = p.db i.display()

Inner Class Program: Version-2 #To understand inner class class Person: def __init__(self): self.name = "Ram" self.db = self.Dob() def display(self): print("Name: ", self.name) #Define an inner class class Dob: def __init__(self): self.dd = 10 self.mm = 2 self.yy = 2002 def display(self): print("DoB: {}/{}/{}" . format(self.dd, self.mm, self.yy)) #Creating Object p = Person() p.display() #Create inner class object i = Person().Dob() i.display()

Python programming : Classes objects

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Python programming : Classes objects