Python Object and Classes 파이썬 객체와 클래스

파이썬 객체와 클래스    Python Object and Classes

객체 및 클래스 만들기  Creating object and classes

파이썬은 객체-지향 언어입니다.
파이썬에서, 모든 것은 객체인데, 즉, int , str , bool  even modules, functions도 또한 객체입니다.
객체 지향 프로그래밍은 프로그램을 만들기 위하여 객체들을 사용하며, 이들 객체는 데이터와 동작을 저장합니다.
Python is an object-oriented language. In python everything is object i.e int , str , bool  even modules, functions are also objects.

Object oriented programming use objects to create programs, and these objects stores data and behaviors.

클래스 정의하기 Defining class

파이썬에서 클래스 이름의 앞에는 키워드 class가 오며 그 뒤에는 콜론(:)이 옵니다.
클래스에는 일반적으로 데이터와 동작을 정의하는 메소드를 저장하는 데이터 필드가 포함되어 있습니다.
또한 파이썬의 모든 클래스에는 초기화자 (또한 일반적으로 생성자라고도 함)라는 특별한 메소드가 포함되어 있는데, 이 초기화자는 새 객체를 만들 때마다 자동으로 invoke 됩니다.

예제를 보자.
Class name in python is preceded with class  keyword followed by colon ( : ). Classes commonly contains data field to store the data and methods for defining behaviors. Also every class in python contains a special method called initializer (also commonly known as constructors), which get invoked automatically every time new object is created.

Let’s see an example.<span style="font-family: Tahoma;">class Person:        # constructor or initializer       def __init__(self, name):             self.name = name # name is data field also commonly known as instance variables       # method which returns a string      def whoami(self):            return "You are " + self.name</span>

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class Person:          # 생성자(constructor) 또는 초기화자(initializer)       def __init__(self, name):             self.name = name # name은 일반적으로 인스턴스 변수로도 알려진 data field 입니다         # string을 리턴하는 메소드      def whoami(self):            return "You are " + self.name

 

here we have created a class called Person  which contains one data field called name  and method whoami().

What is self ??

All methods in python including some special methods like initializer have first parameter self . This parameter refers to the object which invokes the method. When you create new object the self parameter in the __init__  method is automatically set to reference the object you have just created.

Creating object from class

<span style="font-family: Tahoma;">p1 = Person('tom') # now we have created a new person object p1 print(p1.whoami()) print(p1.name)</span>

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p1 = Person('tom') # now we have created a new person object p1 print(p1.whoami()) print(p1.name)

Expected Output:

<span style="font-family: Tahoma;">You are tom tom</span>

1 2	You are tom tom

Note: When you call a method you don’t need to pass anything to self  parameter, python automatically does that for you behind the scenes.

You can also change the name  data field.

<span style="font-family: Tahoma;">p1.name = 'jerry' print(p1.name)</span>

1 2	p1.name = 'jerry' print(p1.name)

Expected Output:

<span style="font-family: Tahoma;">jerry</span>

1

jerry

Although it is a bad practice to give access to your data fields outside the class. We will discuss how to prevent this next.

Hiding data fields

To hide data fields you need to define private data fields. In python you can create private data field using two leading underscores. You can also define a private method using two leading underscores.

Let’s see an example

<span style="font-family: Tahoma;">class BankAccount:      # constructor or initializer     def __init__(self, name, money):          self.__name = name          self.__balance = money   # __balance is private now, so it is only accessible inside the class     def deposit(self, money):          self.__balance += money     def withdraw(self, money):          if self.__balance > money :              self.__balance -= money              return money          else:              return "Insufficient funds"     # method which returns a string     def checkbalance(self):          return self.__balance b1 = BankAccount('tim', 400) print(b1.withdraw(500)) b1.deposit(500) print(b1.checkbalance()) print(b1.withdraw(800)) print(b1.checkbalance())</span>

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class BankAccount:        # constructor or initializer     def __init__(self, name, money):          self.__name = name          self.__balance = money   # __balance is private now, so it is only accessible inside the class       def deposit(self, money):          self.__balance += money       def withdraw(self, money):          if self.__balance > money :              self.__balance -= money              return money          else:              return "Insufficient funds"       # method which returns a string     def checkbalance(self):          return self.__balance   b1 = BankAccount('tim', 400) print(b1.withdraw(500)) b1.deposit(500) print(b1.checkbalance()) print(b1.withdraw(800)) print(b1.checkbalance())

Expected Output:

<span style="font-family: Tahoma;">Insufficient funds 900 800 100</span>

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Insufficient funds 900 800 100

Let’s try to access __balance  data field outside of class.

<span style="font-family: Tahoma;">print(b1.__balance)</span>

1	print(b1.__balance)

Expected Output:

<span style="font-family: Tahoma;">AttributeError: 'BankAccount' object has no attribute '__balance'</span>

1	AttributeError: 'BankAccount' object has no attribute '__balance'

As you can see now __balance  is not accessible outside the class.

In next chapter we will learn about operator overloading.

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This entry was posted in Python Basics on August 20, 2015 by admin.