[파이썬300제] 251~276 정리

1. Class vs Instance vs Object

• Object
  • Everything in python is an object
  • built-in data types(e.g. integers, strings, lists…) & user-defined data types(e.g. classes)
  • An object consists of:
    • State: It is represented by the attributes of an object. It also reflects the properties of an object.
    • Behavior: It is represented by the methods of an object. It also reflects the response of an object to other objects.
    • Identity: It gives a unique name to an object and enables one object to interact with other objects.
• Class
  • a collection of objects(instances)
  • attributes 는 클래스에 속한 variables
  • attributes are always public and can be accessed using the dot(.) Eg.:my_class.my_attribute
• Instance

  • objects of a class

  • object가 어떤 클래스의 객체인지 관계를 설명할때 사용

    class Cookie:
    	pass
    
    a = Cookie() # a는 object, Cookie의 instance
    b = Cookie() # b는 object, Cookie의 instance

2. Constructor vs Method

• Constructor
  • the init() method
  • always called when an object is created
  • types
    • default constructor : doesn’t accepts any arguments(has only one argument → self)
    • parameterized constructor : takes its first argument as a reference to the instance being constructed known as self and the rest of the arguments are provided by the programmer
• Method

  • 클래스 안에서 구현하는 함수 ⇒ tied to objects or classes(need an object or class instance to be invoked)

  • 첫번째 매개변수의 이름은 클래스의 인스턴스 자신 → self

    # 1.
    
    class GeekforGeeks:
    
    # default constructor
    		def __init__(self):
            self.geek = "GeekforGeeks"
    
    # a method for printing data members
        def print_Geek(self):
            print(self.geek)
    
    obj = GeekforGeeks()
    
    ------------------------------------------------------------
    
    # 2.
    
    class Addition:
        first = 0
        second = 0
        answer = 0
    
    # parameterized constructor
    		def __init__(self, f, s):
            self.first = f
            self.second = s
    
    obj1 = Addition(1000, 2000)

3. random() function

• Functions for integers

• random.randint(a, b)

  • Return a random integer N such that a <= N <= b.
  • Alias for randrange(a, b+1)

• random.randrange(start, stop[, step])

  • Return a randomly selected element from range(start, stop, step)

4. zfill() method

• string.zfill(len)

  • adds zeros (0) at the beginning of the string, until it reaches the specified length
  • len < len(str) : no filling is done

• 클래스 안의 변수를 method(constructor)안에서 사용하려면 “클래스이름.변수명” 이렇게 사용해야함!

  class Hello:
      # class variable
      greeting_count = 0
  
  		def __init__(self, name):
  			self.name = name
  
  			print(f"Hello, {name}!")
  
  			# greeting_count += 1       이렇게 사용못함
  			Hello.greeting_count += 1 # 이렇게 사용해야함!
  
  K = Hello("K")
  print(Hello.greeting_count) #greeting_count 변수 불러오기
  J = Hello("J")
  print(J.greeting_count)  
  #J가 Hello 클래스 instance니까 J.greeting_count해도 같음!

5. String format() method

• formats the specified value(s) and insert them inside the string's placeholder.
  • The placeholder is defined using curly brackets: {}
    • The placeholders can be identified using named indexes {price}
    • numbered indexes {0},
    • or even empty placeholders {}.

     txt1 = "My name is {fname}, I'm {age}".format(fname = "John", age = 36)
      txt2 = "My name is {0}, I'm {1}".format("John",36)
      txt3 = "My name is {}, I'm {}".format("John",36)

  • returns the formatted string

  Formatting Types

  • :, Use a comma as a thousand separator

  • Output formatting : f-strings

    universe_age = 13800000000
    print(f"The universe is {universe_age:,} years old.")
    
    #... The universe is 13,800,000,000 years old.