diff --git a/4. code_james_refactoring.py b/4. code_james_refactoring.py
new file mode 100644
index 0000000..a7727f1
--- /dev/null
+++ b/4. code_james_refactoring.py	
@@ -0,0 +1,213 @@
+# 예외처리(PostNotFoundError, IndexError, ValueError)
+# 예외처리(없는글번호)
+# 사용자 입력받는 함수 작성(m,n)
+# 데코레이터(실행권한)
+# Logical delete 구현
+
+import os
+import datetime
+import hashlib
+import getpass
+
+SAVE_FILE_PATH = 'save.csv'
+
+def autosave(func):
+    def decorated(caller, *args, **kwargs):
+        func(caller, *args, **kwargs)
+        caller.save()
+    return decorated
+
+class User():
+    def __init__(self, sign_in_id=None):
+            self.sign_in_id = sign_in_id
+
+    def signup(self):
+        uid = input('아이디 : ')
+        upasswd = getpass.getpass(prompt = '비밀번호 : ', stream = None)
+        hexdigest_upw = hashlib.sha256(upasswd.encode()).hexdigest()
+        data = {uid:hexdigest_upw}
+        with open('account.csv', 'a', encoding = "euc-kr") as f:
+            # [f.write('{0},{1}\n'.format(key, value)) for key, value in data.items()]
+            [f.write(f'{key},{value}\n') for key, value in data.items()] # python 3.6↑부터 지원
+        self.sign_in_id = uid
+        
+    def signin(self):
+            self.sign_in_id = input('아이디 : ')
+            sign_in_pw = getpass.getpass(prompt = '비밀번호 : ', stream = None)
+            hexdigest_pw = hashlib.sha256(sign_in_pw.encode()).hexdigest()
+            # account = '{0},{1}'.format(self.sign_in_id,hexdigest_pw)
+            account = f'{self.sign_in_id},{hexdigest_pw}'
+            with open('account.csv', mode = 'r', encoding = 'euc-kr') as f:
+                account_list = [line.rstrip() for line in f] # 개행문자 제거한 리스트 생성
+            if account not in account_list: # 아이디,패스워드 값을 account_list와 비교
+                clear()
+                print_login_message()
+                m = int(input(' >'))
+                if m == 1 :
+                    user.signin()
+                if m == 2 :
+                    user.signup()
+            clear()
+            return
+                    
+class Board():
+    post_per_pages = 15
+    
+    def __init__(self):
+        self.posts = []
+        self.current_page = 1
+        try:
+            self.load()
+        except FileNotFoundError:
+            pass
+    def search_post_by_id(self, pid):
+        for index, post in enumerate(self.posts):
+            if post.has_pid(pid):
+                return index, post
+   
+    def list_posts(self):
+        if self.posts == []:
+            print('게시판에 아직 작성된 글이 없습니다.')
+            print('한 번 작성해보는 것은 어떨까요?')
+            return
+        
+        page_from = (self.current_page - 1) * Board.post_per_pages
+        page_to = self.current_page * Board.post_per_pages
+        for post in self.posts[page_from : page_to]:
+            print(post)
+            
+    @autosave
+    def create_post(self): #1
+        title = input('작성 > ')
+        self.posts.insert(0, Post(title))
+
+    @autosave
+    def modify_post(self): #잘못된 글 번호 입력에 대한 예외처리 필요 #2
+        pid = int(input('몇번 글을 수정할까요? '))
+        new_title = input('수정 > ')
+        _, post = self.search_post_by_id(pid)
+        post.modify(new_title)
+
+    @autosave         
+    def delete_post(self): #삭제가 아닌 hidden 처리 필요 #잘못된 글 번호 입력에 대한 예외처리 필요 #3
+        pid = int(input('몇번 글을 지울까요? '))
+        index, _ = self.search_post_by_id(pid)
+        del self.posts[index]
+
+    def load(self):
+        global SAVE_FILE_PATH
+        f = open(SAVE_FILE_PATH, 'r', encoding='euc-kr')
+        posts = f.readlines()
+        for post in posts:
+            self.posts.append(Post.from_csv(post))
+        f.close()
+    
+    def save(self):
+        global SAVE_FILE_PATH
+        f = open(SAVE_FILE_PATH, 'w', encoding='euc-kr')
+        for post in self.posts:
+            print(post.to_csv(), file=f)
+        f.close()
+
+    def next_page(self): #4
+        if self.current_page * Board.post_per_pages < len(self.posts):
+            self.current_page += 1
+
+    def previous_page(self): #5
+        if self.current_page > 1:
+            self.current_page -= 1
+
+class Post():
+    pid = 1
+
+    def __init__(self, title, pid=None, created_at=None):
+        self.pid = Post.pid
+        self.title = title
+        self.created_at = datetime.datetime.now().strftime("%Y-%m-%d %H:%M")
+        self.sign_id = user.sign_in_id
+        Post.pid += 1
+
+    def __str__(self):
+        return '{:^8} | {:60.60} | {} | {}'.format(self.pid, self.title, self.sign_id, self.created_at)
+
+    def modify(self, new):
+        self.title = new
+
+    def has_pid(self, pid):
+        return self.pid == pid
+
+    @classmethod
+    def from_csv(cls, csv):
+        pid, title, created_at = csv[:-1].split(',')
+        return cls(title, pid, created_at)
+    
+    def to_csv(self):
+        return '{},{},{}'.format(self.pid, self.title, self.created_at)
+    
+
+def print_greeting_message():
+    greeting_message = '''
+    본 프로그램은 광일공방에서 중고거래 사이트를 기웃기웃 거리던 광일이가
+    어느날 삘이 꽂혀 '아! 여기가 돈 나오는 방석이다!' 생각하자마자 아주
+    급하게 만들어낸 불안정한 중고거래 사이트입니다. 물론 아직은 허접한 
+    게시판이지만 1000만 사용자를 꿈꾸는 광일이는 커다란 희망의 씨앗을 마음에
+    품고서 오늘도 밤을 지새우며 코드를 작성합니다.
+    '''
+    print(greeting_message)
+    input('프로그램을 시작하시려면 로그인하세요...')
+
+def print_login_message():
+    login_message = '''
+    -------------------------------------------------------------
+    1. 로그인
+    2. 회원가입
+    -------------------------------------------------------------
+    '''
+    print(login_message)
+
+
+def show_actions():
+    show_actions = '''
+    1. 작성하기
+    2. 수정하기
+    3. 삭제하기
+    4. 다음 페이지로
+    5. 이전 페이지로
+    '''
+    print(show_actions)
+    print('--------------------------------------------------------')
+
+def success_signin_message():
+    # print('로그인 성공 : 환영합니다', '{}님!'.format(user.sign_in_id))
+    print('로그인 성공 : 환영합니다', f'{user.sign_in_id}님!')
+
+def clear():
+    os.system('cls' if os.name == 'nt' else 'clear')
+
+
+if __name__ == "__main__":
+    print_greeting_message()
+    user = User()
+    print_login_message()
+    m = int(input(' >'))
+    if m == 1 :
+        user.signin()
+    if m == 2 :
+        user.signup()
+    success_signin_message()
+    board = Board()
+    while True:
+        ##clear()
+        board.list_posts()
+        show_actions()
+        n = int(input(' >'))
+        if n == 1 :
+             board.create_post()
+        elif n == 2:
+             board.modify_post()
+        elif n == 3:
+             board.delete_post()
+        elif n == 4:
+             board.next_page()
+        elif n == 5:
+             board.previous_page()