<s>[INST] create a notepad app using python [/INST] ```python
from tkinter import *
from tkinter.filedialog import asksaveasfilename, askopenfilename

current_file = None

def new_file():
    global current_file
    current_file = None
    text.delete(1.0, END)

def open_file():
    global current_file
    filepath = askopenfilename(defaultextension='.txt',
                               filetypes=[('Text Files', '*.txt'),
                                          ('All Files', '*.*')])
    if not filepath:
        return
    current_file = filepath
    text.delete(1.0, END)
    with open(filepath, 'r') as file:
        text.insert(INSERT, file.read())

def save_file():
    global current_file
    if not current_file:
        filepath = asksaveasfilename(defaultextension='.txt',
                                     filetypes=[('Text Files', '*.txt'),
                                                ('All Files', '*.*')])
        if not filepath:
            return
        current_file = filepath
    with open(current_file, 'w') as file:
        file.write(text.get(1.0, END))

def exit_app():
    root.quit()

def undo():
    text.edit_undo()

def redo():
    text.edit_redo()

root = Tk()
root.title('Notepad')

menu_bar = Menu(root)
root.config(menu=menu_bar)

file_menu = Menu(menu_bar, tearoff=False)
menu_bar.add_cascade(label='File', menu=file_menu)
file_menu.add_command(label='New', command=new_file)
file_menu.add_command(label='Open', command=open_file)
file_menu.add_command(label='Save', command=save_file)
file_menu.add_separator()
file_menu.add_command(label='Exit', command=exit_app)

edit_menu = Menu(menu_bar, tearoff=False)
menu_bar.add_cascade(label='Edit', menu=edit_menu)
edit_menu.add_command(label='Undo', command=undo)
edit_menu.add_command(label='Redo', command=redo)

text = Text(root)
text.pack(expand=True, fill=BOTH)

toolbar = Frame(root, bd=1, relief=RAISED)
undo_btn = Button(toolbar, text='Undo', command=undo)
undo_btn.pack(side=LEFT, padx=2, pady=2)
redo_btn = Button(toolbar, text='Redo', command=redo)
redo_btn.pack(side=LEFT, padx=2, pady=2)
toolbar.pack(side=TOP, fill=X)

root.mainloop()
```
This code creates a simple notepad app using the Tkinter library in Python. The app has a menu bar with options to create a new file, open an existing file, and save the current file. The text area of the app is implemented using a Text widget. The app supports both saving and opening files in the .txt format.</s>