如何学习重构代码？ Python

大家好！我在tkinter中编写了一个简单的程序。本质上是一个带有数据库（sqlite3）的注册程序。

所以现在，在我看来，它看起来不太好。是的，有人建议我重构代码，但我不知道该怎么做。虽然我理解OOP的本质。

代码：

from tkinter import Tk, Label, Entry, Button, StringVar, Toplevel, messagebox
import sqlite3

def check_user(login, password):
    conn = sqlite3.connect('users.db')
    c = conn.cursor()

    c.execute('''CREATE TABLE IF NOT EXISTS users
    (id INTEGER PRIMARY KEY, 
    login TEXT, 
    password TEXT)''')

    c.execute("SELECT login, password FROM users WHERE login = ?", (login, ))
            
    data_select_user = c.fetchone()
    data_submit_uesr = (login, password)

    if data_select_user == data_submit_uesr:
        messagebox.showinfo(title="Welcome!",  message="It's Ok!")
    else:
        messagebox.showerror(title="REGISTER!",  message="REGISTER PLEASE!")
                
            
    conn.commit()
    conn.close()


def insert_user(login, password):
    conn = sqlite3.connect('users.db')
    c = conn.cursor()

    c.execute('''CREATE TABLE IF NOT EXISTS users
    (id INTEGER PRIMARY KEY, 
    login TEXT, 
    password TEXT)''')

    c.execute("INSERT INTO users (login, password) VALUES (?,?)", (login, password))
            
    conn.commit()
    conn.close()

def submit():
    login = login_var.get()
    password = password_var.get()
    # print('its ok')
    check_user(login, password)
    login_var.set("")
    password_var.set("")

class root2:
    def __init__(self):
        super().__init__()
            
        def submit2():
            login = login_var2.get()
            password = password_var2.get()
            insert_user(login, password)
            login_var2.set("")
            password_var2.set("")
                    
        root2 = Toplevel(root)
        root2.title("Register Form")

        def call_func2():
            close2()
            submit2()

        def close2():
            root2.destroy()
                
        login_var2 = StringVar()
        password_var2 = StringVar()

        Label(root2, text="Login:").grid(row=0, column=0)
        Entry(root2, textvariable=login_var2).grid(row=0, column=1)

        Label(root2, text="Password:").grid(row=1, column=0)
        Entry(root2, textvariable=password_var2, show='*').grid(row=1, 
        column=1)

        Button(root2, text="Submit", command=call_func2).grid(row=2, column=1)



root = Tk()
root.title("Login Form")
root.resizable(False, False)

login_var = StringVar()
password_var = StringVar()

def click():
    root = root2()

def close():
    root.destroy()

def call_func():
    # close()
    submit()


Label(root, text="Login:").grid(row=0, column=0)
Entry(root, textvariable=login_var).grid(row=0, column=1)

Label(root, text="Password:").grid(row=1, column=0)
Entry(root, textvariable=password_var, show='*').grid(row=1, column=1)

Button(root, text="Submit", command=call_func).grid(row=2, column=1)
Button(root, text="Create new account", command=click).grid(row=3, 
column=1)

root.mainloop()