Tkinter Atlernative

from tkinter import *
from tkinter import messagebox


class Add:

    def __init__(self, master):
        self.var_1 = StringVar()
        self.var_2 = StringVar()

        self.frame = Frame(master)
        self.frame.grid()

        self.entry_1 = Entry(self.frame, textvariable=self.var_1)
        self.entry_1.grid(row=0, column=0)

        self.entry_2 = Entry(self.frame, textvariable=self.var_2)
        self.entry_2.grid(row=1, column=0)

        self.button = Button(self.frame, text='Submit', command=self.submit)
        self.button.grid(row=0, column=1)

    def submit(self):
        try:
            if len(self.entry_1.get()) == 0:
                self.var_1.set('0')
            elif len(self.entry_2.get()) == 0:
                self.var_2.set('0')
            add_entry_1_and_2 = int(self.entry_1.get()) + int(self.entry_2.get())
            print(add_entry_1_and_2)
        except ValueError:
            if self.entry_1.get().isdigit() is False or self.entry_2.get().isdigit() is False:
                messagebox.showerror('Error', 'Invalid Character(s).')


root = Tk()
Add(root)
root.mainloop()