import tkinter as tk
class Window(tk.Tk):
    def __init__(self):
        self.root = tk.Tk()
    def const(self):
        self.entry = tk.Entry(self.root).pack()
        self.button = tk.Button(self.root, text="Get", command=self.on_button).pack()
    def on_button(self):
        print(self.const.entry.get())
    def run(self):
        self.const()
        self.root.mainloop()
win = Window()
win.run()

 import tkinter as tk
class Window(tk.Tk):
    def __init__(self):
        super().__init__()
        self.entry = tk.Entry()
        self.entry.pack()
        self.button = tk.Button(text="Get", command=self.on_button)
        self.button.pack()
    def on_button(self):
        out = self.entry.get()
        print(out if out else 'Пустая строка')
    def run(self):
        self.mainloop()
win = Window()
win.run()