from tkinter import *
    
class Application(Frame):
    def __init__(self, master):
        super(Application, self).__init__(master)
        self.grid()
    def create_widgets(self):
        self.button_list = []
        for i in range(9):
            button = Button(text = '',
                            font = 'arial 28',
                            width = 3,
                            height = 1)
            self.button_list.append(button)
            row = i // 3
            column = i - 3 * row
            self.button_list[i].grid(row = row, column = column)
            self.button_list[i]['command'] = lambda: self.return_number(i)        
    def return_number(self, i):
        print(i)
    
root = Tk()
app = Application(root)
app.create_widgets()
root.mainloop()

self.button_list[i]['command'] = lambda x=i: self.return_number(x)