python - Updating the Tkinter button error with code structure Explanation required -

so i'm teaching myself python, , developing simple gui restaurant menu. came across error i'm hoping explain, i've solved doing self.total_button option, rather putting options in parentheses (like did button above it), thing have changed , program runs no errors.

the line in total method self.total_button["text"] = "total £"+ str(self.cash_total) works when declare self.total_button way do, if declare each option in parentheses states nonetype' object not support item assignment.

does layout matter buttons in tkinter when updating them later in program?

#order up! #a simple gui program, presents simple restaurant menu. #it lists items, , prices. #let user select different items , show user total bill.  tkinter import *  class application(frame):     """create gui application."""     def __init__(self, master):         """initialises frame"""         super(application, self).__init__(master)         self.grid()         self.cash_total = 0          self.total_list = []         self.create_widgets()       def create_widgets(self):         """creates widgets creates menu ordering systems"""         #create instruction label         label(self,               text = "click desired buttons order something"               ).grid(row = 0, column = 0, columnspan = 4, sticky = n)          #create burger button         button(self,                text = "hamburger no bun",                command = self.hamburger_no_bun                ).grid(row = 1, column = 0, sticky = w)         #creates total button         #super weird bug have set out can use total method later.         self.total_button = button(self)         self.total_button["text"] = "total: £"         self.total_button["command"] = self.total         self.total_button.grid(row = 5, column = 5, sticky = w)          #create text box show current order         self.order_txt = text (self, width = 100, height = 8, wrap = word)         self.order_txt.grid(row = 1, column = 5, sticky = w)        def hamburger_no_bun(self):         """creates hamburger tuple added list."""         self.hamburger_no_bun = ("hamburger no bun, £", 2.95)         self.total_list.append(self.hamburger_no_bun)         self.order_txt.insert(0.2, str(self.hamburger_no_bun))      def total(self):         #the affected method         """adds total amount due taken total_list"""         in self.total_list:             self.cash_total += i[1]         print(self.cash_total)         self.total_button["text"] = "total £"+ str(self.cash_total)         self.cash_total = 0  #main root = tk() root.title("order up! - restaurant menu gui") app = application(root) root.mainloop() 

you wrote code this

self.total_button = button(self, self, text="total: £", command=self.total).grid(row = 5, column = 5, sticky = w) 

you may suprised learn self.total_button holds value none. because holding return value of grid method of button not button reference itself.

later when try use self.total_button throw exception, because value none , none has no attribute "text".

to resolve issue must correctly capture reference button , split line creating , setting button 2 lines.

self.total_button = button(self, text="total: £", command=self.total) self.total_button.grid(row = 5, column = 5, sticky = w) 

now have correct reference button, usable later in total method.