更新Tkinter按钮错误，需要代码结构解释

Question

所以我现在正在自学python，我正在开发一个简单的GUI菜单。但是我遇到了一个错误，我希望有人能解释，我已经解决了它，我做了self.total_button按选项，而不是把选项放在括号里（就像我做的按钮上面），但我唯一改变的是，程序运行没有错误。你知道吗

total方法self.total_button["text"] = "Total £"+ str(self.cash_total)中的行仅在我以我的方式声明self.total_button时有效，如果我在括号中声明每个选项，它将声明一个Nonetype' object does not support item assignment。你知道吗

以后在程序中更新按钮时，布局是否与tkinter中的按钮有关？你知道吗

#Order Up!
#A Simple GUI program, that presents a simple restaurant menu.
#It lists items, and prices.
#Let the user select different items and then show the user the total bill.

from tkinter import *

class Application(Frame):
    """Create a GUI application."""
    def __init__(self, master):
        """Initialises the frame"""
        super(Application, self).__init__(master)
        self.grid()
        self.cash_total = 0

        self.total_list = []
        self.create_widgets()


    def create_widgets(self):
        """Creates the widgets that creates the menu ordering systems"""
        #Create a instruction label
        Label(self,
              text = "Click the desired buttons to 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 a total button
        #Super weird bug have to set it out like this so i can use the 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 to 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 a hamburger tuple to be added to the 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 the total amount due taken from the total_list"""
        for i 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! - A Restaurant Menu GUI")
app = Application(root)
root.mainloop()

Answer 1

你可能写了这样的代码

self.total_button = Button(self, self, text="Total: £", command=self.total).grid(row = 5, column = 5, sticky = W)

您可能会惊讶地发现self.total_button现在拥有值None。这是因为它保存的是Button的grid方法的返回值，而不是按钮引用本身。你知道吗

稍后当您尝试使用self.total_button时，它将抛出一个异常，因为值是None，并且None没有属性"Text"。你知道吗

要解决此问题，必须正确捕获对按钮的引用，并将创建和设置按钮的行拆分为两行。你知道吗

self.total_button = Button(self, text="Total: £", command=self.total)
self.total_button.grid(row = 5, column = 5, sticky = W)

现在您有了对按钮的正确引用，该引用将在total方法中稍后使用。你知道吗