如何将标签/条目设置为等于在函数中进行的计算

Question

抱歉，代码太糟糕了，我对编程是全新的，这是我第一次尝试使用tkinter。这是一个小计算器，我正试图为我的工作的某个方面组装起来，更多的是为了乐趣和经验，而不是其他任何东西。我在TR55课程中遇到了一个难题，我想将max_retention设置为1000/curve_number-10，然后将结果显示在网格上“径流开始后的潜在最大保留率（以英寸为单位）”旁边的灰色条目上

我认为问题在于将回调函数外的变量设置为等于回调函数中的计算，但我在这里有点深奥。我知道该函数正确地计算了max_retention，因为它打印了正确的数字，所以我的问题是如何将该数字显示在gui的网格上

import tkinter as tk
from tkinter import ttk

import numpy as np

LARGE_FONT = ("Verdana", 12)


class Calculator(tk.Tk):

    def __init__(self, *args, **kwargs):
        tk.Tk.__init__(self, *args, **kwargs)

        tk.Tk.wm_title(self, "Max flow calculator for drainage analysis of a proposed culvert installation")

        container = tk.Frame(self)
        container.pack(side="top", fill="both", expand=True)
        container.grid_rowconfigure(0, weight=1)
        container.grid_columnconfigure(0, weight=1)

        self.frames = {}

        for F in (StartPage, TR55):
            frame = F(container, self)

            self.frames[F] = frame

            frame.grid(row=0, column=0, sticky="nsew")

        self.show_frame(StartPage)

    def show_frame(self, cont):
        frame = self.frames[cont]
        frame.tkraise()


class StartPage(tk.Frame):

    def __init__(self, parent, controller):
        tk.Frame.__init__(self, parent)
        label = tk.Label(self, text="How large is the drainage area for the project?", font=LARGE_FONT)
        label.pack(pady=10, padx=10)

        button = ttk.Button(self, text="Drainage area is over 200 acres",
                            command=lambda: controller.show_frame(TR55))
        button.pack()



class TR55(tk.Frame):

    def __init__(self, parent, controller):
        tk.Frame.__init__(self, parent)
        label = tk.Label(self, text="TR-55 Method", font=LARGE_FONT)
        label.grid(columnspan=2)

        max_retention = ""

        def callback(*args):
            global max_retention

            drainage_area = drainage_area_entry.get()
            drainage_area = float(drainage_area)
            print "DA=", drainage_area

            curve_number = curve_number_entry.get()
            curve_number = float(curve_number)
            print "CN=", curve_number

            max_retention = (1000.0/curve_number) - 10
            print "S=", max_retention


        drainage_area_label = tk.Label(self, text="What is the drainage area of the project?")
        drainage_area_label.grid(row=1, column=0, sticky='w')
        drainage_area_entry = tk.StringVar()
        drainage_area_entry.trace("w", callback)
        da_entry = tk.Entry(self, textvariable=drainage_area_entry)
        da_entry.grid(row=1, column=1)

        curve_number_label = tk.Label(self, text="What is the Curve Number?")
        curve_number_label.grid(row=2, column=0, sticky='w')
        curve_number_entry = tk.StringVar()
        curve_number_entry.trace("w", callback)
        cn_entry = tk.Entry(self, textvariable=curve_number_entry)
        cn_entry.grid(row=2, column=1)

        max_retention_label = tk.Label(self, text="Potential maximum retention after runoff begins in inches:")
        max_retention_label.grid(row=3, column=0, sticky='w')
        max_retention_entry = tk.Entry(self, textvariable=max_retention, state=tk.DISABLED)
        max_retention_entry.grid(row=3, column=1, sticky='e')


app = Calculator()
app.mainloop()

再一次，为我确信的糟糕代码感到抱歉。任何建设性的批评都是受欢迎的