tkinter Entry validatecommand cannot call Entry.config()

Question

In Python 3.6.6, I would like to configure a tkinter Entry widget whenever another Entry widget is modified. Entry widgets don't have a callback function, but they do have validatecommand, which can work the same way if validate='focus'. However, the interpreter doesn't recognize the to-be-modified Entry widget within the validatecommand function. Here is my minimal example:

import tkinter as tk

class app:
    def __init__(self,window):
        self.root = window

        self.tv1 = tk.DoubleVar(None,2)
        self.tv2 = tk.DoubleVar(None,3)
        self.tv3 = tk.DoubleVar(None,0)

        self.entry1 = tk.Entry(self.root, textvariable=self.tv1,
                               validate='focus', validatecommand=self.valid)
        self.entry2 = tk.Entry(self.root, textvariable=self.tv2,
                               validate='focus', validatecommand=self.valid)
        self.label1 = tk.Label(self.root, textvariable=self.tv3)

        self.entry1.pack()
        self.entry2.pack()
        self.label1.pack()

    def valid(self):
        self.tv3.set(self.tv1.get()*self.tv2.get())
        self.label1.config(fg='red')
        return True

main = tk.Tk()
win = app(main)
main.mainloop()

which returns

AttributeError: 'app' object has no attribute 'label1'

which is obviously not true.

Answer 1

The validate command is being called for the first time before the label is created.

You can solve this by creating the label before creating the entry widgets.