无法在tkinter python中将一个函数的变量用于另一个函数

Question

I am trying to create the GUI, I am unable to use the value of a variable in one function on the other function in tkinter

But, in normal functions, when we declare the variable as global, it is accessible.

Here is my code,

from tkinter import *
global chosen_option
root = Tk()
root.title("Drop-down boxes for option selections.")

var = StringVar(root)
var.set("drop down menu button")

def grab_and_assign(event):
    grab_and_assign.var = var.get()
    label_chosen_variable= Label(root, text=chosen_option)
    label_chosen_variable.grid(row=1, column=2)
    print(chosen_option)

def pri():
    print(grab_and_assign.chosen_option)

drop_menu = OptionMenu(root, var,  "one", "two", "three", "four", "meerkat", "12345", "6789", command=grab_and_assign)
drop_menu.grid(row=0, column=0)

label_left=Label(root, text="chosen variable= ")
label_left.grid(row=1, column=0)
pri()

root.mainloop()

Answer 1

You have two problems:

First: mainloop() starts window so your pri() is executed before you see window - so before you could selecte anything. I add pri to button so you can also see how it works after selecting option.

Second: all variables create outside functions/classes are global and they don't need keyword global . You have to use global inside function to inform function that when you will assign value using = then you what to use external/global variable instead of creating local one.

In code I have global variable normal_var and I use global normal_var inside grab_and_assign to inform function to assign value to global variable and not to create local variable in line

 normal_var = var.get()

I don't have to use global inside pri() because I only get value from variable nromal_var and don't use = to assign new value

I don't have to use global to assign new value for:

• var because it uses set() instead of = .
• label_selected["text"] because it assign value to key in dictionary.

But I would have to use global for label_selected if I would like to assign new Label inside function - ie. label_selected = tk.Label(..)

import tkinter as tk

# --- functions ---

def grab_and_assign(event):
    global normal_var

    normal_var = var.get()

    label_selected["text"] = var.get()

def pri():
    print('normal_var:', normal_var)
    print('var.get():', var.get())
    print('---')

# --- main ---

# global variable
normal_var = "- empty -"

root = tk.Tk()
root.title("Drop-down boxes for option selections.")

# it is global variable
var = tk.StringVar(root)
var.set("drop down menu button")

drop_menu = tk.OptionMenu(root, var,  "one", "two", "three", "four", "meerkat", "12345", "6789", command=grab_and_assign)
drop_menu.grid(row=0, column=0)

label_left = tk.Label(root, text="chosen variable = ")
label_left.grid(row=1, column=0)

# create it only once
label_selected = tk.Label(root, text="?")
label_selected.grid(row=1, column=1)

# execute `pri` after button press
b = tk.Button(root, text='PRI', command=pri)
b.grid(row=2, column=0)

# execute `pri` before window starts
pri()

root.mainloop()

---

BTW: Label has textvariable= and you can assign var which you use in OptionMenu and then it will automatically display selected value

import tkinter as tk

# --- main ---

root = tk.Tk()

var = tk.StringVar(root)
var.set("drop down menu button")

label_selected = tk.Label(root, textvariable=var)
label_selected.pack()

drop_menu = tk.OptionMenu(root, var,  "one", "two", "three", "four", "meerkat", "12345", "6789")
drop_menu.pack()

root.mainloop()