获取python中的combobox值

我正在开发一个简单的程序，我需要从组合框中获取值。当combobox位于第一个创建的窗口中时，这很容易，但例如，如果我有两个窗口，而combobox位于第二个窗口中，则我无法读取值

例如：

from tkinter import *
from tkinter import ttk

def comando():
    print(box_value.get())

parent = Tk() #first created window
ciao=Tk()     #second created window
box_value=StringVar()
coltbox = ttk.Combobox(ciao, textvariable=box_value, state='readonly')
coltbox["values"] = ["prova","ciao","come","stai"]
coltbox.current(0)
coltbox.grid(row=0)
Button(ciao,text="Salva", command=comando, width=20).grid(row=1)
mainloop()

如果我将小部件的父级从ciao更改为父级，它就会工作！ 谁能给我解释一下吗？ 提前感谢，并为我的英语不好表示歉意

您不能有两个Tk（）窗口。一个人必须是顶级的

要获取变量，请执行box_value.get（）

下拉框示例

class TableDropDown(ttk.Combobox):
    def __init__(self, parent):
        self.current_table = tk.StringVar() # create variable for table
        ttk.Combobox.__init__(self, parent)#  init widget
        self.config(textvariable = self.current_table, state = "readonly", values = ["Customers", "Pets", "Invoices", "Prices"])
        self.current(0) # index of values for current table
        self.place(x = 50, y = 50, anchor = "w") # place drop down box 

---

print（self.current\u table.get（））

Tkinter不能很好地处理两个主窗口，因此第二个主窗口应该是第一个主窗口的顶层。

from tkinter import *
from tkinter import ttk
from tkinter import messagebox


root = Tk()

root.geometry("400x400")
#^ Length and width window :D


cmb = ttk.Combobox(root, width="10", values=("prova","ciao","come","stai"))
#^to create checkbox
#^cmb = Combobox


#now we create simple function to check what user select value from checkbox

def checkcmbo():

if cmb.get() == "prova":
     messagebox.showinfo("What user choose", "you choose prova")

#^if user select prova show this message 
elif cmb.get() == "ciao":
    messagebox.showinfo("What user choose", "you choose ciao")

 #^if user select ciao show this message 
elif cmb.get() == "come":
    messagebox.showinfo("What user choose", "you choose come")

elif cmb.get() == "stai":
    messagebox.showinfo("What user choose", "you choose stai")

elif cmb.get() == "":
    messagebox.showinfo("nothing to show!", "you have to be choose something")




cmb.place(relx="0.1",rely="0.1")

btn = ttk.Button(root, text="Get Value",command=checkcmbo)
btn.place(relx="0.5",rely="0.1")

root.mainloop()