如何在python中使用Tkinter制作3（或4）个嵌套的绑定组合框？

我正在尝试制作一个具有绑定下拉列表的GUI，该下拉列表依赖于以前的下拉列表。我试图实现的格式如下：

1. 选择一个{level 1}
2. 选择一个{level 2}-绑定到level 1
3. 选择一个{level 3}-绑定到level 2
4. 选择一个{level 4}-绑定到level 3

我一直在努力的代码如下所示。这是从这里派生的：

import tkinter
from tkinter import ttk
root = tkinter.Tk()
''' 
widgets are added here 
'''
brands = ["Bugatti","BMW"]

models = [["Veyron","Golf"],
          ["330M"]]

Zip = [[["TEST1"],["TEST2"]],[["TEST3"]]]

car_brand = ttk.Combobox(root, width=37, value=(brands))
car_brand.grid(row=3, column=1, columnspan=2, padx=10, pady=2, sticky='w')

def callback(eventObject):
    abc = eventObject.widget.get()
    car = car_brand.get()
    index=brands.index(car)
    car_model.config(values=models[index])

car_model = ttk.Combobox(root, width=37)
car_model.grid(row=4, column=1, columnspan=2, padx=10, pady=2, sticky='w')
car_model.bind('<Button-1>', callback)

def callback2(eventObject):
    abc = eventObject.widget.get()
    y = car_model.get()
    index=models.index(y)
    xx.config(values=Zip[index])

xx = ttk.Combobox(root, width=37)
xx.grid(row=5, column=1, columnspan=2, padx=10, pady=2, sticky='w')
xx.bind('<Button-1>', callback2)

root.mainloop()

我可能真的很亲密，但是我需要一些帮助才能使其正常工作。

更新：谢谢@JacksonPro的回答。我需要稍微调整一下以允许网格放置。见下文：

import tkinter as tk
from tkinter import ttk



cars = {"Bugatti":["Veyron","Golf"] ,"BMW":["330M"]}

_zip = [[["TEST1"],["TEST11"]],[["TEST2"],["Hello"],["World"]],["TEST3"]]

lst = [x for y in list(cars.values()) for x in y]


def change_dropdown(*args):

    print("Chosen brand " + tkvar.get())

    if args[0] == 'PY_VAR0':
        for x, y in cars.items():
            if tkvar.get() == x:
                tkvar2.set(y[0])
                popupMenu2.configure(values=y)
        
    if args[0] == 'PY_VAR1':
        for x, y in zip(lst, _zip):
            if tkvar2.get() == x:            
                tkvar3.set(y[0])
                popupMenu3.configure(values=y)
        
    return
    
root = tk.Tk()
# canvas = tk.Canvas(root, height=500, width= 500, bg="white")
root.geometry('500x100')
font = ("Courier", 16, "bold")
# canvas.pack()

tkvar = tk.StringVar(root)
tkvar.trace('w', change_dropdown)

tkvar2 = tk.StringVar(root)
tkvar2.trace('w', change_dropdown)

tkvar3 = tk.StringVar(root)

labelTop = tk.Label(root, text = "Select a State")
labelTop.grid(column=0, row=0)
popupMenu1 = ttk.Combobox(root, textvariable=tkvar, values=list(cars.keys()))
popupMenu1.grid(row=0, column=1)
# popupMenu1.pack()

popupMenu2 = ttk.Combobox(root, textvariable=tkvar2, values=[])
popupMenu2.grid(row=1, column=1)
# popupMenu2.pack()

popupMenu3 = ttk.Combobox(root, textvariable=tkvar3, values=[])
popupMenu3.grid(row=2, column=1)
# popupMenu3.pack()

root.mainloop()