NameError: name 'messagebox' is not defined 错误处理

　　写了段代码，想在按下button的时候跳个提示框出来，调试的时候提示了messagebox未定义

from tkinter import *

def test_show():
    messagebox.showinfo(None, 'first')

root = Tk()

one = Label(root, text = 'First')
one.pack()

two = Button(root, text = 'First', command=test_show)
two.pack()

mainloop()

而后在CSDN上看到有人说messagebox输入子模块，不会默认导入，然后在开头加入了messagebox的导入，程序就可以正常执行了

from tkinter import *
from tkinter import messagebox

def test_show():
    messagebox.showinfo(None, 'first')

root = Tk()

one = Label(root, text = 'First')
one.pack()

two = Button(root, text = 'First', command=test_show)
two.pack()

mainloop()