[Tkinter] how to assign a command to a button

[ChickenWithRice]

Hello!
I'm having some trouble with my code. I can't find how to assign a command to a button. It keeps coming up with an error saying " line 5, in greeting label.config(text = ":)") AttributeError: 'NoneType' object has no attribute 'config'

import tkinter
import _tkinter

def greeting():
    label.config(text = "hi")

def bye():
    label.config(text = "bye")

from tkinter import *
root = Tk()
label=Label(root, text = "hello").pack()
button=Button(root, text = "hi", command = greeting).pack()
button2=Button(root, text = "bye",command = bye).pack()
root.mainloop()

Thanks for any help you can give!

[buran]

as is at the moment, label, button and button2 are not widgets, but what pack() returns, i.e. None
do the pack on separate line

[ChickenWithRice]

Thank you!