[Python] Set button command to None but no effect in python tkinter

I want to remove button's click event when click it, but this code has no effect. Every button popup msgbox again and again:

from tkinter import *
from tkinter import messagebox

root = Tk()
root.geometry("500x500+400+200")

def show_btn(b):
messagebox.showinfo(message=f"{b.cget('text')}")
b.config(command=None)

for x in range(3):
btn = Button(root, text=f"{x}")
btn.config(command=lambda b=btn: show_btn(b))
btn.pack()

root.mainloop()

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