from tkinter import *
root = Tk()
def buttonchange(button):
    if button["text"] == " ":
      button["text"] = "X"
button = StringVar()
btn1 = Button(text=" ", command=lambda:buttonchange(btn1))
btn1.pack()
btn2 = Button(text=" ", command=lambda:buttonchange(btn2))
btn2.pack()
if btn2["text"] == "X":
    print("This is a test.")
root.mainloop()

from tkinter import *
root = Tk()
def buttonchange(button):
    if button["text"] == " ":
      button["text"] = "X"
      print("This is a test.")

button = StringVar()
btn1 = Button(text=" ", command=lambda:buttonchange(btn1))
btn1.pack()
btn2 = Button(text=" ", command=lambda:buttonchange(btn2))
btn2.pack()

root.mainloop()