python 3.x – input an error or a button

while the user types an entry in the tkinter box, I wish that a red button with an X appear, informing him that something bad is happening, for example:
I want the entry to be just digits and point to just one point distinguishing a float, but if the user types something else, X appears and the corrections disappear. I did not put an attempt code because I do not even know how to start. if anyone can help me, if i press the button, i wish to bring up an error in the box itself, which i have not yet been able to & # 39; Send to these two types or just limit the number of entries and indicate something that I do not know yet. that's it?