from tkinter import ttk, messagebox
import tkinter as tk
 
def validar_entrada(previous_text, text, index, action):
    if text.isdigit():
        if ((int(index) == 0) and (previous_text.startswith('-'))):
            return False
        else:
            return True
    elif text == '.':
        if action == '0':
            return True
        else:
            return previous_text.count(text) == 0
    elif text == '-':
        if action == '0':
            return True
        else:
            if len(previous_text) == 0:
                return True
            else:
                return (previous_text[0] != text) and (int(index) == 0)
    else:
        return False
 
def main():
    root = tk.Tk()
    root.geometry("300x100+500+300")
    root.title("Unicamente decimales.")
 
    ingreso = ttk.Entry(root, validate="key", validatecommand=(root.register(validar_entrada), "%s", "%S", "%i", "%d"))
    ingreso.place(x=10, y=20, width=150)
 
    ingreso.focus()
    root.mainloop()
 
 
if __name__ == '__main__':
    main()