Let's assume that we have a form that asks for a username. We want users to input only alphabetical or space characters in the name. Thus, a number of special characters should not be allowed, as shown in the following screenshot of the widget:

This is clearly a case for the key validation mode because we want to check whether an entry is valid after every keypress. The percent substitution that we need to check is %S because it yields the text string being inserted or deleted in
the Entry widget. Therefore, the code that validates the Entry widget is as follows (see the 10.05_key_validation.py code):

import tkinter as tk

class KeyValidationDemo():
  def __init__(self):
    root = tk.Tk()
    tk.Label(root, text='Enter your name / only alpabets & space 
      allowed').pack()
    vcmd = (root.register(self.validate_data), '%S')
    invcmd = (root.register(self.invalid_name), '%S')
    tk.Entry(root, validate="key",validatecommand=vcmd, 
               invalidcommand=invcmd).pack(pady=5, padx=5)
    self.error_message = tk.Label(root, text='', fg='red')
    self.error_message.pack()
    root.mainloop()

  def validate_data(self, S):
    self.error_message.config(text='')
    return (S.isalpha() or S == ' ')

  def invalid_name(self, S):
    self.error_message.config(text='Invalid character %s 
 
                     name can only have alphabets and spaces' % S)
    app = KeyValidationDemo()

The description of the preceding code is as follows:

• We first register two options, namely validatecommand ( vcmd ) and invalidcommand ( invcmd ).
• In the example, validatecommand is registered to call the validate_data method, and the invalidcommand option is registered to call another method named invalid_name.
• The validatecommand option specifies a method that needs to be evaluated, which will validate the input. The validation method must return a Boolean value, where True signifies that the data entered is valid, and a False return value signifies that the data is invalid.
• In case the validate method returns False (invalid data), no data is added to the Entry widget and the script registered for invalidcommand is evaluated. In our case, a False validation will call the invalid_name  method. The invalidcommand method is generally responsible for displaying error messages or setting back the focus to the Entry widget.