How to validate text entry in a form in Tkinter

Overview

We can validate a text entry in Tkinter with the help of callback functions of button widget in tkinter.

The `Button` class

The Button class is used to create buttons in tkinter.

The syntax to create a button is as follows:

ttk.Button(parent, options)

Parameters

parent: This is the parent component on which the button will be placed.
options: There are a lot of options. Some of the important options we use in this answer are as follows:

Option	Description
`text`	The text to display on the button.
`padx`	Padding to the button in `x` or horizontal direction.
`pady`	Padding to the button in `y` or vertical direction.
`command`	The callback function to be called.

Let’s learn this with the help of an example.

We build a simple GUI using Python Tkinter. Firstly, we create a simple interface to accept the user input and a submit button. When the user clicks on the submit button, a validation function is invoked.

Example

import tkinter as tk
from tkinter import messagebox

window_screen = tk.Tk()
window_screen.title('Validation Demo')
window_screen.geometry('300x200')

def validation():
    text = text_tf.get()
    msg = "The input text is correct"

    if len(text) == 0:
        msg = "The text can\'t be empty"
    elif len(text) > 20:
        msg = "The text can\'t be greater than 20 characters"    
        
    messagebox.showinfo('message', msg)
    
frame = tk.Frame(window_screen, padx=10, pady=10)
frame.pack(pady=20)
tk.Label(frame, text='Enter Text').grid(row=0, column=0)
text_tf = tk.Entry(frame, font=('calibre',10,'normal'))
text_tf.grid(row=0, column=1)

tk.Button(frame, text='Submit', pady=10, padx=20, command=validation).grid(row=1, columnspan=2)

window_screen.mainloop()

Tkinter text field validation

Explanation

Lines 1–2: We import the relevant dependencies.
Line 4: We create an instance of Tk() called window_screen.
Line 5: We set the title of the window_screen using the title() method.
Line 6: We set the shape of the window_screen using the geometry() method.
Lines 8–17: We define a method called validation() which gets called when the submit button is clicked.
- Line 9: We retrieve the content of the text entry using the get() method.
- Line 10: A variable called msg is defined that contains the message to be shown to the user after the validation.
- Lines 12–15: We check if the input text is empty or more than 20 characters.
- Line 17: We use the message box to show the validation result to the user.
Line 19: A frame called frame is created.
Line 22: A label for the text entry widget is created.
Line 23: A text entry widget is created with the help of the Entry class.
Line 25: A button is created with the command parameter set to the validation function. The validation function is invoked once the button is clicked.