11. Using Entry Boxes in Tkinter

The tk.Entry widget creates a text box that allows users to type in information (like names, ages, or scores).

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11.1. Validating Input (Numbers Only)

Sometimes you want to force the user to type only numbers (for example, when asking for an age).

We can block letters from being typed entirely.

To do this, we use two special settings: * validate='key': Checks the text every single time a key is pressed. * validatecommand: Tells Tkinter which function to use to check the text. * '%P': A special code that tells Tkinter to pass the proposed new text to our function before it appears on the screen.

11.1.1. Simple Number Validation

This script only allows the user to type numbers. If they press a letter key, nothing will happen!

import tkinter as tk

def check_number(new_text):
    # Allow the box to be empty, OR check if the new text is a number
    if new_text == "" or new_text.isdigit():
        return True   # True means "allow the change"
    else:
        return False  # False means "reject the keystroke"

root = tk.Tk()
root.title("Numbers Only")
root.geometry("400x200")

# Register our validation function with Tkinter
vcmd = (root.register(check_number), '%P')

# Create the entry box with validation turned on
entry = tk.Entry(root, font=("Arial", 20), validate='key', validatecommand=vcmd)
entry.pack(pady=40)

root.mainloop()

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Tasks

1. Modify the validation rules so that a user can type a number, but limits the entry to a maximum of 2 digits total.

Task 1 Solution

We use len(new_text) to ensure the user cannot type more than 2 characters.

import tkinter as tk

def validate_two_digits(new_text):
    if new_text == "":
        return True  # Allow deleting text completely
    # Check if it is a number AND has 2 or fewer digits
    if new_text.isdigit() and len(new_text) <= 2:
        return True
    else:
        return False  # Reject letters or a 3rd digit

root = tk.Tk()
root.title("2-Digit Validator")
root.geometry("400x200")

vcmd = (root.register(validate_two_digits), '%P')

entry = tk.Entry(root, font=("Arial", 20), validate='key', validatecommand=vcmd)
entry.pack(pady=40)

root.mainloop()

2. Modify the validation rules so that the entry box accepts numbers, but limits the entry to a maximum age of 120.

Task 2 Solution

We can convert new_text into an integer using int() to check if it fits within our 0-120 range.

import tkinter as tk

def validate_age(new_text):
    if new_text == "":
        return True  # Allow deleting text completely
    if new_text.isdigit():
        age = int(new_text)
        # Only allow if the number is between 0 and 120
        if 0 <= age <= 120:
            return True
        else:
            return False
    else:
        return False   # Reject letters

root = tk.Tk()
root.title("Age Validator (0-120)")
root.geometry("400x200")

vcmd = (root.register(validate_age), '%P')

entry = tk.Entry(root, font=("Arial", 20), validate='key', validatecommand=vcmd)
entry.pack(pady=40)

root.mainloop()

3. Modify the validation rules so that the entry box only accepts numbers from -9 to +9.

Task 3 Solution

We allow the user to type a single - or + sign first, and then check if the total value is between -9 and +9.

import tkinter as tk

def validate_range(new_text):
    if new_text == "":
        return True  # Allow deleting text completely
    # Allow just a minus or plus sign so the user can start typing
    if new_text in ["-", "+"]:
        return True
    # Check if the text is a valid number
    try:
        num = int(new_text)
        # Only allow if the number is between -9 and +9
        return -9 <= num <= 9
    except ValueError:
        return False  # Reject letters or invalid symbols

root = tk.Tk()
root.title("Single Digit Validator (-9 to +9)")
root.geometry("400x200")

vcmd = (root.register(validate_range), '%P')

entry = tk.Entry(root, font=("Arial", 20), validate='key', validatecommand=vcmd)
entry.pack(pady=40)

root.mainloop()