Allowing only a range of inputs within an entry field in Tkinter

Question

I need to implement an entry box that accepts only a range of DoubleVar values. I have referenced this question that was asked How to only allow certain parameters within an entry field on Tkinter, but I want the user to be notified (using a change in font colour or anything) while they are entering the values. I've read the documentation but this is something that I haven't come across. I'm new to Tkinter, so please excuse me if this sounds very stupid

Answer 1

You can bind KeyRelease event of the Entry to a callback and check whether the input value is valid and within the required range, then update the foreground color of the Entry accordingly:

import tkinter as tk

root = tk.Tk()

def check_value(entry, min_value, max_value):
    try:
        value = float(entry.get().strip())
        valid = min_value <= value <= max_value
    except ValueError:
        valid = False
    entry.config(fg='black' if valid else 'red')
    return valid  # in case you want the checking result somewhere else

entry = tk.Entry(root)
entry.pack()
entry.bind('<KeyRelease>', lambda e: check_value(e.widget, 10, 20))

root.mainloop()

Answer 2

Use the validatecommand option for the Entry.

Some code fragments to show how it works:

root = tk.Tk()

vcmd = root.register(is_number)

e = ttk.Entry(pressf, justify='right', validate='key', validatecommand=(vcmd, '%P'))

def is_number(data):
    """Validate the contents of an entry widget as a float."""
    if data == '':
        return True
    try:
        rv = float(data)
        if rv < 0:
            return False
    except ValueError:
        return False
    return True

This basically calls the validation function on every keypress. Only if the validation succeeds is the character added to the entry.

You can find a complete working example here.

Edit

Above is the "canonical" example of a validator. It allows or disallows characters into the Entry.

But you can also use it in other ways. For example, you can always return True, but e.g. change the text color of the Entry to red if the value is not within the limits you want.