How to validate tkinter entry

Question

I'm working on a tkinter app that has an entry widget and a method to validate it whenever the user types something in it. the problem is that the <Key> binding runs the validate method before self.entry_str is updated so it tests what was previously in the entry. what I want it to do is run the validate method after self.entry_str is updated, anyone know how I could do this? here's the simplified program:

import sys
import tkinter as tk


class GUI:
    def __init__(self, master):
        # frame
        self.frame = tk.Frame(master)
        self.frame.pack()
        # StringVars
        self.error = tk.StringVar()
        self.error.set('')
        self.entry_str = tk.StringVar()
        self.entry_str.set('')
        # widgets
        self.entry = tk.Entry(self.frame, textvariable=self.entry_str)
        self.entry.bind('<Key>', lambda _: self.validate_entry())
        self.entry.grid(row=0, column=0)
        self.error_label = tk.Label(self.frame, textvariable=self.error, fg='red')
        self.error_label.grid(row=1, column=0)
        self.bttn = tk.Button(self.frame, text='continue', command=sys.exit)
        self.bttn.grid(row=2, column=0)

    def validate_entry(self):
        try:
            _ = int(self.entry_str.get())
        except ValueError:# entry is not valid: disable button and show error
            self.error.set('entry has to be an integer')
            self.bttn.config(state='disabled')
        else:# entry is valid: enable button and hide error
            self.error.set('')
            self.bttn.config(state='normal')


root = tk.Tk()
gui = GUI(root)
root.wm_title('entry validation test')
root.mainloop()

score 2 · Accepted Answer · answered Mar 27 '21 at 15:17

Use <KeyRelease> instead of <Key>

A better way would be to use .trace

self.entry_str = tk.StringVar()
self.entry_str.set('')
self.entry_str.trace('w', self.validate_entry)

...

def validate_entry(self, *event):
        try:
            _ = int(self.entry_str.get())
         ...

An alternative is to disable the user from entering anything but integer.

self.entry = tk.Entry(self.frame, textvariable=self.entry_str, validate='key', validatecommand=(master.register(self.validate), "%P"))
...
def validate(self, char):  # this function must return only True or False
        return char.isdigit() or char==''

_{Do note self.entry_str will always be up-to date}

@TheLizzard unless you have non-digit in the clipboard the paste operation will work fine — JacksonPro, Mar 27 '21 at 15:27

TheLizzard · Answer 2 · 2021-03-27T19:00:11.553

You have 2 options. First one: save the entry data before the user inputs the data and restore the data if needed like this:

import sys
import tkinter as tk


class GUI:
    def __init__(self, master):
        # frame
        self.frame = tk.Frame(master)
        self.frame.pack()
        # StringVars
        self.error = tk.StringVar()
        self.error.set('')
        self.entry_str = tk.StringVar()
        self.entry_str.set('')
        # widgets
        self.entry = tk.Entry(self.frame, textvariable=self.entry_str)
        self.entry.bind('<Key>', self.validate_entry)
        self.entry.grid(row=0, column=0)
        self.error_label = tk.Label(self.frame, textvariable=self.error, fg='red')
        self.error_label.grid(row=1, column=0)
        self.bttn = tk.Button(self.frame, text='continue', command=sys.exit)
        self.bttn.grid(row=2, column=0)

        self.current_entry_state = ""
        self.error_occured = False

    def validate_entry(self, event):
        current_entry_state = self.entry_str.get()
        # Save the state only if there are no invalid characters:
        try:
            _ = int(self.entry_str.get())
            self.current_entry_state = current_entry_state
            self.error_occured = False
        except:
            self.error_occured = True
        # Wait for tkinter to display the character
        self.entry.after(1, self._validate_entry)

    def _validate_entry(self):
        try:
            _ = int(self.entry_str.get())
        except ValueError:# entry is not valid: disable button and show error
            self.error.set('entry has to be an integer')
            # Restore the entry's text:
            self.entry.delete("0", "end")
            self.entry.insert("end", self.current_entry_state)
        else:
            if not self.error_occured:# entry is valid: enable button and hide error
                self.error.set('')


root = tk.Tk()
gui = GUI(root)
root.wm_title('entry validation test')
root.mainloop()

There is another method (which isn't documented very well) but look at this and this.

How to validate tkinter entry

2 Answers2