Messagebox in ttk.Spinbox bound command causes an infinite stream of pop up windows

Question

I am working on a personal learning project that consists in a simple GUI that generates passwords. The user can either select how many characters of each type (letters, numbers and symbols) to include in the password or simply let the application choose a random combination. There is a password length constraint of 128 characters. What I am trying to do is to show a message when the user tries to increase the number of characters beyond the 128 limit.

What I've tried so far is to include a check in the function bound to the <<Increment>> event that gets called when the increment button in the spinbox is pressed. There is an if statement that checks the available characters left, if the value is zero then a messagebox.showwarnning pops up informing the user that the maximum amount of chars is 128.

The message appears but when the ok button is clicked the message closes just to pop up again and again and the only way to stop it is to kill the whole thing. I have also tried to move the messagebox to the function that is bound to the spinbox command with the same result. Any ideas on how to solve this or what am I doing terribly wrong? Thanks

This is a simplify version of the widget:

from tkinter import *
from tkinter import ttk
from tkinter import messagebox


class App(Tk):
    def __init__(self):
        super().__init__()
        self.title('Sample')
        self.geometry('300x100')
        Selector(self).pack()

class Selector(ttk.Frame):
        def __init__(self, container):
            super().__init__(container)
            self.max_length = 10
            self.input = IntVar(0);

            self.label = ttk.Label(self, text="Quantity: ").pack()
            self.spinbox = ttk.Spinbox(self, from_=0,
                                       to=self.max_length,
                                       textvariable=self.input)
            self.spinbox.pack()
            self.spinbox.bind('<<Increment>>', lambda e: self.check_length(e))

        def check_length(self, e):
            if self.input.get() == 10:
                messagebox.showwarning("Max length reached", "Too much!")
            

        
if __name__ == "__main__":
    app = App()
    app.mainloop()

It would be more helpful to have a minimal example reproducing the issue (see https://stackoverflow.com/help/minimal-reproducible-example) rather than the full code of the widget. — j_4321, Feb 09 '22 at 08:25
Fair enough, that was me being lazy. I edited the post hopefully someone can help me to understand the behavior. Thanks for the suggestion @j_4321. — MrG-O-L, Feb 09 '22 at 10:29
Thanks, it's easier to see what happens now. It seems that closing the messagebox retriggers the `<>` event (it does not happen if I print the message instead). — j_4321, Feb 09 '22 at 10:41

j_4321 · Accepted Answer · 2022-02-09T16:16:42.590

It seems that closing the messagebox retriggers the <<Increment>> event. I think I have found a workaround: the focus is removed from the spinbox after closing the messagebox and the value of the spinbox is checked only if the spinbox has the focus (which means that the user has actually clicked on it).

from tkinter import *
from tkinter import ttk
from tkinter import messagebox


class App(Tk):
    def __init__(self):
        super().__init__()
        self.title('Sample')
        self.geometry('300x100')
        Selector(self).pack()

class Selector(ttk.Frame):
        def __init__(self, container):
            super().__init__(container)
            self.max_length = 10
            self.input = IntVar(self, 0);

            self.label = ttk.Label(self, text="Quantity: ").pack()
            self.spinbox = ttk.Spinbox(self, from_=0,
                                       to=self.max_length,
                                       textvariable=self.input)
            self.spinbox.pack()
            self.spinbox.bind('<<Increment>>', lambda e: self.check_length(e))

        def check_length(self, e):
            if str(self.spinbox) != str(self.focus_get()):
                return
            if self.input.get() == 10:
                messagebox.showwarning("Max length reached", "Too much!")
                self.focus_set()



if __name__ == "__main__":
    app = App()
    app.mainloop()

Note: This does not solve the underlying problem (the fact that <<Increment>> is endlessly triggered after closing the messagebox), it only prevents the endless reopening of the messagebox.

Brilliant! It works. Not sure why the messagebox gets wired like that to the focus event instead to the increment arrow button. Maybe the spinbox was not designed in the first place to do this sort of validation. Thanks. — MrG-O-L, Feb 09 '22 at 12:50
I don't know why this happens, it looks like a bug in tk or tkinter where the content is revalidated somehow after closing the messagebox. I get the same bug when using a trace on the `self.input` variable or when replacing the messagebox by a Toplevel and calling `grab_set()`. — j_4321, Feb 09 '22 at 13:22
Your solution works in the sense that the windows stop popping up, however the `check_length` method keeps getting called in the background. I added to the Selector class a counter, a button bound to a method to print the counter. And one line to increase the counter at the top of the check_length method. I found that after closing the firs pop up the counter keeps increasing. So probably not use the spinbox like this. — MrG-O-L, Feb 09 '22 at 13:50
This is a bug at tkinter or tk level, you should probably report it, the `<>` event is endlessly triggered once you close the messagebox. — j_4321, Feb 09 '22 at 14:24

Messagebox in ttk.Spinbox bound command causes an infinite stream of pop up windows

1 Answers1