[Tkinter-discuss] Exception protection to tk calls

[Vasilis Vlachoudis]

Thank you Michael,

>> grab_release() on a widget that has already been destroyed or, in the/

this is also my understanding, however the places that happens are on
Dialog code(s) like

import tkinter as tk

class Dialog(tk.Toplevel):
	width = -1
	height = -1
	# ----------------------------------------------------------------------
	def __init__(self, master, **kw):
		super().__init__(master, class_="MyDialog", **kw)
		self.transient(master)
		self.title("My title")
		frame = tk.Frame(self)
		frame.pack(side=tk.TOP, fill=tk.BOTH)
		# ... various widget+buttons creation
		#...
		#...
		self.protocol("WM_DELETE_WINDOW", self.close)

		if Dialog.width>0:
			self.geometry("%dx%d"%(Dialog.width, Dialog.height))
		self.wait_visibility()
		self.focus_set()
		self.grab_set()
		self.wait_window()

	#-----------------------------------------------------------------------
	def close(self, event=None):
		Dialog.width  = self.winfo_width()
		Dialog.height = self.winfo_height()
		self.grab_release()
		self.destroy()

My impression is that on slow network connections (using remote X11 session),
sometimes the dialog is displayed before the wait_visibility is being executed and
if the user closes the window from the window manager, it arrives to the focus_set,
grab_set or wait_window with a window that doesn't exist and then I get the error message.

I've once got also the TclError on the "frame = tk.Frame(self)" command

Maybe the self.geometry() is the culprit?

This types of errors are not something frequent. They are rare, but they happen. We have
several thousands active users around the world and I get an automatic report maybe once
every a couple of weeks.

Vasillis

________________________________________
From: Tkinter-discuss [tkinter-discuss-bounces+vasilis.vlachoudis=cern.ch at python.org] on behalf of Michael Lange [klappnase at web.de]
Sent: Wednesday, March 31, 2021 10:10
To: tkinter-discuss at python.org
Subject: Re: [Tkinter-discuss] Exception protection to tk calls

Hi,

On Wed, 31 Mar 2021 06:53:26 +0000
Vasilis Vlachoudis <Vasilis.Vlachoudis at cern.ch> wrote:

> Is there any recommendation which tkinter calls have to be protected
> with an exception handling I occasionally receive traceback errors from
> my users on various calls like: grab_current, grab_release, focus_get
> most of the times I protect them with with a TclError handler catching
> exceptions like_tkinter.TclError: bad window path name
>
> Also which type of exception has to be protected, this is the first
> time I've got a KeyError from a tkinter call, which normally should be
> innocent focus_get()"

it is a bit hard to guess what's going on without a real code example.
It seems like you get these errors when either calling for example
grab_release() on a widget that has already been destroyed or, in the
last example, calling focus_get() while a messagebox is being displayed
(not sure how this can actually happen in real-world code, I was only
able to reproduce that error message with a non-sensical example, like

>>> def foo():
...     root.after(1000, lambda: print(root.focus_get()))
...     messagebox.showinfo()
...
) .

I know that this is not really an answer to your question, but my guess
is that all of these situations may actually be caused by programming
errors which, once they have been discovered, should rather be fixed
properly than worked around with a try-except clause.

For example, if the window on which grab_release() is called might no
longer exist, something like

if widget.winfo_exists():
    widget.grab_release()

might do the trick.

If you want to use try-except protection anyway, I don't think that there
is any general rule which methods should be protected with which
exception type. I think the only way to find out is to examine the
tkinter code and find out for yourself which exception(s) are likely to
occur. For example with focus_get() I think the KeyError caused by
nametowidget() is probably the only exception one might expect.
Of course one could always play it safe with something like:

try:
    focus = self.focus_get()
except:
    focus = None

Best regards

Michael


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