Unjustified Screenshot error after closing a Tkinter Toplevel (Linux)

[kelltom]

General information:

• OS name: Ubuntu Linux
• OS version: 22.04
• OS architecture: 64 bits
• Resolutions:
  • Monitor 1: 3440x1440 (issue occurs at all resolutions)
• Python version: 3.10.6
• MSS version: 6.1.0 & 7.0.1

For GNU/Linux users:

• Display server protocol and version, if known: X11 server
• Desktop Environment: ubuntu:GNOME
• Composite Window Manager name and version: X.Org v: 1.21.1.3, compositor: gnome-shell v: 42.5

Description of the warning/error

--- The issue (and solution) is described in detail here ---

In summary, the following error occurs when trying to instantiate an mss object after opening/closing a Tkinter Toplevel window. This error does not occur on Windows.

Full message

  File "/home/my_project/src/utilities/geometry.py", line 64, in screenshot
    with mss.mss() as sct:
  File "/home/my_project/env/lib/python3.10/site-packages/mss/factory.py", line 34, in mss
    return linux.MSS(**kwargs)
  File "/home/my_project/env/lib/python3.10/site-packages/mss/linux.py", line 297, in __init__
    self.root = self.xlib.XDefaultRootWindow(self._get_display(display))
  File "/home/my_project/env/lib/python3.10/site-packages/mss/linux.py", line 184, in validate
    raise ScreenShotError(f"{func.__name__}() failed", details=details)
mss.exception.ScreenShotError: XDefaultRootWindow() failed

Screenshot error: XDefaultRootWindow() failed, {'retval': <mss.linux.LP_XWindowAttributes object at 0x7f31d8d38ac0>, 
    'args': (<mss.linux.LP_Display object at 0x7f31d8d38740>,)}

Other details

The following code can be used to reproduce the error. Simply run this program and use the GUI button to open a new window that allows you to take a screenshot - then close the pop-up window and attempt to repeat the process.

import tkinter as tk

import mss
import mss.tools


def take_screenshot():
   with mss.mss() as sct:
       screen_part = {"top": 370, "left": 1090, "width": 80, "height": 390}
       sct_img = sct.grab(screen_part)
       mss.tools.to_png(sct_img.rgb, sct_img.size, output="./output.png")

def create_top_level_win():
   top_level_win = tk.Toplevel(root)

   take_screenshot_btn = tk.Button(top_level_win, text="Take screenshot", command=take_screenshot)
   take_screenshot_btn.pack()

root = tk.Tk()

btn = tk.Button(root, text="Open TopLevel", command=create_top_level_win)
btn.pack()

root.mainloop()

A temporary workaround is to reuse a single mss object throughout the application, like so:

sct = mss.mss()

def take_screenshot():
    global sct
    screen_part = {"top": 370, "left": 1090, "width": 80, "height": 390}
    sct_img = sct.grab(screen_part)
    mss.tools.to_png(sct_img.rgb, sct_img.size, output="./output.png")

On StackOverflow, one answer suggests that the bug is due to "the current implementation of MSS changing the current X11 error handler and leaving it afterwards, which causes a conflict with Tcl/Tk (the backend of Python Tkinter), see here for details"

[kelltom]

I should also note that it's not as simple as the opening/closing of a Toplevel causing this issue. In a project I'm working on, this issue only occurs the 2nd time I open/close a Toplevel window (see my video linked in the StackOverflow post). The project is quite large so I'll only make effort to create a reproducible example out of it if the fix isn't obvious with the information already provided.

[BoboTiG]

I run the test script on Python 3.11 with MSS 7.0.1, and so far no luck reproducing the problem. Even with the video, it's not really clear how to reproduce 🤔

[BoboTiG]

And sorry for the late answer. I'll try to fix it ASAP so that nobody needs to use hacky code around MSS.

[BoboTiG]

To try making some progress, in you code, when the screenshot fails, can you add a call to sct.get_error_details() and copy/paste the result here?

Something like:

# I see `sct` is global in your project
try:
    # MSS stuff
except mss.ScreenShotError:
    global sct
    details = sct.get_error_details()
    
    import pprint
    pprint.pprint(details)

[BoboTiG]

Better, I pushed some changes on #224. Would you be able to simply try that version, and still print the details, but now as follow:

try:
    # MSS stuff
except mss.ScreenShotError as exc:
    print(exc.details)

🙏🏻 ?

[kelltom]

Hey! Actually, if it makes a difference, my code base is locked to Python 3.10 for the time being. I haven't had a chance to look at it, but do you think your fix would apply for it?

[BoboTiG]

Yes, the fix would be for all supported Python versions :)

[kelltom]

Cool, I'll try it out as soon as I get a chance, thanks!

[BoboTiG]

If #224 may not fix the issue, it will likely fix another issue regarding that any X11 error would not be cleared, and so subsequent legitimate calls to any X11 function would fail. I refactored that part, and I think it will help in your case, and your tests will be critical :)

You should just use the version of MSS from the master branch because I merged the PR.

[kelltom]

I've installed MSS (8.0.0) directly from the master branch and used the code I provided as a reproducible sample and the problem appears to persist.

However, more interestingly, I've also installed it into my large project and reverted my workaround to the code that existed at the time I created my Stack Overflow post, and while I still see the error message appear when I expect it to, my program can recover from it. To better explain, I see the error message when I press the Play button in my program after fiddling with some Toplevel views, but if I just press Play a second time it works perfectly fine. In comparison, when using MSS 7.0.1, my program is unable to continue as soon as I see that error message. It doesn't crash the program, but it is unable to do anything meaningful beyond that point until I restart it.

So, looks like progress, but not quite 100% solved :)

[BoboTiG]

We are moving in the right direction then, thanks for the testing :)

When the error happens, could you print, and copy-paste here, the exception details? And all that is printed in the output, to have a full overview.

[BoboTiG]

I think I've an idea of the root cause, but will need time to investigate, and create a POC. So, your help about giving the details, will be useful 👍🏻

[kelltom]

Will do