[XServer] Library makes program hard crash when monitor is out of bounds

[RedFantom]

General information:

• OS name: Ubuntu (ubuntu-desktop)
• OS version: 18.04 LTS
• OS architecture: 64 bits
• Resolutions:
  • Monitor 1: 1920x1080 (laptop built-in)
  • Monitor 2: 1366x768 (external)
• Python version: 3.6.5
• MSS version: 3.2.0

For GNU/Linux users:

• Display server protocol and version, if known: X server (X.Org version: 1.19.6)
• Desktop Environment: Gnome 3
• Composite Window Manager name and version: ??? (The default one, I guess?)

Description of the warning/error

This error occurs when the a monitor that is out of bounds is specified in the grab() call.

from mss import mss

screenshotter = mss()
monitor = {
    "left": -30,
    "top": 0,
    "width": 100,
    "height": 100,
}
try:
    screenshotter.grab(monitor)  # X Error
except BaseException as e:
    # Never reached
    print("Handling error")

Using a monitor like {"left": 0, "top": 0, "width": 10000, "height": 100} has the same result.

Full message

X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  73 (X_GetImage)
  Serial number of failed request:  91
  Current serial number in output stream:  91

The message is printed to stderr, then the Python interpreter quits with exit code 1.

Other details

I discovered this because I let my program generate a box based on the location of the cursor. Then I take the screenshot of only the cursor area.

While it might be better for efficiency not to check the bounds of the monitor, I personally don't think causing this type of hard crash is acceptable. It's up to you. If you decide not to check bounds, I think a mention of this in the documentation would be appropriate.

[BoboTiG]

Thank you for the report.

You are totally right that this kind of crash is not acceptable. It will be fixed in the next release: the error will raise a specific exception instead of crashing the interpreter.