Screen reconfiguration not handled correctly

[andreasbuhr]

General informations:

• OS name: Linux Mint
• OS version: xenial
• OS architecture: 64 bit
• Resolutions:
  • Monitor 1: 1920x1080
  • Monitor 2: 1920x1080
  • Monitor 3: 1920x1080
• Python version: 2.7.12
• MSS version: 3.3.1

Description of the warning/error

1. start a python script while only one monitor connected
2. take a screenshot
3. connect more monitors
4. take a screenshot of monitor "-1"
5. only the monitor attached when taking the first screenshot is screenshoted.

Full message

no message

Other details

The reason is that the "_monitors" is a class varable of MSSBase

python-mss/mss/base.py

Line 19 in f3c83cf

_monitors = [] # type: List[Dict[str, int]]

and it is only initialized once per script run:

python-mss/mss/linux.py

Line 347 in f3c83cf

if not self._monitors:

One possible solution is to turn "_monitors" into a member variable.

[BoboTiG]

Hello,

Thanks for the report and analysis!

Turning _monitors to a member variable would slow the screen capture by a too important factor. An idea would be to regularly set sct._monitor = [] to force an update. But this will need to be done outside MSS, in your code.

If we could set a hook to know when a monitor is added or removed, it would be the best way: efficient and easy. I do not know of such API but some research will have to be done.

Do you have time to investigate? The process can be only related to one OS, others will eventually come later.

[andreasbuhr]

What about having the monitor detection in the MSS constructor? That way it is not done in every screenshot, but only when the MSS object is created? That way users could decide how often monitor detection is done, so to have a script taking screenshots every minute and doing monitor detection every time, one would have:

while True:
    time.sleep(60)
    with mss() as sct:
        sct.shot()

and for low overhead one would use:

with mss() as sct:
    while True:
        time.sleep(1)
        sct.shot()

Would that be a solution?

[BoboTiG]

Hm this is already the case :)
The first snippet will detect new monitors as expected because when the context manager ends, it will freed resources.

[andreasbuhr]

I'm sorry, but it is not the case. You are mistaken.

The first snippet will not detect new monitors as expected, because _monitors is a class variable. It is a member of the class, not a member of the object. So _monitors will persist until the end of the interpreter run, unless it is deleted explicitly. MSSBase.__exit__ does not delete it:

python-mss/mss/base.py

Line 27 in f3c83cf

def __exit__(self, *_):

Inserting a _monitors = [] into MSSBase.__exit__ would solve this problem.

If you like I could prepare a pull request.

[BoboTiG]

Good catch, I stand corrected.

I would be very happy to merge your PR :)
Could you add a simple test to validate the fix also?

[BoboTiG]

Release 3.3.2 is published with your patch :)

[andreasbuhr]

amazing!!1!! :-)