[HOW-TO] Is there an exception hook to catch exceptions during capture

[Elaschatz]

Is there an exception hook to catch exceptions during capture?

I have a program, which starts a recording via a method like this:

def startVideo(self):
         self.isRecording=True
         #if the outdirectory does not exist, create it
         if not os.path.exists(self.out_directory):
             os.makedirs(self.out_directory)
         self.currentFilename=self.generateFilename()
         self.encoder.output = FileOutput(self.out_directory+"/"+self.currentFilename+".h264")
         self.picam2.start_encoder() 

When I want to stop the recording, I use the following function

def stopVideo(self):
         self.isRecording=False
         self.picam2.stop_encoder()
         self.currentFilename= None 

This all works fine. As I write the videos to a USB stick, sometimes there is no space left on this device and an exception is raised during recording:

Traceback (most recent call last):
  File "/usr/lib/python3.9/threading.py", line 954, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.9/threading.py", line 892, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3/dist-packages/picamera2/encoders/v4l2_encoder.py", line 188, in thread_poll
    self.outputframe(b, keyframe, (buf.timestamp.secs * 1000000) + buf.timestamp.usecs)
  File "/usr/lib/python3/dist-packages/picamera2/encoders/encoder.py", line 214, in outputframe
    out.outputframe(frame, keyframe, timestamp)
  File "/usr/lib/python3/dist-packages/picamera2/outputs/fileoutput.py", line 88, in outputframe
    self._write(frame, timestamp)
  File "/usr/lib/python3/dist-packages/picamera2/outputs/fileoutput.py", line 118, in _write
    self._fileoutput.flush()
OSError: [Errno 28] No space left on device

Now my Question.

Is there any option to catch this exception?

At the moment the exception is printed to the console and I have no possibility to catch it with a try ...except. The python script hangs up, and this is not good, as it is a longtime surveillance and the console is not monitored.

I read about excepthook in the threading module. Is there anything similar which I can use?

[davidplowman]

Hi, so far as I understand it you can use threading.excepthook and when the output thread signals an exception your hook should be called. Could you try if this works?

[Josee-xav]

I can confirm that threading.excepthook works but this method sucks as theirs no proper way of handling the issue thats causing the exception as u cannot pass the exception down to the calling/main thread. Unless their is a way then i would love to know.

[davidplowman]

Hi, and thanks for the comment. I'm not really too sure what the right answer is here (or if there is one). Can you explain a bit more about what you think should happen?

AIUI, one of the background threads, that's writing out the encoded video frames, is throwing an exception. Might there be a solution where you could provide a callback to the output object which is invoked whenever an error occurs? There's already something like that in the FfmpegOutput which lets the user set an error_callback, but maybe we could extend it to other outputs? Which one are you using, by the way?

Or if you have any other suggestions, I'd be interested in those too. Thanks!