Modifying or passing some argument to `logger.catch` to *only* log the exception message, rather than the full traceback?

[svioletg]

My searching through the docs and issues didn't lead me to much, but it's always possible I've just missed something, so I apologize in advance if that has been asked before. In most cases I have no issue with having the logger.catch decorator print out a full traceback, but right now I'm working on a GUI version of a script I'm writing, and within the GUI there's a "console window" which I have another log handler (in addition to the previous two sys.stdout and log file handlers already present) set up to route log messages to a callback that tacks on the message to this area, so relevant messages can be displayed in the app itself.

Tracebacks for this project can get particularly large since there's a number of things working in tandem here, and for that handler specifically I wanted to see if there was possibly some way to have logger.catch still log the full trace as normal to stdout and the current log file, but for that GUI-related hander, I'd prefer it only showing the final exception message.

For example, as it is now:

ERROR: An error has been caught in function 'run', process 'MainProcess' (38996), thread 'Thread-2 (create_image)' (59604):
Traceback (most recent call last):
  File "C:\Users\Violet\Documents\Code\Python\py-squaremap-combiner\.venv\Lib\site-packages\PIL\JpegImagePlugin.py", line 639, in _save
    rawmode = RAWMODE[im.mode]
KeyError: 'RGBA'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "C:\Users\Violet\AppData\Local\Programs\Python\Python312\Lib\threading.py", line 1030, in _bootstrap
    self._bootstrap_inner()
  File "C:\Users\Violet\AppData\Local\Programs\Python\Python312\Lib\threading.py", line 1073, in _bootstrap_inner
    self.run()
> File "C:\Users\Violet\AppData\Local\Programs\Python\Python312\Lib\threading.py", line 1010, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\Violet\Documents\Code\Python\py-squaremap-combiner\src\squaremap_combine\gui\actions.py", line 182, in create_image
    result.img.save(out_file)
  File "C:\Users\Violet\Documents\Code\Python\py-squaremap-combiner\.venv\Lib\site-packages\PIL\Image.py", line 2568, in save
    save_handler(self, fp, filename)
  File "C:\Users\Violet\Documents\Code\Python\py-squaremap-combiner\.venv\Lib\site-packages\PIL\JpegImagePlugin.py", line 642, in _save
    raise OSError(msg) from e
OSError: cannot write mode RGBA as JPEG

...versus what my goal would be:

ERROR: OSError: cannot write mode RGBA as JPEG

Is this something I'd be able to set up either in the handler itself, or some argument I can pass to catch()? For reference, my handlers are being added like so:

stdout handler:

stdout_handler = logger.add(sys.stdout, colorize=True, format="<level>[{time:HH:mm:ss}] {level}: {message}</level>", level=stdout_level, diagnose=False)

Log file handler:

file_handler = logger.add(LOGS_DIR / '{time:YYYY-MM-DD_HH-mm-ss}.log', format="[{time:HH:mm:ss}] {level}: {message}", level='DEBUG', mode='w', retention=5, diagnose=False)

GUI "console" updater handler:

logger.add(actions.update_console, format='<level>{level}: {message}</level>', level='GUI_COMMAND')

[sillydan1]

I found that you can do something along the lines of:

def handler(e: BaseException):
    logger.error(e)
    sys.exit(1)

@logger.catch(level="DEBUG", onerror=handler)
def main():
    a = 1 / 0

That way, if you set the verbosity level of your logger to DEBUG or more, it will print the stacktrace as a debug message and the error message is logged as an error (assuming you dont raise any errors before setting the verbosity level).

[Delgan]

Although you can reduce the verbosity of traceback formatting with diagnose=False and backtrace=False, you can also disable it entirely. To do this, you need to remove the “{exception}” field from the format used by your handler.

For convenience, "\n{exception}" is automatically added at the end of the format string-parameter by default. In your case, you have to use a custom formatter function to prevent that. In this function, you can replace the "{exception}" with a custom field, such as the error message alone.

For example:

from loguru import logger
import sys

def formatter(record):
    base = "{time} {level} {message}\n"
    if record["exception"] is not None:
        error_type, error_message, _ = record["exception"]
        record["extra"]["error"] = f"{error_type.__name__}: {error_message}"
        base += "{extra[error]}\n"
    return base

@logger.catch
def main():
    logger.remove()
    logger.add(sys.stderr)  # Full traceback
    logger.add("file.log", format=formatter)  # Only error message

    logger.info("Hello")

    1 / 0  # Oops!

if __name__ == "__main__":
    main()

The suggestion from @sillydan1 is another elegant solution.