Finding the source of format errors when using python logging

Rather than editing installed python code, you can also find the errors like this:

    def handleError(record):
        raise RuntimeError(record)
    handler.handleError = handleError

where handler is one of the handlers that is giving the problem. Now when the format error occurs you'll see the location.

The logging module is designed to stop bad log messages from killing the rest of the code, so the emit method catches errors and passes them to a method handleError. The easiest thing for you to do would be to temporarily edit /usr/lib/python2.6/logging/__init__.py, and find handleError. It looks something like this:

def handleError(self, record):
    """
    Handle errors which occur during an emit() call.

    This method should be called from handlers when an exception is
    encountered during an emit() call. If raiseExceptions is false,
    exceptions get silently ignored. This is what is mostly wanted
    for a logging system - most users will not care about errors in
    the logging system, they are more interested in application errors.
    You could, however, replace this with a custom handler if you wish.
    The record which was being processed is passed in to this method.
    """
    if raiseExceptions:
        ei = sys.exc_info()
        try:
            traceback.print_exception(ei[0], ei[1], ei[2],
                                      None, sys.stderr)
            sys.stderr.write('Logged from file %s, line %s\n' % (
                             record.filename, record.lineno))
        except IOError:
            pass    # see issue 5971
        finally:
            del ei

Now temporarily edit it. Inserting a simple raise at the start should ensure the error gets propogated up your code instead of being swallowed. Once you've fixed the problem just restore the logging code to what it was.

It's not really an answer to the question, but hopefully it will be other beginners with the logging module like me.

My problem was that I replaced all occurrences of print with logging.info , so a valid line like print('a',a) became logging.info('a',a) (but it should be logging.info('a %s'%a) instead.

This was also hinted in How to traceback logging errors? , but it doesn't come up in the research