"None" in python log-file

Question

I'm using the logging module in python. When I call the script with the wrong arguments on the command-line, the log-file contains at some point the single word "None" and I don't know where it comes from.

Here's my code cutting, where I do the logging.exception:

# Show script where to find blank- and viva-data / set argv

vz = glob.glob("/home/jw/cmp_blank_viva/*csv")
skript, s1, m1 = argv

# Define script-functions and logging-config

logging.basicConfig(level=logging.ERROR, filename='cmp_blank_viva.log', format='%(asctime)s:%(message)s', datefmt='%m/%d/%Y %I:%M:%S %p')

def open_data(vz, s1, m1):

    try:

        checker = []

        for i in vz:
            if "drhot_viva_check_%s" % m1 in i:
                blank_data = csv.reader(open(i, 'r'))
                checker.append(1)
            else:
                pass

        if 1 not in checker:
            logging.exception("Could not open viva file with %s %s.\n" % (s1, m1))

        checker = []

        for i in vz:
            if "hoteldaten_%s_%s" % (m1, s1) in i:
                viva_data = csv.DictReader(open(i, 'r'), delimiter = ';')
                checker.append(1)
            else:
                pass

        if 1 not in checker:
            logging.exception("Could not open blank file with %s %s.\n" % (s1, m1))

        return blank_data, viva_data

    except IOError:
        logging.exception("Could not open file:\n " + traceback.format_exc())
    except IndexError:
        logging.exception("Could not open file:\n " + traceback.format_exc())

After I run the script (with the wrong arguments), the log-file looks like:

Could not open viva file with arg1 arg2.

None
Could not open blank file with arg1 arg2.

None

Any ideas?

Answer 1

You are using logging.exception() when there was no exception:

logging.exception("Could not open viva file with %s %s.\n" % (s1, m1))

Since there is no exception to log, None is added instead. Use logging.error() here to just log a message:

logging.error("Could not open viva file with %s %s", s1, m1)

You don't need to interpolate the string elements, logging does this for you when needed. I also removed the \n newline, that too is added for you.

Elsewhere you are including the exception manually when you are in a exception handler:

logging.exception("Could not open file:\n " + traceback.format_exc())

You don't need to append the traceback manually, logging.exception() handles this for you and includes the traceback, properly formatted.

Just log the message, nothing more:

logging.exception("Could not open file")

From the logging.exception() documentation:

Exception info is always added to the logging message. This method should only be called from an exception handler.

Bold emphasis mine; you used it outside an exception handler so no exception was available to be logged.

It does so by setting the exc_info keyword argument for you:

• exc_info* which, if it does not evaluate as false, causes exception information to be added to the logging message. If an exception tuple (in the format returned by sys.exc_info()) is provided, it is used; otherwise, sys.exc_info() is called to get the exception information.