Date Tags Python

In a Python command-line interface (CLI) utility, you will want to inform your users about what your program is doing. Your will also want to give your users the ability to tweak how verbose that output is. Now there is a de-facto standard convention for doing that, which most CLIs — Python or otherwise — tend to adhere to:

  • By default, show messages only about errors and warning conditions.
  • Define a -v or --verbose option that makes your program also show messages that are merely informative in nature.
  • Optionally, allow users to repeat the -v option, making the program even more verbose (to include, for example, debug output).
  • Conversely, also define a -q or --quiet (alternatively -s/--silent) option that, when set, makes the program suppress warnings and show only errors — i.e. the stuff that your program shows if it exits with a nonzero exit code.
  • Log output that tells users about what the program is doing, as it goes along, to the standard error (stderr) stream, whereas the output related to the program’s results goes to standard output (stdout). This gives your users the ability to pipe stdout to a file or another program, and your progress or status messages won’t interfere with that.

And in Python it’s not at all difficult to do that!

argparse options

First, we’ll want to define a couple of options for our argparse.ArgumentParser object, which in the following snippet I’ve named parser. Define two options, like so:1

parser.add_argument('-v', '--verbose',
                    action='count',
                    dest='verbosity',
                    default=0,
                    help="verbose output (repeat for increased verbosity)")
parser.add_argument('-q', '--quiet',
                    action='store_const',
                    const=-1,
                    default=0,
                    dest='verbosity',
                    help="quiet output (show errors only)")

From this, we get two command-line options:

  • -v or --verbose, which can be repeated, sets verbosity, which defaults to 0. action='count' means that if you invoke your CLI with -v, verbosity is 1, -vv sets verbosity to 2, etc.

  • -q or --quiet also sets verbosity, but to a constant value, -1, via store_const.

Setting up the logging subsystem

What we’ll want to do is use the logging subsystem to send our status, progress, and error messages to stderr.

First, you can translate verbosity into a logging level understood by the logging module. Here’s a little convenience method that achieves that:

def setup_logging(verbosity):
    base_loglevel = 30
    verbosity = min(verbosity, 2)
    loglevel = base_loglevel - (verbosity * 10)
    logging.basicConfig(level=loglevel,
                        format='%(message)s')

Now what does this do? Python log levels go from 10 (logging.DEBUG) to 50 (logging.CRITICAL) in intervals of 10; our verbosity argument goes from -1 (-q) to 2 (-vv).2 We never want to suppress error and critical messages, and default to 30 (logging.WARNING). So we multiply verbosity by 10, and subtract that from our base loglevel of 30.

With -v, that sets our effective log level to 20 (logging.INFO); with -vv, to 10 (logging.DEBUG). And with -q (i.e. verbosity==-1), our log level becomes 40 (logging.ERROR).

Now we can use logging.basicConfig() to configure the logging subsystem to send unadorned log messages with the desired loglevel or above, to stderr: basicConfig(), by default, sets up a StreamHandler whose output stream is sys.stderr, so it already does what we want here. And setting format='%(message)s' strips the LEVEL:logger: prefix that basicConfig() would otherwise include in the log line (and which is helpful for log files, but not so much for CLI output).

From then on, every time your program should write an informational message to stderr, you just use logging.info(), for a debug message, logging.debug(), and so on.

Adding an environment variable

In some circumstances you might always want debug output, and invoking your CLI with -vv all the time might not be practical. (CI systems are an example — you generally want your build logs as verbose as possible.) You can make your users’ lives easier by optionally fixing up your logging subsystem with an environment variable, like so:

def setup_logging(verbosity):
    base_loglevel = int(os.getenv('LOGLEVEL', 30)) 
    verbosity = min(verbosity, 2)
    loglevel = base_loglevel - (verbosity * 10)
    logging.basicConfig(level=loglevel,
                        format='%(message)s')

This way, if you invoke your CLI with LOGLEVEL=10 in its environment, it will always use debug output.

Perhaps you’d like to make this even easier, allowing your users to also set LOGLEVEL to debug, INFO, erRoR and whatever else. That you could do like this:3

def setup_logging(verbosity):
    base_loglevel = gettattr(logging, 
                             (os.getenv('LOGLEVEL', 'WARNING')).upper()) 
    verbosity = min(verbosity, 2)
    loglevel = base_loglevel - (verbosity * 10)
    logging.basicConfig(level=loglevel,
                        format='%(message)s')

Parting thought

One of the many ways in which using logging comes in handy in a CLI is in a catch-all exception handler:

if __name__ == '__main__':
    try:
        main()
    except Exception as e:
        logging.error(str(e))
        logging.debug('', exc_info=True)
        try:
            sys.exit(e.errno)
        except AttributeError:
            sys.exit(1)

This way, unhandled exceptions will show merely the exception message by default, but if and only if debug logging is enabled, your users will also see a stack trace.


  1. This is used here

  2. There is, to the best of my knowledge, no way to limit the number of repeats for an argument with action='count'. Hence the construct with the min() built-in function. 

  3. A variation of this is used here