Fun With **kwargs

Ever want to write functions that flexibly receive arguments? What about getting defaults from a configuration on the module level, then have those defaults automatically used by functions in your module?

Well kwargs might be just what you need. “kwarg” is a nickname for “keyword arguments”, which you’re likely already familiar with in a format like this:

def func(first='ask questions', second='shoot'):
    print '{0} first'.format(first)
    print '{0} second'.format(second)

if __name__ == '__main__':
    func()

Let’s try it out!

$ python test.py
ask questions first
shoot second

Now that’s all well and good, of course, but what if you you need to set those defaults on the fly, say, from a config file that your program reads on startup. Let’s say your config file looks like this.

[Defaults]
first = shoot
second = ask_questions

And this is the code that will read the config.

from ConfigParser import ConfigParser
conf = ConfigParser()
conf.read('test.cfg')
conf_dict = dict(conf.items('Defaults'))


def func(**kwargs):
    # the name selected for kwargs is not important, what's special is the **
    # any arguments we need that weren't in kwargs, get them from conf_dict
    kwargs.update(conf_dict)
    print '{0} first'.format(kwargs['first'])
    print '{0} second'.format(kwargs['second'])

if __name__ == '__main__':
    func()
    print 'maybe something more conservative...'
    func(second='more questions', first='ask questions')

The program now gives different output.

$ python with_kwargs.py
shoot first
ask questions second
maybe something more conservative...
ask questions first
more questions second

An alternative to having default arguments from a config file would be something like this. The values remain hardcoded, unfortunately, but it allows you to have defaults for kwargs.

def func(**kwargs):
    # the .get() function takes a second argument, which is returned by default
    # if there is no value for that key
    print '{0} first'.format(kwargs.get('first', 'shoot'))
    print '{0} second'.format(kwargs.get('second', 'ask questions'))

So now we have a program that does a similar thing to our original, but none of the defaults are hardcoded (except the config file, but let’s ignore that).

The uses of **kwargs go beyond this, of course, they’re great for making extremely flexible functions that are tolerant to non-consistent use.