Python and Arbitrary Function Arguments – **kwargs

Python has a pretty useful policy: named arguments. When you call a function, you can explicitly say that such-and-such value is what you’re providing for a particular argument, and can even include them in any order:

def hello(first, last):
    print 'Hello %s %s' % (first, last)
    hello(last='Lecocq', first='Dan')

In fact, you can programmatically gain insight into functions with the . But suppose you want to be able to accept an arbitrary number of parameters. For example, for a printf equivalent. Or where I encountered it in wanting to read a module name from a configuration file, as well as the arguments to instantiate it. In this case, you’d get the module and class as a string and then a dictionary of the arguments to make an instance of it. Of course, Python always has a way. In this case, **kwargs.

This is actually dictionary unpacking, taking all the keys in a dictionary and mapping them to argument names. For example, in the above example, I could say:

hello(**{'last':'Lecocq', 'first':'Dan'})

Of course, in that case it’s a little verbose, but if you’re getting a dictionary of arguments programmatically, then it’s invaluable. But wait, there’s more! Not only can you use the **dict operator to map a dictionary into parameters, but you can accept arbitrary parameters with it, too!

def kw(**kwargs):
    for key, value in kwargs.items():
        print '%s => %s' % (key, value)

kw(**{'hello':'Howdy!', 'first':'Dan'})
kw(hello='Howdy!', first='Dan')

Magic! No matter how you invoke the function, it has access to the parameters. You can even split the difference, making some parameters named and some parameters variable. For example, if you wanted to create an instance of a class that you passed a name in for, initialized with the arguments you give it:

def factory(module, cls, **kwargs):
    # The built-in __import__ does just what it sounds like
    m = __import__(module)
    # Now get the class in that module
    c = getattr(m, cls)
    # Now make an instance of it, given the args
    return c(**kwargs)  

factory('datetime', 'datetime', year=2011, month=11, day=8)
factory('decimal', 'Decimal', value=7)

This is one place where Python’s flexibility is extremely useful.

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