Using *args and **kwargs in python

please read the update to this post : Using *args, reloaded!

So we’ve been actively working on our latest project in django – and I came across this weird syntax in views
def view_function(request, *args, **kwargs)

Digging in further, I found that *args stands for argument list, and **kwargs stands for keyword argument list ( well, only the asterik and double asterik matters, the names can be anything ). So, why is this useful again?

For example:

def foo(*args):
  for i, arg in enumerate(*args):
    print "Argument ", i, " : ", arg

and call it by

foo("a", "b", "c")

Simply put, we now have a way to call a function with arbitrary number of parameter. The function foo can be passed with any number of arguments.

So, that’s about *args. Now what is **kwargs? It is the keyword argument list – which means you can pass the keywords as well as their values as a dictionary.

For example:

class Foo(object):
  def __init__(self, value):
    print value

class DerivedFoo(Foo):
  def __init__(self, *args, **kwargs):
    print 'DerivedFoo'
    super(DerivedFoo, self).__init__(*args, **kwargs)

myFoo = DerivedFoo("Calling Foo through DerivedFoo")

The super keyword can be used to call the methods of superclass – in this case the init of class Foo. The parameters which are passed to inherited class can be passed to the base class as shown above. This can be used to extend the behavior of the base class, without knowing anything about base class.


2 thoughts on “Using *args and **kwargs in python

  1. Nice little post. The last bit would probably be a bit clearer if you did something like:
    myFoo = DerivedFoo(“Calling Foo through DerivedFoo”). This will output:
    Calling Foo through DerivedFoo
    Basically, you are calling the constructor of the super class of DerivedFoo, which is Foo. You do this by creating variable length placeholders for any arguments or keyword arguments that super might have. I guess this is a way of subclassing a class in which the subclass doesn’t need to know anything about the number or type of arguments the constructor needs.

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