What do *args and **kwargs mean?


Question

What exactly do *args and **kwargs mean?

According to the Python documentation, from what it seems, it passes in a tuple of arguments.

def foo(hello, *args):
    print hello

    for each in args:
        print each

if __name__ == '__main__':
    foo("LOVE", ["lol", "lololol"])

This prints out:

LOVE
['lol', 'lololol']

How do you effectively use them?

1
263
5/14/2013 10:55:11 AM

Accepted Answer

Putting *args and/or **kwargs as the last items in your function definition’s argument list allows that function to accept an arbitrary number of arguments and/or keyword arguments.

For example, if you wanted to write a function that returned the sum of all its arguments, no matter how many you supply, you could write it like this:

def my_sum(*args):
    return sum(args)

It’s probably more commonly used in object-oriented programming, when you’re overriding a function, and want to call the original function with whatever arguments the user passes in.

You don’t actually have to call them args and kwargs, that’s just a convention. It’s the * and ** that do the magic.

The official Python documentation has a more in-depth look.

249
3/30/2013 10:48:15 PM

Also, we use them for managing inheritance.

class Super( object ):
   def __init__( self, this, that ):
       self.this = this
       self.that = that

class Sub( Super ):
   def __init__( self, myStuff, *args, **kw ):
       super( Sub, self ).__init__( *args, **kw )
       self.myStuff= myStuff

x= Super( 2.7, 3.1 )
y= Sub( "green", 7, 6 )

This way Sub doesn't really know (or care) what the superclass initialization is. Should you realize that you need to change the superclass, you can fix things without having to sweat the details in each subclass.


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