*args and **kwargs in Python, with Examples

When you start with a programming language, like Python, we usually learn it hands-on (by programming), skipping over the theory of the language itself. One of the things we all overlook when starting to learn Python, and one of the most puzzling, is functions (or methods) that take *args and **kwargs as parameters. To understand how they work, you need to be clear on the concepts of List and Dictionary in Python.

Note that the syntax is * and **, while the names args and kwargs are just a convention and not a strict requirement; that is, if we pass a function *my_list or **my_dictionary as an argument, it will have the same effect.

*args is used when you are not sure how many arguments will need to be passed to the function; that is, it lets you pass an arbitrary number of arguments. Let’s look at an example:

def read_list_args(*args):
    for count, arg in enumerate(args):
        print '%d - %s' % (count, arg)

print 'First'
read_list_args('Ricardo', 'jarroba.com')

print 'Second'
read_list_args('Ricardo', 23, 'Ramon', [1, 2, 3], 'jarroba.com')

As we can see, we call the function with a different number of arguments, and the output of the function is the following:

First
0 - Ricardo
1 - jarroba.com
Second
0 - Ricardo
1 - 23
2 - Ramon
3 - [1, 2, 3]
4 - jarroba.com

Similarly, when you don’t know how many arguments a function is going to receive, **kwargs accepts arguments for parameters with names that haven’t been defined beforehand. Let’s look at an example:

def read_dict_args(**kwargs):
    for key, value in kwargs.items():
        print '%s - %s' % (key, value)

print 'First'
read_dict_args(name1='Ricardo', name2='Ramon', web='jarroba.com')

print 'Second'
read_dict_args(Team='FC Barcelona', player='Iniesta', demarcation='Right winger', number=8)

The output of this code is the following:

First
web - jarroba.com
name2 - Ramon
name1 - Ricardo
Second
player - Iniesta
number - 8
demarcation - Right winger
Team - FC Barcelona

Both * and ** can be used together and alongside other parameters defined in the function, but in this case order does matter. First come the parameters defined for the function, second *args, and third **kwargs:

def do_something(a, b, *args, **kwargs):

def something(parameter1, parameter2):
...

something(parameter1=argument1, parameter2=argument2)

References: