In this tutorial we are going to learn about args and kwargs in Python with Syntax and Examples.

args and kwargs

Work with Python long enough, and eventually you will encounter args and kwargs. These strange terms show up as parameters in function definitions.

What do they do?

Let’s review a simple function:

def myfunc(a,b):
    return sum((a,b))*.05

myfunc(40,60)
5.0

This function returns 5% of the sum of a and b. In this example, a and b are positional arguments; that is, 40 is assigned to a because it is the first argument, and 60 to b. Notice also that to work with multiple positional arguments in the sum() function we had to pass them in as a tuple.

What if we want to work with more than two numbers? One way would be to assign a lot of parameters, and give each one a default value.

def myfunc(a=0,b=0,c=0,d=0,e=0):
    return sum((a,b,c,d,e))*.05

myfunc(40,60,20)
6.0

Obviously this is not a very efficient solution, and that’s where *args comes in.

args

When a function parameter starts with an asterisk, it allows for an arbitrary number of arguments, and the function takes them in as a tuple of values. Rewriting the above function:

def myfunc(*args):
    return sum(args)*.05

myfunc(40,60,20)
6.0

Notice how passing the keyword “args” into the sum() function did the same thing as a tuple of arguments.

It is worth noting that the word “args” is itself arbitrary - any word will do so long as it’s preceded by an asterisk. To demonstrate this:

def myfunc(*spam):
    return sum(spam)*.05

myfunc(40,60,20)
6.0

kwargs

Similarly, Python offers a way to handle arbitrary numbers of keyworded arguments. Instead of creating a tuple of values, kwargs builds a dictionary of key/value pairs.

For example:

def myfunc(**kwargs):
    if 'fruit' in kwargs:
        print(f"My favorite fruit is {kwargs['fruit']}")  # review String Formatting and f-strings if this syntax is unfamiliar
    else:
        print("I don't like fruit")
        
myfunc(fruit='pineapple')
My favorite fruit is pineapple
myfunc()
I don't like fruit

args and kwargs combined You can pass args and kwargs into the same function, but args have to appear before kwargs

def myfunc(**args**, **kwargs**):
    if 'fruit' and 'juice' in kwargs:
        print(f"I like {' and '.join(args)} and my favorite fruit is {kwargs['fruit']}")
        print(f"May I have some {kwargs['juice']} juice?")
    else:
        pass
        
myfunc('eggs','spam',fruit='cherries',juice='orange')
I like eggs and spam and my favorite fruit is cherries
May I have some orange juice?

Placing keyworded arguments ahead of positional arguments raises an exception:

myfunc(fruit='cherries',juice='orange','eggs','spam')
  File "<ipython-input-8-fc6ff65addcc>", line 1
    myfunc(fruit='cherries',juice='orange','eggs','spam')
                                          ^
SyntaxError: positional argument follows keyword argument

As with “args”, you can use any name you’d like for keyworded arguments - “kwargs” is just a popular convention.

That’s it! Now you should understand how *args and **kwargs provide the flexibilty to work with arbitrary numbers of arguments!