In function definitions, parameters are named entities that specify an argument that a given function can accept.
When programming, you may not be aware of all the possible use cases of your code, and may want to offer more options for future programmers working with the module, or for users interacting with the code. We can pass a variable number of arguments to a function by using *args
and **kwargs
in our code.
You should have Python 3 installed and a programming environment set up on your computer or server. If you don’t have a programming environment set up, you can refer to the installation and setup guides for a local programming environment or for a programming environment on your server appropriate for your operating system (Ubuntu, CentOS, Debian, etc.)
In Python, the single-asterisk form of *args
can be used as a parameter to send a non-keyworded variable-length argument list to functions. It is worth noting that the asterisk (*
) is the important element here, as the word args
is the established conventional idiom, though it is not enforced by the language.
Info: To follow along with the example code in this tutorial, open a Python interactive shell on your local system by running the python3
command. Then you can copy, paste, or edit the examples by adding them after the >>>
prompt.
Let’s look at a typical function that uses two arguments:
def multiply(x, y):
print (x * y)
In the code above, we built the function with x
and y
as arguments, and then when we call the function, we need to use numbers to correspond with x
and y
. In this case, we will pass the integer 5
in for x
and the integer 4
in for y
:
def multiply(x, y):
print (x * y)
multiply(5, 4)
Now, we can run the above code:
- python lets_multiply.py
We’ll receive the following output, showing that the integers 5 and 4 were multiplied as per the multiply(x,y)
function:
Output20
What if, later on, we decide that we would like to multiply three numbers rather than just two? If we try to add an additional number to the function, as shown below, we’ll receive an error.
def multiply(x, y):
print (x * y)
multiply(5, 4, 3)
OutputTypeError: multiply() takes 2 positional arguments but 3 were given
So, if you suspect that you may need to use more arguments later on, you can make use of *args
as your parameter instead.
We can essentially create the same function and code that we showed in the first example, by removing x
and y
as function parameters, and instead replacing them with *args
:
def multiply(*args):
z = 1
for num in args:
z *= num
print(z)
multiply(4, 5)
multiply(10, 9)
multiply(2, 3, 4)
multiply(3, 5, 10, 6)
When we run this code, we’ll receive the product for each of these function calls:
Output20
90
24
900
Because we used *args
to send a variable-length argument list to our function, we were able to pass in as many arguments as we wished into the function calls.
With *args
you can create more flexible code that accepts a varied amount of non-keyworded arguments within your function.
The double asterisk form of **kwargs
is used to pass a keyworded, variable-length argument dictionary to a function. Again, the two asterisks (**
) are the important element here, as the word kwargs
is conventionally used, though not enforced by the language.
Like *args
, **kwargs
can take however many arguments you would like to supply to it. However, **kwargs
differs from *args
in that you will need to assign keywords.
First, let’s print out the **kwargs
arguments that we pass to a function. We’ll create a short function to do this:
def print_kwargs(**kwargs):
print(kwargs)
Next, we’ll call the function with some keyworded arguments passed into the function:
def print_kwargs(**kwargs):
print(kwargs)
print_kwargs(kwargs_1="Shark", kwargs_2=4.5, kwargs_3=True)
Let’s run the program above and look at the output:
- python print_kwargs.py
Output{'kwargs_3': True, 'kwargs_2': 4.5, 'kwargs_1': 'Shark'}
Depending on the version of Python 3 you are currently using, the dictionary data type may be unordered. In Python 3.6 and above, you’ll receive the key-value pairs in order, but in earlier versions, the pairs will be output in a random order.
What is important to note is that a dictionary called kwargs
is created and we can work with it just like we can work with other dictionaries.
Let’s create another short program to show how we can make use of **kwargs
. Here we’ll create a function to greet a dictionary of names. First, we’ll start with a dictionary of two names:
def print_values(**kwargs):
for key, value in kwargs.items():
print("The value of {} is {}".format(key, value))
print_values(my_name="Sammy", your_name="Casey")
We can now run the program and look at the output:
- python print_values.py
OutputThe value of your_name is Casey
The value of my_name is Sammy
Again, because dictionaries may be unordered, your output may be with the name Casey
first or with the name Sammy
first.
Let’s now pass additional arguments to the function to show that **kwargs
will accept however many arguments you would like to include:
def print_values(**kwargs):
for key, value in kwargs.items():
print("The value of {} is {}".format(key, value))
print_values(
name_1="Alex",
name_2="Gray",
name_3="Harper",
name_4="Phoenix",
name_5="Remy",
name_6="Val"
)
When we run the program at this point, we’ll receive the following output, which may again be unordered:
OutputThe value of name_2 is Gray
The value of name_6 is Val
The value of name_4 is Phoenix
The value of name_5 is Remy
The value of name_3 is Harper
The value of name_1 is Alex
Using **kwargs
provides us with flexibility to use keyword arguments in our program. When we use **kwargs
as a parameter, we don’t need to know how many arguments we would eventually like to pass to a function.
When ordering arguments within a function or function call, arguments need to occur in a particular order:
*args
**kwargs
In practice, when working with explicit positional parameters along with *args
and **kwargs
, your function would look like this:
def example(arg_1, arg_2, *args, **kwargs):
...
And, when working with positional parameters along with named keyword parameters in addition to *args
and **kwargs
, your function would look like this:
def example2(arg_1, arg_2, *args, kw_1="shark", kw_2="blobfish", **kwargs):
...
It is important to keep the order of arguments in mind when creating functions so that you do not receive a syntax error in your Python code.
We can also use *args
and **kwargs
to pass arguments into functions.
First, let’s look at an example with *args
.
def some_args(arg_1, arg_2, arg_3):
print("arg_1:", arg_1)
print("arg_2:", arg_2)
print("arg_3:", arg_3)
args = ("Sammy", "Casey", "Alex")
some_args(*args)
In the function above, there are three parameters defined as arg_1
, arg_
, and arg_3
. The function will print out each of these arguments. We then create a variable that is set to an iterable (in this case, a tuple), and can pass that variable into the function with the asterisk syntax.
When we run the program with the python some_args.py
command, we’ll receive the following output:
Outputarg_1: Sammy
arg_2: Casey
arg_3: Alex
We can also modify the program above to an iterable list data type with a different variable name. Let’s also combine the *args
syntax with a named parameter:
def some_args(arg_1, arg_2, arg_3):
print("arg_1:", arg_1)
print("arg_2:", arg_2)
print("arg_3:", arg_3)
my_list = [2, 3]
some_args(1, *my_list)
If we run the program above, it will produce the following output:
Outputarg_1: 1
arg_2: 2
arg_3: 3
Similarly, the keyworded **kwargs
arguments can be used to call a function. We will set up a variable equal to a dictionary with 3 key-value pairs (we’ll use kwargs
here, but it can be called whatever you want), and pass it to a function with 3 arguments:
def some_kwargs(kwarg_1, kwarg_2, kwarg_3):
print("kwarg_1:", kwarg_1)
print("kwarg_2:", kwarg_2)
print("kwarg_3:", kwarg_3)
kwargs = {"kwarg_1": "Val", "kwarg_2": "Harper", "kwarg_3": "Remy"}
some_kwargs(**kwargs)
Let’s run the program above with the python some_kwargs.py
command:
Outputkwarg_1: Val
kwarg_2: Harper
kwarg_3: Remy
When calling a function, you can use *args
and **kwargs
to pass arguments.
We can use the special syntax of *args
and **kwargs
within a function definition in order to pass a variable number of arguments to the function.
Creating functions that accept *args
and **kwargs
are best used in situations where you expect that the number of inputs within the argument list will remain relatively small. The use of *args
and **kwargs
is primarily to provide readability and convenience, but should be done with care.
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Brilliant refresher.
Thank you for your tutorial. Simple to understand and comprehensive.
This is fascinating. Like the other commenters said, simple, brief, and explanatory. My question is what would a function invocation look like which uses each of the four argument types? You showed the function signature though I’d like to see the invocation.
This is very simple to recap and also for new starters. Lisa deserves hike for this article :-).
Thank you so much. All Digitalocean tutorials are very clear and easy to understand.
Thanks
Thanks , helped me alot
I appreciated this overview. Thank you.
The dictionary is actually just called
kwargs
, isn’t it?As of 3.6 dicts stay in order. :)