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How To Use *args and **kwargs in Python 3

PostedMay 9, 2017 14.9k views Python Development

Introduction

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.

Understanding *args

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.

Let’s look at a typical function that uses two arguments:

lets_multiply.py
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:

lets_multiply.py
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:

Output
20

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.

lets_multiply.py
def multiply(x, y):
    print (x * y)

multiply(5, 4, 3)
Output
TypeError: 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:

lets_multiply.py
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:

Output
20 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.

Understanding **kwargs

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 simply print out the **kwargs arguments that we pass to a function. We’ll create a short function to do this:

print_kwargs.py
def print_kwargs(**kwargs):
        print(kwargs)

Next, we’ll call the function with some keyworded arguments passed into the function:

print_kwargs.py
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'}

Because the dictionary data type is unordered, we received the key-value pairs in a random order, but it is important to note 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:

print_values.py
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
Output
The value of your_name is Casey The value of my_name is Sammy

Again, because dictionaries are 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:

print_values.py
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 is again unordered:

Output
The 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.

Ordering Arguments

When ordering arguments within a function or function call, arguments need to occur in a particular order:

  1. Formal positional arguments
  2. *args
  3. Keyword arguments
  4. **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.

Using *args and **kwargs in Function Calls

We can also use *args and **kwargs to pass arguments into functions.

First, let’s look at an example with *args.

some_args.py
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:

Output
arg_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:

some_args.py
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:

Output
arg_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:

some_kwargs.py
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:

Output
kwarg_1: Val kwarg_2: Harper kwarg_3: Remy

When calling a function, you can use *args and **kwargs to pass arguments.

Conclusion

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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