What are *args and **kwargs in Python

In this post, I'm going to talk about *args and **kwargs in Python which is a useful concept while handling multiple arguments.

They are called *args and **kwargs in Python which allows the function to accept optional arguments(positional and keyword). Let’s see with an example. At first, let’s make a simple function that accepts a single argument, first_name.

>>> def person(first_name):
...     print(first_name)
>>> person("Ram")
Ram
>>> person()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: person() missing 1 required positional argument: 'first_name'

What we can see is first_name is a compulsory argument. So, it is not possible to call the function without arguments. Now, what we are going to do is make a function that accepts first_name as compulsory arguments followed by last_name as an optional argument. One way to do this would be

>>> def person(first_name, last_name=""):
...     print(first_name, last_name)
>>> person("Ram") # Called with 1 argument
Ram
>>> person("Ram", "Shrestha") # Called with 2 arguments
Ram Shrestha

So far everything is going well. But now we want to scale the same function to accept first_name(compulsory) and last_name and middle_name as positional arguments. One simple trick is to add another argument middle_name.

>>> def person(first_name, last_name="", middle_name=""):
...     print(first_name, middle_name, last_name)

But there is a more concise way to scale the function to accept multiple optional arguments called *args in Python.

1. *args

*args collects extra positional arguments as a tuple

The idea is simple. We modify our function definition as:

>>> def person(first_name, *args):
...     print(first_name, end = " ")
...     for names in reversed(args):
...             print(names, end=" ")
...     print()
>>> # Accepts single argument
>>> person("Ram")
Ram
>>> # Accepts 2 arguments
>>> person("Ram", "Chhetri")
Ram Chhetri
>>> # Accepts 3 arguments
>>> person("Ram", "Chhetri", "Lal")
Ram Lal Chhetri
>>> person("Ram", "Chhetri", "Lal", "Krishna")
Ram Krishna Lal Chhetri

We can see that now our function can accept first_name followed by last_name, middle_name as optional positional arguments. To see if args is indeed a tuple you can add

print(type(args))

inside of function definition Let’s see another example:

Task 1: Write a function to add N integers which are passed as arguments ( N >= 1) Let’s do it for 1 integer

>>> def add(a):
...     return a
>>> add(8)
8

Nothing complicated. Now do it for 2 integers

>>> def add(a, b):
...     return a + b
>>> add(8, 2)
10
>>> add(8)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: add() missing 1 required positional argument: 'b'

Notice?? We lost the compatibility for 1 argument. This is where *args comes handy:

>>> def add(a, *args):
...     sum = a
...     print(type(args))
...     for num in args:
...         sum += num
...     return sum
>>> add(8)
8
>> add(8, 2)
10
>> add(8, 2, 5)
15

See how flexible it is using *args. Note it is not necessary to name *args as args . We can name it whatever we like. But it must be preceded with a *.

2. **kwargs

**kwargs collects extra positional arguments as a dictionary.

Let’s return to our previous examples of function person:

Task 2: Modify function person so that it accepts additional optional parameters phone_num, email and so on…as keywords. We modify our original function definition to accept a new argument **kwargs which is a keyword argument as:

>>> def person(first_name, *args, **kwargs):
...     print(first_name, end=" ")
...     for names in reversed(args):
...             print(names, end=" ")
...     print()
...     # Since kwargs is a dictionary
...     # we can iterate using .items()
...     for key, value in kwargs.items():
...             print(key, value)
>>> person("Ram", "Chhetri", "Lal", phone_num="981128331", email="[email protected]")

Ram Lal Chhetri
phone_num 981128331
email anon@testmail.com

Since kwargs is of dictionary type. we can iterate over key, value using .items() Notes:

• *args and **kwargs are used to allow the function to accept a variable number of arguments
• *args collects positional arguments as a tuple whereas **kwargs as dictionary
• args and kwargs are just naming convention. You can name it whatever you like.
• * and ** are also used in value unpacking