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Python Concepts for Data Science: *args and **kwargs

General introduction

In this article we will be interested in the use of *args and **kwargs.

But first we will try to know what it represents and the different cases where we could be trying to use them.

Let me tell you that it is not require to write *args or **kwargs and only the "*" is require. You could have also *vars and **vars or with the name that is best suited to the situation.

Usually *args and **kwargs allow you to pass unspecified number of argument. You don't need to know how many arguments will be used.

Using of *args

In a function

*args is use for non-keyworded argument.

For clarity, we'll illustrate this with a few examples. First we're going to use a function that doesn't use *args and then another that uses it.

def my_sum(a, b):
    return a + b

print(my_sum(1, 4)) # output 5

With the previous example we see that the different variables used in the program are already known. If we want the same function to be able to calculate the sum of 3 numbers we will add another argument to the function.

def my_sum(*args):
    result = 0
    for x in args:
        result += x
    return result

print(my_sum(1, 2, 3, 4)) # output 10

With our new function we have no argument limit that we want to use in the function. We find that the body of our function has changed a bit, and *args behaved as if we had passed a list to our function. This function could be compared to this one.

def my_sum(to_add_up):
    result = 0
    for x in to_add_up:
        result += x
    return result

print(my_sum([1, 2, 3, 4])) # output 10

In summary when *args is used in a function it could be likened to as if we are using a list in a function parameter. Except that the arguments are added one by one to our function.

unpacking operators

Unpacking operators are operators that unpack the values from iterable objects in Python.

The single asterisk operator "*" can be used on any iterable that Python provides.

my_list = [1, 2, 3, 4]
print(*my_list) # output 1 2 3 4

In the previous example we were able to display all the values ​​of our list without having to browse the values ​​inside.

my_list = [1, 2, 3, 4]
first, *new_list, last = my_list

print(first) # output 1
print(new_list) # output [2, 3]
print(*new_list) # output 2 3
print(last) # output 4

If the number of variables is not equal as in the following example we get an error.

my_list = [1, 2, 3, 4]
first, second, last = my_list #error

We note then that thanks to the unpacking operator "*" we can retrieve the value of several variables directly inside a list.

my_first_list = [1, 2, 3, 4]
my_second_list = [5, 6, 7, 8, 9]

my_list = [*my_first_list, *my_second_list]

print(my_list) # output [1, 2, 3, 4, 5, 6, 7, 8, 9]

Finally we can use this operator to fill a list

Using of **kwargs

in a function

**kwargs is use for keyworded argument like dictionnary.

Trying to use this to a function which concatenates the words we have.

  • without using **kwargs

def concatenate(a='', b='', c=''):
    return a + b + c

print(concatenate('Kwargs', ' is ', 'awesome')) # output "Kwargs is awesome"

In this function each argument passed has a default value, so the function can be used without parameters

  • using **kwargs

def concatenate(**kwargs):
    result = ""
    for x in kwargs:
        result += x
    return result

print(concatenate(a='Kwargs', b=' is ', c='awesome')) # output "abc"

By default when we are a parameter we access keys and not values, so to have access to values ​​we have this new version of our code.

def concatenate(**kwargs):
    result = ""
    for x in kwargs.values():
        result += x
    return result

print(concatenate(a='Kwargs', b=' is ', c='awesome')) # output "Kwargs is awesome"

If we use our function in the following way we will get an error.

print(concatenate('Kwargs', ' is ', 'awesome')) # error

As with the *args argument, the **kwargs argument acts as a list so each element is a dictionary.

unpacking operators

As with *args, **kwargs can be used to create another dictionary

my_first_dict = {"A": 1, "B": 2}
my_second_dict = {"C": 3, "D": 4}
my_dict = {**my_first_dict, **my_second_dict}

print(my_dict) # output {'A': 1, 'B': 2, 'C': 3, 'D': 4}

Using *args and **kwargs together in function

In the event that we want to use both one or more normal variable with *args and **kwargs, then the order of the variables matters. If they are not placed in the correct order then we will have an error.

Here is the correct order that must be followed to be able to use them at the same time.

def my_func(normal, *args, **kwargs)


We enjoyed defining the *args and **kwargs arguments together,

then reviewed the different use cases. We hope you enjoyed learning with us, so it can be used for you later.


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