*args and **kwargs

Intro

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In this article, I'll explain what *args and **kwargs are, their use, and a common mistake made while using them. To avoid wasting time, let's get right into it!

What are *args and **kwargs?

*args and **kwargs are a special type of argument, used when you don't know yet how many arguments are going to be passed in a function.
For example, say you want to write a function that takes n numbers as input and outputs their sum (different from the python sum() function as this one takes an array as input), you'd have to know what n was exactly to write the function:

def user_sum(n1, n2, n3):
    return n1+n2+n3
    
print(user_sum(1,2,3,4))

Output:

Traceback (most recent call last):
  File "C:\Users\gerva\OneDrive\Bureau\args and kwargs.py", line 4, in 
    print(user_sum(1,2,3,4))
TypeError: user_sum() takes 3 positional arguments but 4 were given

Knowing about *args and **kwargs will let us work around that. Let's start with *args. It is used for handling an unknown amount of positional arguments, let's check out how it works:

def args_function(*args):
    print(args)
    print(type(args))
    
args_function(1,2,3,4)

Output:

(1, 2, 3, 4)
<class 'tuple'>

As we can see, the *args arguments collects all positional arguments and stores them in the args variable as a tuple. Using that knowledge, we can rewrite the previous sum function:

def user_sum(*args):
    total = 0
    for number in args:
        if type(number) is int or type(number) is float:
            total += number
    return total
    
result = user_sum(1,2,3,4,5,6,7,8)
print(result)

Output:

36

The reason I checked the type of each argument here is that, just like a normal iterable, we can put different variable types and the *args argument will handle them all the same:

def check_type(*args):
    for element in args:
        print(element, type(element))
        
    
check_type(1, True, "hello", 0.0, None, b"hello", [1,2], (1,2), 1j)

Output:

1 <class 'int'>
True <class 'bool'>
hello <class 'str'>
0.0 <class 'float'>
None <class 'NoneType'>
b'hello' <class 'bytes'>
[1, 2] <class 'list'>
(1, 2) <class 'tuple'>
1j <class 'complex'>

Now, let's talk about the **kwargs argument. As the name might suggest, this is used to handle multiple keyword arguments. Let's check it out as well:

def check_kwargs(**kwargs):
    print(kwargs)
    print(type(kwargs))
    
check_kwargs(a=1, b=2, c=3)

Output:

{'a': 1, 'b': 2, 'c': 3}
<class 'dict'>

So, to actually get the passed values, we could write:

def get_kwvalues(**kwargs):
    for kw in kwargs:
        print(kwargs[kw])
        
get_kwvalues(a="hello", b=123, c=[0,1])

Output:

hello
123
[0, 1]

Well, this is also pretty interesting, let's try to modify our previous sum function to incorporate keyword arguments!

def user_sum_and_multiply(*args, **kwargs):
    total = 0
    for number in args:
        if type(number) is int or type(number) is float:
            total += number
    for kw in kwargs:
        if type(kwargs[kw]) is int or type(kwargs[kw]) is float:
            total *= kwargs[kw]
    return total
    
result = user_sum_and_multiply(1,2,3,4,5,6,7,8, a=2, b=3)
print(result)

Output:

216

Common mistake

The main thing to remember while using *args and **kwargs is that normal positional arguments and keyword arguments can still be used in the function and will be taken into account, but their position matter:

• positional arguments must go before the *args
• Keyword arguments must go between the *args and **kwargs

Here is an example, this is valid:

def test(a, b, c, *args, kw1="hello", kw2=10, **kwargs):
    print(a)
    print(b)
    print(c)
    print(args)
    print(kw1)
    print(kw2)
    print(kwargs)
    
test(1, 2, 3, "this is in *args", kw1=7, var1=True, var2=False) #for example, kw2 will not be modified

Output:

1
2
3
('this is in *args',)
7
10
{'var1': True, 'var2': False}

While this is not valid:

def test(*args, a, b, c, **kwargs, kw1="hello", kw2=10):
    print(a)
    print(b)
    print(c)
    print(args)
    print(kw1)
    print(kw2)
    print(kwargs)

Output:

  File "C:\Users\gerva\OneDrive\Bureau\args and kwargs.py", line 1
    def test(*args, a, b, c, **kwargs, kw1="hello", kw2=10):
                                       ^^^
SyntaxError: invalid syntax

Thanks a lot for reading until the end, I hope you learned something and consider subscribing to my newsletter to learn more!