function args kwargs intro

December 30, 2023

intro

When writing a function in Python it is often required to pass values to the function, where this values are called function arguments, which can be in fixed or dynamic numbers (Krishna, 2022).
There two types of argument passed to function in Python, which are positional and keyword arguments (Wilkinson, 2023).
One of the reason Python programming languange, which is known for being simple and readable, is its way of handling parameters in very easy and dynamics way using *args and *kwargs (Bolanle, 2023).
Due tue not aware of all possible use cases of a code, and to offer more options for future programmers working with the module or for users interacting with the code, passing a variable number of arguments to a function can be accomodate using *args and **kwargs (Tagliaferri, 2021).
In the Python documentation about its functions it is common to see *args and **args defined as parameters in some functions, which is a major cause of confusion for most Python programmers (Pykes, 2022).
There are some combinations of normal positional arguments and normal keyword arguments that are working, which can be easier to understand by trying them yourself (Gruppeta, 2022).

using *args

Here a problem is presented, where one of the solutions is the use of *args.
The *args is a tuple containing all positional arguments.

problem

Suppose we have to add two numbers using user-defined function add with following code, but we try to give unmatched number of arguments.

def add(x, y):
  z = x + y
  return z

a = 100
b = 1
c = add(a, b)
print(a, "+", b, "=", c)

print()

a = 100
b = 10
c = 1
d = add(a, b, c)
print(a, "+", b, "+", c, "=", d)

url https://onecompiler.com/python/3zxwxjvam [20231230].

Output

100 + 1 = 101


Traceback (most recent call last):
  File "main.py", line 15, in 
    d = add(a, b, c)
TypeError: add() takes 2 positional arguments
  but 3 were given

Notice that 3 positional arguments are given, while it requires only 2.

solution

One way to solve previous problem is by using *args as follow.

def add(*args):
  nums = list(args)
  return sum(nums)

a = 100
b = 1
c = add(a, b)
print(a, "+", b, "=", c)

print()

a = 100
b = 10
c = 1
d = add(a, b, c)
print(a, "+", b, "+", c, "=", d)

print()

a = 10
b = add(a)
print(a, "=", b)

url https://onecompiler.com/python/3zxwxvqru [20231230].

Output

100 + 1 = 101

100 + 10 + 1 = 111

10 = 10

Notice that now the function works with arbitrary number of arguments, 1, 2, or 3.
There might be also some problems arise with current version, e.g. if the arguments are not integer or float.

positional and keyword arguments

In previous section the use of positional arguments has been shown.
For positional arguments position of each parameter as the function called should match the corresponding argument.

positional or keyword argument

Following code will show the importance of position of parameters and how to change it using keyword arguments.

def printArguments(a, b, c, d):
  print("1st =", a)
  print("2dn =", b)
  print("3rd =", c)
  print("4th =", d)

printArguments(1, 10, 100, 1000)

print()

printArguments(a=1, d=4, c=3, b=2)

url https://onecompiler.com/python/3zxx85pme [20231230].

Output

1st = 1
2dn = 10
3rd = 100
4th = 1000

1st = 1
2dn = 2
3rd = 3
4th = 4

Notice that upper part of the output is already clear, but in the lower part it is shown that the order of parameter can be rearranged using keyword argument for corresponding argument.

combinations two types of argument

It is also possible to use both positional and keyword arguments at the same time when calling a function.

def printArguments(a, b, c, d):
  print("1st =", a)
  print("2dn =", b)
  print("3rd =", c)
  print("4th =", d)

printArguments(1, 10, d=1000, c=100)

print()

printArguments(1, 2, 4, c=3)

url https://onecompiler.com/python/3zxx95unj [20231230].

Output

1st = 1
2dn = 10
3rd = 100
4th = 1000


Traceback (most recent call last):
  File "main.py", line 11, in 
    printArguments(1, 2, 4, c=3)
TypeError: printArguments() got multiple
  values for argument 'c'

Notice that the third argument c has been assigned with two different values, first via positional argument with value 4 and second with keyword argument with value 3.

positional and keyword

Positional arguments must come first then followed by keywords arguments.

def printArguments(a, b, c, d):
  print("1st =", a)
  print("2dn =", b)
  print("3rd =", c)
  print("4th =", d)

printArguments(1, 10, c=100, 1000)

url https://onecompiler.com/python/3zxx9rkjq [20231230].

Output

File "main.py", line 7
    printArguments(1, 10, c=100, 1000)
                                 ^
SyntaxError: positional argument follows
  keyword argument

In previous subsection the order between positional arguments and keyword arguments is already right, a parameter is assigned with two arguments.

challenges

Test previous add function for 4, 8, and 20 arguments. Prove that it always works.
Create similar function for calculate multiplication of numbers and name it as mul function, that can receive arbitrary number of arguments.
Create a function named buyItemsWithAmount("orange", 12, "apple", 10, "durian", 1, "papaya", 3), that can have arbitrary number of items in pairs of item and amount.

refs

Bolanle F (2023) “How To Use *args and **kwargs In Python”, DEV Community, 10 Aug, url https://dev.to/faithbolanle/understand-args-and-kwargs-use-it-the-right-way-2d45 [20231230].
Gruppetta S (2022) “Argh! What are args and kwargs in Python? [Intermediate Python Functions Series #4]”, The Python Coding Book, 30 Nov, url https://thepythoncodingbook.com/2022/11/30/what-are-args-and-kwargs-in-python/ [20231230].
Krishna A (2022) “How to Use *args and **kwargs in Python”, freeCodeCamp, 23 Mar, url https://www.freecodecamp.org/news/args-and-kwargs-in-python/ [20231230].
Pykes K (2022) “Python *args & **kwargs”, Geek Culture, Medium, 16 Mar, url https://medium.com/geekculture/1c5c0ec9121 [20231230].
Wilkinson P (2023) “*args, **kwargs, and Everything in Between”, Towards Data Science, Medium, 18 Jul, url https://towardsdatascience.com/ff7d9b536494 [20231230].
Tagliaferri L (2021) “How To Use *args and **kwargs in Python 3”, DigitalOcean, 21 Aug, url https://www.digitalocean.com/community/tutorials/how-to-use-args-and-kwargs-in-python-3 [20231230].