Using $^*$args and $^{**}$kwargs in Python

There are three types of parameters in Python, namely:

Positional-or-Keyword Parameters
Positional-Only Parameters
Keyword-Only Parameters

The Positional-or-Keyword parameter is the most commonly used one, where one can either specify the name of the parameter or not when calling the function.

The $^*$args and $^{**}$kwargs are typically used when a function has a varying (or arbitrary) number of parameters.

The $^*$args is used to pass a varying number of arguments.

The $^{**}$kwargs is used to pass a varying number of dictionary arguments.

Understanding $^*$args

In Python, the single-asterisk form of $^*$args can be used as a parameter to pass a variable-length argument list to a function.

Example: We want to multiply the numbers in a list of unknown length.

def multiplyList(*lst):
  result = 1
  for num in lst:
    result *= num
  return result

multiplyList(10, 25)
multiplyList(4,5,6,7,8,9,10)

When we run the above, we get the following output:

250
604800

Since the function was defined using $^*$args, we were able to pass in arbitrary number of arguments as we wanted into the function (in the first call 2 arguments and in the second call 7 arguments respectively).

Understanding $^{**}$kwargs

In Python, the double-asterisk form of **kwargs can be used as a parameter to pass a variable-length argument dictionary to a function. The main difference between $^*$args and **kwargs is that one needs to assign keys when using **kwargs.

Example:

def printValues(**thisDict):
  for (key, value) in thisDict.items():
    print(key, ':', value)

printValues(a = 10, b = 20)
printValues(w = 101, x = 7, y = 5, z = 13)

When we run the above, we get the following output:

a : 10
b : 20

w : 101
x : 7
y : 5
z : 13

Using `*` and `**` as Arguments to a Function

As opposed to our discussion above, the functionality of * and **, when passed as arguments when calling a function, is to unpacking an iterable data structure (such as a list or a tuple) and a dictionary into individual arguments respectively.

Example:

def printValues(val1, val2, val3):
  print("Value 1: ", val1, " Value 2: ", val2, " Value 3: ", val3)

myList = [100, 200, 300]
printValues(*myList)

When we run the above, we get the following output:

Value 1:  100  Value 2:  200  Value 3:  300

An example usagee of ** is as follows.

myDict = {"val1" : 5, "val2" : 12, "val3" : 40}
printValues(**myDict)

When we run the above, we get the following output:

Value 1:  5  Value 2:  12  Value 3:  40

Note that the keys in myDict corresponds to the names of the arguments defined in the function, namely val1, val2, and val3.

Anusha Murali