When we don't know the number of arguments we will give to a method or a function, we can use *args and **kwargs. We do this by placing * or ** before the argument name.

*args

*args - non-keyword (normal) arguments.

def function(*numbers):
    sum1 = 0
    for x in numbers:
        sum1 += x
    print("Sum: ", sum1)

function(1, 2)
function(1, 2, 3)
function(1, 2, 3, 4)
                                    

**kwargs

**kwargs - keyword arguments. A keyword argument is an argument represented by a key-value pair, similar to a dictionary.

def function(**data):
    for key, value in data.items():
        print(f"{key}: {value}")

function(name = "John", surname = "Anderson", age = 20)
function(name = "Liam", surname = "Brown", age = 20, country = "USA")
                                    

*args and **kwargs

When using *args and **kwargs along with standard arguments, remember to follow this order: standard arguments first, then *args, followed by **kwargs.

def function(argument1, argument2, *args, **kwargs):
    print("Standard argument nr. 1:", argument1)
    print("Standard argument nr. 2:", argument2)
    for x in args:
        print("Args:", x)
    for key, value in kwargs.items():
        print(f"Kwargs: {key} - {value}")

function(1, 2, 3, 4, a = 5, b = 6, c = 7)
                                    

* and ** symbols

A single asterisk can also be used to unpack a list into individual elements.

list1 = ["one", 2]
print(*list1)
print(*list1, sep = ", ") # separating each element with a comma
                                    

Double asterisks can also be used to merge dictionaries.

dict1 = {"one": 1}
dict2 = {"two": 2}
linked = {**dict1, **dict2}
print(linked)
                                    

Below are some examples of unpacking lists using a single asterisk. Remember that an expression can have only one variable with * for it to make sense.

ages_descending = [90, 65, 40, 25, 10]

oldest, *others, youngest = ages_descending
print(oldest)
print(others)
print(youngest)

*others, second_youngest, youngest = ages_descending
print(others)
print(second_youngest)
print(youngest)

def get_values():
    return 1, 2, 3, 4, 5
a, b, *others = get_values()
print(a, b, others)
                                    

Adding a new positional argument to a function with *args can cause subtle errors.

def func(*args):
    print(args)
func(1, 2, 3) # (1, 2, 3)

def func(a, *args):
    print(a, args)
func(1, 2, 3) # 1 (2, 3) – the first value now goes to "a" instead of *args