What does ** (double star) and * (star) do for parameters in Python?

While creating a function, the single asterisk (*) is used to accept any number of positional arguments, and the double asterisk (**) is used to accept any number of keyword arguments. These operators provide flexibility when you don't know in advance how many arguments will be passed to your function.

Using * (Single Asterisk) for Positional Arguments

The single asterisk (*) collects extra positional arguments into a tuple. By convention, this parameter is named *args.

Example

Create a function that accepts an arbitrary number of positional arguments ?

def sum_numbers(*args):
    result = 0
    for number in args:
        result += number
    return result

print(sum_numbers(1, 2, 3, 4, 5, 6, 7))
print(sum_numbers(10, 10, 10)) 
print(sum_numbers(1))
print(sum_numbers())  # No arguments

28
30
1
0

Using ** (Double Asterisk) for Keyword Arguments

The double asterisk (**) collects extra keyword arguments into a dictionary. By convention, this parameter is named **kwargs.

Example

Create a function that accepts an arbitrary number of keyword arguments ?

def sum_values(**kwargs):
    result = 0
    for key, value in kwargs.items():
        print(f"{key}: {value}")
        result += value
    return result

print("Total:", sum_values(a=1, b=2, c=3, d=4))
print("Total:", sum_values(x=10, y=100))

a: 1
b: 2
c: 3
d: 4
Total: 10
x: 10
y: 100
Total: 110

Combining Both *args and **kwargs

You can use both operators in the same function. The order must be: regular parameters, *args, then **kwargs.

Example

def flexible_function(name, *args, **kwargs):
    print(f"Name: {name}")
    print(f"Args: {args}")
    print(f"Kwargs: {kwargs}")

flexible_function("Alice", 1, 2, 3, age=25, city="New York")

Name: Alice
Args: (1, 2, 3)
Kwargs: {'age': 25, 'city': 'New York'}

Unpacking with * and ** Operators

These operators can also unpack collections when calling functions ?

Unpacking Lists and Tuples with *

def greet(first, second, third):
    print(f"Hello {first}, {second}, and {third}!")

names = ["Alice", "Bob", "Charlie"]
greet(*names)

# Also works with tuples
name_tuple = ("David", "Eve", "Frank")
greet(*name_tuple)

Hello Alice, Bob, and Charlie!
Hello David, Eve, and Frank!

Unpacking Dictionaries with **

def calculate(a, b, operation="add"):
    if operation == "add":
        return a + b
    elif operation == "multiply":
        return a * b

params = {'a': 10, 'b': 5, 'operation': 'multiply'}
result = calculate(**params)
print(f"Result: {result}")

Result: 50

Common Use Cases

Conclusion

The *args and **kwargs operators provide powerful flexibility in Python functions. Use *args for variable positional arguments and **kwargs for variable keyword arguments. They're essential for creating flexible APIs and wrapper functions.