How Do Single and Double Star Operators (*) and () Handle Variable Arguments in Python Function Definitions?

Parameter Unpacking: and in Function Definitions*

Function definitions in Python can leverage the (star) and * (double star) operators to handle varied numbers of arguments.

Single Star Operator (*)

The operator (also known as "var-args" or "splat") collects all positional arguments passed to the function into a tuple called args. For instance, in the definition below:

def foo(x, y, *args):
    pass

If args is invoked with foo(1, 2, 3, 4, 5), the args tuple will contain (3, 4, 5).

Double Star Operator ()**

The operator (also known as "var-kwargs" or "keyword splat") captures all keyword arguments as a dictionary named kwargs**. In the following definition:

def bar(x, y, **kwargs):
    pass

When bar is invoked with bar(1, 2, name='Alice', age=30), the kwargs dictionary will contain {'name': 'Alice', 'age': 30}.

Mixing Standard and Variable Arguments

Function definitions can combine normal arguments with variable arguments using *:

def foo(kind, *args, bar=None, **kwargs):
    pass

When invoked with foo(123, 'a', 'b', apple='red'), the variables 'kind', 'args', 'bar', and 'kwargs' will contain:

• kind = 123
• args = ('a', 'b')
• bar = None
• kwargs = {'apple': 'red'}

Unpacking Argument Lists

*: can also be used to unpack argument lists when calling functions:

def foo(bar, lee):
    pass

baz = [1, 2]

foo(*baz) # Unpacks baz into [1, 2] for bar and lee

Extended Iterable Unpacking (Python 3)

In Python 3, *l can be used on the left side of an assignment to unpack iterables into multiple variables:

first, *rest = [1, 2, 3, 4] # first = 1, rest = [2, 3, 4]

Keyword-Only Arguments (Python 3)

Python 3 introduces keyword-only arguments, where after *: only keyword arguments are allowed:

def func(arg1, arg2, arg3, *, kwarg1, kwarg2):
    pass

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