Master Python’s *args and **kwargs: When to Use Each Parameter Style

We start with a simple function that accepts arbitrary positional and keyword arguments:

def test(*args, **kwargs):
    print(args, kwargs)

Two variables are defined: a list l = [1, 2, 3, 4] and a dictionary d = {"a": 1, "b": 2}.

First calling style test(l, d) Both l and d are placed into the args tuple, while kwargs remains empty, producing output similar to ([1, 2, 3, 4], {'a': 1, 'b': 2}) {}.

Second calling style test(*l, **d) The list is unpacked into positional arguments and the dictionary into keyword arguments, resulting in (1, 2, 3, 4) {'a': 1, 'b': 2}.

Next we examine a function that mixes a regular parameter with **kwargs:

def foo(action=None, **kwargs):
    print("action", action, sep="=================>")
    print("kwargs", kwargs, sep="=================>")

Calling foo(d) passes the whole dictionary as the positional action argument, leaving kwargs empty. Calling foo(**d) unpacks the dictionary so that a=1 and b=2 become keyword arguments, while action stays None. The output screenshots illustrate these results:

Finally, a third call demonstrates a dictionary that also contains an action key:

d = {"action": "action", "a": 1}
foo(**d)

The resulting output (shown in the second image) confirms that the action parameter receives the value from the dictionary, while the remaining items appear in kwargs:

Through these examples you should now understand how Python’s universal parameters work and how to choose the appropriate calling syntax.