*args and **kwargs

Sometimes you want a function that accepts a variable number of arguments. Python gives you two tools: *args (gathers positional arguments into a tuple) and **kwargs (gathers keyword arguments into a dict).

*args: variable positional arguments

def somma_tutto(*nums):
    totale = 0
    for n in nums:
        totale += n
    return totale

somma_tutto(1, 2, 3)         # 6
somma_tutto(1, 2, 3, 4, 5)   # 15
somma_tutto()                # 0

Inside the function, nums is a tuple with all the values passed. The name is just a convention: *args is the most common, but *nums or *valori are perfectly valid.

**kwargs: variable keyword arguments

def crea_config(**opzioni):
    return opzioni

crea_config(debug=True, port=8080)
# {'debug': True, 'port': 8080}

Inside the function, opzioni is a dict with {param_name: value}.

Combining everything

The fixed order is: **regular parameters, *args, default parameters, **kwargs**.

def f(primo, *args, opzione="x", **kwargs):
    print(primo, args, opzione, kwargs)

f(1, 2, 3, opzione="y", extra=10)
# 1 (2, 3) y {'extra': 10}

Unpacking in the call: * and **

The same operator works the other way around: to "explode" a list/tuple into positional arguments or a dict into keyword arguments.

def punto(x, y, z):
    return (x, y, z)

coords = [1, 2, 3]
punto(*coords)        # punto(1, 2, 3)  →  (1, 2, 3)

opts = {"x": 1, "y": 2, "z": 3}
punto(**opts)         # punto(x=1, y=2, z=3)

Wrapper pattern

def log_chiamata(fn, *args, **kwargs):
    print(f"chiamo {fn.__name__} con {args=} {kwargs=}")
    return fn(*args, **kwargs)

log_chiamata(max, 3, 1, 2)             # calls max(3, 1, 2)
log_chiamata(sorted, [3, 1, 2], reverse=True)

Parameter ordering in function signatures

When defining a function that accepts multiple argument types, you must adhere to a strict order in the signature:

1. Standard positional parameters.
2. Positional varargs *args.
3. Keyword parameters with default values.
4. Keyword varargs **kwargs.

Try it

Review exercise

Additional challenge