18. Function packing and unpacking

18.1. Packing arguments of a function

When a function has a parameter preceded by an asterisk *, it can accept a variable number of arguments.
Pass zero, one, or more arguments to the *args parameter.
The arguments are packed into a tuple.

def polygon_perimeter(*args):
    return sum(args)

perimeter = polygon_perimeter(3, 4, 5, 6)
print(perimeter)

The ouput is: 18

18.2. Unpacking for a function

The function below, tri_perimeter, requires 3 arguments, (a, b, c).

The list, sides, can be passed to the function by unpacking the list using *sides.

def tri_perimeter(a, b, c):
    return a + b + c

sides = [7, 12, 13]
perimeter = tri_perimeter(*sides)
print(perimeter)

The ouput is: 32

18.3. Combining unpacking and packing for a function

The three lists are unpacked into a tuple and passed to the function.

A tuple is printed that contains the args and the result.

def sum_all(*args):
    result = 0
    for x in args:
        result += x
    return args, result

list1 = [1, 2, 3]
list2 = [4, 5]
list3 = [6, 7, 8, 9]

print(sum_all(*list1, *list2, *list3))

The ouput is: ((1, 2, 3, 4, 5, 6, 7, 8, 9), 45)

18.4. Built in functions: Unpacking with more than one value

The range function below can take two values, the start and stop values.
The for-loop code below would normally look like: for x in range(0, 5).
The unpacking operator, *, can be used to unpack the variable vals, so that the tuple (0, 5) is turned into 2 separate arguments, 0 and 5, instead of staying as the single tuple (0, 5).

The code below runs as if the arguments to the range function were 0 and 5.

vals = (0, 5)
for x in range(*vals):
    print(x, end=", ")

The code prints out: 0, 1, 2, 3, 4,

The code below runs as if the arguments to the range function were 0, 5 and 2.

vals = (0, 5, 2)
for x in range(*vals):
    print(x, end=", ")

The code prints out: 0, 2, 4,

18.5. Built in functions: Unpacking with just one value

For tuples with just one value, a trailing comma is required. e.g (5,)

The range function expects a tuple for unpacking via (*vals), so (5,) is needed.

vals = (5, )
for x in range(*vals):
    print(x, end=", ")

The code prints out: 0, 1, 2, 3, 4,