Why use packed *args/**kwargs instead of passing list/dict?

Question

If I don't know how many arguments a function will be passed, I could write the function using argument packing:

def add(factor, *nums):
    """Add numbers and multiply by factor."""
    return sum(nums) * factor

Alternatively, I could avoid argument packing by passing a list of numbers as the argument:

def add(factor, nums):
    """Add numbers and multiply by factor.

    :type factor: int
    :type nums: list of int
    """
    return sum(nums) * factor

Is there an advantage to using argument packing *args over passing a list of numbers? Or are there situations where one is more appropriate?

Answer 1

*args/**kwargs has its advantages, generally in cases where you want to be able to pass in an unpacked data structure, while retaining the ability to work with packed ones. Python 3's print() is a good example.

print('hi')
print('you have', num, 'potatoes')
print(*mylist)

Contrast that with what it would be like if print() only took a packed structure and then expanded it within the function:

print(('hi',))
print(('you have', num, 'potatoes'))
print(mylist)

In this case, *args/**kwargs comes in really handy.

Of course, if you expect the function to always be passed multiple arguments contained within a data structure, as sum() and str.join() do, it might make more sense to leave out the * syntax.