Python 9.5 : Variable number of parameters

We already know that Python functions can take a fixed number of parameters, or none at all. A simple function such as def ab(): print("Hello") takes no argument; another one such as print(name, height, age, married) mixes a string, a float, an integer and a boolean. But what if we want to write a function that sums up the marks of a student, without knowing in advance how many subjects they have?

Accepting any number of arguments

Hard-coding parameters like a, b, c, d, e, f quickly becomes impractical and crashes as soon as the caller passes a different number of values. The clean solution in Python is to declare a special parameter prefixed with a star, conventionally written *args (here we use *numbers). Inside the function, numbers becomes a tuple containing every value passed at the call site, and we iterate over it with a normal for loop to compute the sum.

def sum_marks(title, *numbers):
    print(title)
    note = 0
    for i in numbers:
        note += i
    return note

print(sum_marks("Sum of the notes:", 12, 13, 15, 8, 11))

Calling the function with two, three or ten numbers now works without changing its signature. If no numbers are passed at all, the loop simply does not execute and the function returns the initial 0. We can also add a regular positional parameter such as title before *numbers to print a header.

There is also a double-star variant **kwargs that works like a dictionary of keyword arguments, for example physics=12, math=16, history=15, french=11. To sum the values we have to iterate over kwargs.values(), otherwise Python complains because the iteration variable would be the key, not a number. In our case the simple single-star version is enough, so we stick with it. In the next section we will move on to collections.