Suppose we want to have, not a fixed number of arguments, of which some can be omitted, but a list of arbitrarily many arguments. The * notation for arguments allows us to read in such a list.
def sum_many_numbers(*numbers): ... x = 0 ... for n in numbers: ... x = x + n ... return x
sum_many_numbers(1) 1 sum_many_numbers(1, 1, 2) 4 sum_many_numbers(1, 0.5, 0.25, 0.125, 0.0625) 1.9375 sum_many_numbers() 0
Note that passing in an actual list will not work correctly:
sum_many_numbers([0, 1]) Traceback (most recent call last): ... TypeError: unsupported operand type(s) for +: 'int' and 'list'
However, you can pass in a list of all the individual arguments, by using * when you call the function.
sum_many_numbers(*[0, 1]) 1
This use of the * notation also works for functions which are defined in the normal way:
def sum_three_numbers(first, second, third): ... return first + second + third
sum_three_numbers([10, 11, 12]) 33 sum_three_numbers([10, 11, 12, 13]) Traceback (most recent call last): ... TypeError: sum_three_numbers() takes exactly 3 arguments (4 given) sum_three_numbers(*[]) Traceback (most recent call last): ... TypeError: sum_three_numbers() takes exactly 3 arguments (0 given)
Got that? * when you define the function, means 'take the arguments, and put them into a list'.
- when you call the function, means 'take this list, and use the members as successive arguments'. Using * in the definition doesn't mean you do or don't have to use it in the call, and vice versa.