Variadic Function

A function that accepts any number of arguments.

Also known as: varargs, rest parameters, *args, **kwargs

A variadic function accepts any number of arguments.

def total(*numbers):              # numbers is a tuple
    return sum(numbers)

total(1, 2, 3)        # 6
total()               # 0

function total(...numbers) {      // numbers is an array
  return numbers.reduce((a, b) => a + b, 0);
}
total(1, 2, 3);

print, max, console.log and format functions are all variadic.

Keyword arguments

Python’s **kwargs collects extra named arguments into a dictionary:

def log(message, **fields):
    print(message, fields)

log("saved", user=42, ms=17)      # fields = {'user': 42, 'ms': 17}

The reverse operation unpacks a collection into separate arguments (spread and rest):

nums = [1, 2, 3]
total(*nums)          # same as total(1, 2, 3)

total(...nums);

Fixed parameters should come first, then the variadic one.
Typos pass silently. A **kwargs function accepts misspelled options. Validate known names.
Unclear signatures. If a function can take “anything”, callers can’t see what’s valid. When arguments have different meanings, use named parameters or an options object (default parameters).
Wrappers use *args, **kwargs to pass everything through, which is common in decorators but hides the real signature.
Very long argument lists have limits in some languages.