How to use variable length arguments (*args, **kwargs) in Python

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The following function arguments are probably the most common ones that stump you when you look at Python code and say, “What's this?

  • *args
  • **kwargs

Any number of arguments (variable-length arguments) can be specified by appending an asterisk to the argument in the function definition as follows

  • *
  • **

The names *args,**kwargs are often used as a convention. However, other names are acceptable as long as * and ** are at the beginning. The following sample code uses the names *args,**kwargs.

The following details are described below.

  • *args:Accepts multiple arguments as a tuple
  • **kwargs:Accepts multiple keyword arguments as a dictionary

*args: Accepts multiple arguments as a tuple

Arbitrary number of arguments can be specified by defining arguments with *, as in *args.

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

print(my_sum(1, 2, 3, 4))
# 10

print(my_sum(1, 2, 3, 4, 5, 6, 7, 8))
# 36

Multiple arguments are received as a tuple in the function. In the example, the sum() function is passed a tuple to calculate the sum.

def my_sum2(*args):
    print('args: ', args)
    print('type: ', type(args))
    print('sum : ', sum(args))

my_sum2(1, 2, 3, 4)
# args:  (1, 2, 3, 4)
# type:  <class 'tuple'>
# sum :  10

It can also be combined with a position argument.

The value specified after (to the right of) the positional argument is passed to args as a tuple. If there is only a positional argument, it is an empty tuple.

def func_args(arg1, arg2, *args):
    print('arg1: ', arg1)
    print('arg2: ', arg2)
    print('args: ', args)

func_args(0, 1, 2, 3, 4)
# arg1:  0
# arg2:  1
# args:  (2, 3, 4)

func_args(0, 1)
# arg1:  0
# arg2:  1
# args:  ()

Arguments marked with * may be defined first. In this case, however, arguments defined later than *args must be specified in keyword form. Incidentally, the keyword format is the “argument name = value” form.

The last value is not automatically passed to the positional argument. Therefore, if it is not specified as a keyword argument, a TypeError error will result.

def func_args2(arg1, *args, arg2):
    print('arg1: ', arg1)
    print('arg2: ', arg2)
    print('args: ', args)

# func_args2(0, 1, 2, 3, 4)
# TypeError: func_args2() missing 1 required keyword-only argument: 'arg2'

func_args2(0, 1, 2, 3, arg2=4)
# arg1:  0
# arg2:  4
# args:  (1, 2, 3)

If only * arguments are specified, subsequent arguments must always be specified as keyword arguments.(keyword-only argument)

def func_args_kw_only(arg1, *, arg2):
    print('arg1: ', arg1)
    print('arg2: ', arg2)

# func_args_kw_only(100, 200)
# TypeError: func_args_kw_only() takes 1 positional argument but 2 were given

func_args_kw_only(100, arg2=200)
# arg1:  100
# arg2:  200

**kwargs: Accepts multiple keyword arguments as a dictionary

Arbitrary number of keyword arguments can be specified by defining arguments with ,** as in **kwargs.

In the function, the name of the argument is received as a dictionary whose key is the key and whose value is the value.

def func_kwargs(**kwargs):
    print('kwargs: ', kwargs)
    print('type: ', type(kwargs))

func_kwargs(key1=1, key2=2, key3=3)
# kwargs:  {'key1': 1, 'key2': 2, 'key3': 3}
# type:  <class 'dict'>

It can also be used in conjunction with a position argument.

def func_kwargs_positional(arg1, arg2, **kwargs):
    print('arg1: ', arg1)
    print('arg2: ', arg2)
    print('kwargs: ', kwargs)

func_kwargs_positional(0, 1, key1=1)
# arg1:  0
# arg2:  1
# kwargs:  {'key1': 1}

By specifying the dictionary object with ** as an argument when calling the function, it is possible to expand it and pass it as the respective argument.

d = {'key1': 1, 'key2': 2, 'arg1': 100, 'arg2': 200}

func_kwargs_positional(**d)
# arg1:  100
# arg2:  200
# kwargs:  {'key1': 1, 'key2': 2}

Arguments marked with ** may only be defined at the end of the argument. Defining another argument after the argument marked with ** will result in a SyntaxError error.

# def func_kwargs_error(**kwargs, arg):
#     print(kwargs)

# SyntaxError: invalid syntax