How to use *args and **kwargs in Python

This also means how to use variable length argument lists in Python.

The special syntax,

*args

and

**kwargs

in function definitions is used to pass a variable number of arguments to a function. The single asterisk form (

*args

) is used to pass anon-keyworded, variable-length
argument list, and the double asterisk form is used to pass akeyworded, variable-length argument list. Here is an example of how to use the non-keyworded form. This example passes one formal (positional) argument, and two more variable length arguments.

def test_var_args(farg, *args):
print "formal arg:", farg
for arg in args:
print "another arg:", arg

test_var_args(1, "two", 3)

Results:

formal arg: 1
another arg: two
another arg: 3

Here is an example of how to use the keyworded form. Again, one formal argument and two keyworded variable arguments are passed.

def test_var_kwargs(farg, **kwargs):
print "formal arg:", farg
for key in kwargs:
print "another keyword arg: %s: %s" % (key, kwargs[key])

test_var_kwargs(farg=1, myarg2="two", myarg3=3)

Results:

formal arg: 1
another keyword arg: myarg2: two
another keyword arg: myarg3: 3

Using

*args

and

**kwargs

when calling a function

This special syntax can be used, not only in function definitions, but also whencalling a function.

def test_var_args_call(arg1, arg2, arg3):
print "arg1:", arg1
print "arg2:", arg2
print "arg3:", arg3

args = (1, "two", 3)
test_var_args_call(*args)

Results:

arg1: 1
arg2: two
arg3: 3

Here is an example using the keyworded form when calling a function:

def test_var_args_call(arg1, arg2, arg3):
print "arg1:", arg1
print "arg2:", arg2
print "arg3:", arg3

kwargs = {"arg3": 3, "arg2": "two", "arg1": 1}
test_var_args_call(**kwargs)

Results:

arg1: 1
arg2: two
arg3: 3

See also (updated 2009-04-12):
Section 4.7More on Defining Functions in the
Official Python Tutorial
PEP 3102 Keyword-Only Arguments
Section 5.3.4Calls in the
Python Reference Manual