What does ** (double star/asterisk) and * (star/asterisk) do for parameters?

What do *args and **kwargs mean in these function definitions?

def foo(x, y, *args):
    pass

def bar(x, y, **kwargs):
    pass

See What do ** (double star/asterisk) and * (star/asterisk) mean in a function call? for the complementary question about arguments.

The *args and **kwargs are common idioms to allow an arbitrary number of arguments to functions, as described in the section more on defining functions in the Python tutorial.

The *args will give you all positional arguments as a tuple:

def foo(*args):
    for a in args:
        print(a)        

foo(1)
# 1

foo(1, 2, 3)
# 1
# 2
# 3

The **kwargs will give you all
keyword arguments as a dictionary:

def bar(**kwargs):
    for a in kwargs:
        print(a, kwargs[a])  

bar(name='one', age=27)
# name one
# age 27

Both idioms can be mixed with normal arguments to allow a set of fixed and some variable arguments:

def foo(kind, *args, bar=None, **kwargs):
    print(kind, args, bar, kwargs)

foo(123, 'a', 'b', apple='red')
# 123 ('a', 'b') None {'apple': 'red'}

It is also possible to use this the other way around:

def foo(a, b, c):
    print(a, b, c)

obj = {'b':10, 'c':'lee'}

foo(100, **obj)
# 100 10 lee

Another usage of the *l idiom is to unpack argument lists when calling a function.

def foo(bar, lee):
    print(bar, lee)

baz = [1, 2]

foo(*baz)
# 1 2

In Python 3 it is possible to use *l on the left side of an assignment (Extended Iterable Unpacking), though it gives a list instead of a tuple in this context:

first, *rest = [1, 2, 3, 4]
# first = 1
# rest = [2, 3, 4]

Also Python 3 adds a new semantic (refer PEP 3102):

def func(arg1, arg2, arg3, *, kwarg1, kwarg2):
    pass

Such function accepts only 3 positional arguments, and everything after * can only be passed as keyword arguments.
Note:

A Python dict, semantically used for keyword argument passing, is arbitrarily ordered. However, in Python 3.6+, keyword arguments are guaranteed to remember insertion order.
"The order of elements in **kwargs now corresponds to the order in which keyword arguments were passed to the function." - What’s New In Python 3.6.
In fact, all dicts in CPython 3.6 will remember insertion order as an implementation detail, and this becomes standard in Python 3.7.

Stack Overflow Q#36901, score: 3342