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

Question

In the following method definitions, what does the * and ** do for param2?

def foo(param1, *param2): def bar(param1, **param2): 

Answer 1

The *args and **kwargs is a common idiom to allow arbitrary number of arguments to functions as described in the section more on defining functions in the Python documentation.

The *args will give you all function parameters 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 except for those corresponding to a formal parameter 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, **kwargs):    pass 

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)  l = [1,2]  foo(*l) # 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, *l, last = [1,2,3,4] 

Also Python 3 adds 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, are 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, this becomes standard in Python 3.7.