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

in following method definitions, * , ** param2?

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

the *args , **kwargs common idiom allow arbitrary number of arguments functions described in section more on defining functions in python documentation.

the *args give function parameters as tuple:

in [1]: def foo(*args):    ...:     in args:    ...:         print    ...:             ...:           in [2]: foo(1) 1   in [4]: foo(1,2,3) 1 2 3 

the **kwargs give keyword arguments except corresponding formal parameter dictionary.

in [5]: def bar(**kwargs):    ...:     in kwargs:    ...:         print a, kwargs[a]    ...:             ...:           in [6]: bar(name='one', age=27) age 27 name 1 

both idioms can mixed normal arguments allow set of fixed , variable arguments:

def foo(kind, *args, **kwargs):    pass 

another usage of *l idiom unpack argument lists when calling function.

in [9]: def foo(bar, lee):    ...:     print bar, lee    ...:         ...:       in [10]: l = [1,2]  in [11]: foo(*l) 1 2 

in python 3 possible use *l on left side of assignment (extended iterable unpacking), though gives list instead of tuple in 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 3 positional arguments, , after * can passed keyword arguments.