What do * and ** before a variable name mean in a function signature? [duplicate]

Question

Possible Duplicate:
Understanding kwargs in Python

I have read a piece of python code, and I don't know what does * and ** mean in this code :

def functionA(self, *a, **kw):    // code here 

I just know about one use of *: extract all attribute it has to parameter of method or constructor.

If this true for above function, so what does the rest : ** ?

Answer 1

Inside a function header:

* collects all the positional arguments in a tuple.

** collects all the keyword arguments in a dictionary.

>>> def functionA(*a, **kw):        print(a)        print(kw)   >>> functionA(1, 2, 3, 4, 5, 6, a=2, b=3, c=5) (1, 2, 3, 4, 5, 6) {'a': 2, 'c': 5, 'b': 3} 

In a function call:

* unpacks a list or tuple into position arguments.

** unpacks a dictionary into keyword arguments.

>>> lis=[1, 2, 3, 4] >>> dic={'a': 10, 'b':20} >>> functionA(*lis, **dic)  #it is similar to functionA(1, 2, 3, 4, a=10, b=20) (1, 2, 3, 4) {'a': 10, 'b': 20} 

Answer 2

** takes specified argument names and puts them into a dictionary. So:

def func(**stuff):     print(stuff)  func(one = 1, two = 2) 

Would print:

{'one': 1, 'two': 2}