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Peter Hoffmann

Software Engineer
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What does ** (double star) and * (star) do for python parameters?

Posted on August 31, 2008
#stackoverflow #python

This my Answer to the stackoverflow question: What does ** (double star) and * (star) do for python parameters?:

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

The *args will give you all funtion parameters a a list:

In [1]: def foo(*args):
   ...:     for a in args:
   ...:         print a
   ...:         
   ...:         

In [2]: foo(1)
1


In [4]: 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.

In [5]: def bar(**kwargs):
   ...:     for a in kwargs:
   ...:         print a, kwargs[a]
   ...:         
   ...:         

In [6]: bar(name="one", age=27)
age 27
name one

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

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

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

In [9]: def foo(bar, lee):
   ...:     print bar, lee
   ...:     
   ...:     

In [10]: l = [1,2]

In [11]: foo(*l)
1 2

In the upcoming python 3.0 it will be possible to use *l on the left side of an assignment (Extended Iterable Unpacking):

first, *rest = [1,2,3,4]
first, *l, last = [1,2,3,4]