One thing that I always loved about Python was its readability. As I
got into it more and more, though, I frequently ran into things that
seemed pretty cryptic; one of them was the use of * and **, especially
in function calls. This message on the Python list explains things
pretty clearly (assuming you're familiar with the python 'zip'
function!).
Begin forwarded message:
> From: Christopher T King <squirrel (at) WPI .DOT EDU>
> Date: June 30, 2004 4:29:23 PM EDT
> To: python-list@python.org
> Subject: Re: zip asterisk syntax
> X-Spambayes-Classification: ham; 0.00
>
> garett wrote:
>
>> Hello, I have been reading text processing in python and in the
>> appendix
>> the author describes:
>>>>> sides = [(3, 4), (7, 11), (35, 8)]
>>>>> zip(*zip(*sides))
>>
>> what is this asterisk-list syntax called? Any suggestions for finding
>> more
>> information about it? Thanks. -Garett
>
> It's called the "extended function call" syntax. Basically what it
> does
> is take the items of a list and use them as arguments to a function.
> Let's take the example above:
>
>>>> sides = [(3, 4), (7, 11), (35, 8)]
>>>> zip(*zip(*sides))
>
> is the same as:
>
>>>> zip(*zip((3,4),(7,11),(35,8)))
>
> zip() mangles its arguments in such a way that the above call is
> equivalent to this:
>
>>>> zip(*[(3,7,35),(4,11,8)])
>
> which is the same as:
>
>>>> zip((3,7,35),(4,11,8))
>
> which evaluates to [(3,4),(7,11),(35,8)].
>
> Back on topic, the dictionary equivalent of * is **; that is, ** takes
> the
> key-value pairs from a dictionary and uses them as arguments to a
> function. * and ** must appear after other arguments to the function,
> and
> you may only have at most one each of * and **.
>
> There's another context other than function calls in which * and **
> can be
> used, and that's function definitions:
>
>>> def(a,b=5,*c,**d): pass
>
> will pack any extra arguments (beyond a and b) into list c and
> dictionary
> d.
>
> For (very) detailed information, see
> http://docs.python.org/ref/calls.html
> and http://docs.python.org/ref/function.html.
>
> --
> http://mail.python.org/mailman/listinfo/python-list
___/
/
__/
/
____/
Ed Leafe
http://leafe.com/
http://dabodev.com/
©2004 Ed Leafe |
