Function params with **? what do these mean?

[J Rice]

I'm sorry for such a basic question, but I haven't been able to phrase
a search that gets me an answer and my books are totally silent on
this. I have seen a number of python function defs that take
parameters of the form (**param1). Looks like a pointer... but my
books on python (basic as they are) don't make a mention. What is
this?

Jeff

[Dave Hansen]

Re: Function params with **? what do these mean?

On 20 Mar 2006 12:46:43 -0800 in comp.lang.pytho n, "J Rice"
<rice.jeffrey@g mail.com> wrote:
[color=blue]
>I'm sorry for such a basic question, but I haven't been able to phrase
>a search that gets me an answer and my books are totally silent on
>this. I have seen a number of python function defs that take
>parameters of the form (**param1). Looks like a pointer... but my
>books on python (basic as they are) don't make a mention. What is
>this?[/color]

It's a way of accepting a varying number of named arguments. In the
function, the parameter becomes a dictionary with parameter names as
the keys corresponding to the passed parameter values.

It's harder to explain than understand. Try playing with the
following function in the python interpreter:

def test(a,b='b', *c, **d):
print a,b,c,d

A couple suggestions for tests:

test(1,2,3,4)
test(a=1,b=2,c= 3,d=4)
test(2,4,6,8,10 ,12,ralph=23,to ny=45)

See what happens. Should be mostly self-explanatory.

Regards,
-=Dave

--
Change is inevitable, progress is not.

[Jordan Greenberg]

Re: Function params with **? what do these mean?

in the parameter list, **param gets a dict of arguments that dont
correspond to somthing in the formal parameter list.

More & examples in the python docs:

http://docs.python.org/tut/node6.html#SECTION006720000000000000000

--
Jordan T. Greenberg

[Larry Bates]

Re: Function params with **? what do these mean?

J Rice wrote:[color=blue]
> I'm sorry for such a basic question, but I haven't been able to phrase
> a search that gets me an answer and my books are totally silent on
> this. I have seen a number of python function defs that take
> parameters of the form (**param1). Looks like a pointer... but my
> books on python (basic as they are) don't make a mention. What is
> this?
>
> Jeff
>[/color]
There are too forms that you may be confusing. First
[color=blue][color=green][color=darkred]
>>> def foo(x,y,z):
>>> return x+y+z[/color][/color][/color]
[color=blue][color=green][color=darkred]
>>> t=[1,2,3][/color][/color][/color]
[color=blue][color=green][color=darkred]
>>> foo(*t)[/color][/color][/color]

6


This tells python to expand t and pass it as as 3
separate arguments

The second is:
[color=blue][color=green][color=darkred]
>>> def foo(*args):
>>> return sum(args)[/color][/color][/color]
[color=blue][color=green][color=darkred]
>>> foo(1,2,3)[/color][/color][/color]

6

This allows you to treat all the arguments to a function
as a list of arguments no matter how many there are.

or
[color=blue][color=green][color=darkred]
>>> def bar(**kwargs):
>>> for key, value in kwargs.items():
>>> print "key=%s, value=%s" % (key, value)
>>> return[/color][/color][/color]
[color=blue][color=green][color=darkred]
>>> bar(a=1, b=2, c="this is a test")[/color][/color][/color]
key=a, value=1
key=c, value=this is a test
key=b, value=2


This allows you to access keyword arguments via a dictionary
in the function, instead of individually.


You can combine these to make very powerful functions/methods
that can handle different numbers of arguments and keyword
arguments.

def foo(*args, **kwargs):
....


Larry Bates

[J Rice]

Re: Function params with **? what do these mean?

Wow, this is incredibly useful! I can understand why an introductory
book wouldn't make use of them, but I am really glad to know about
them. I can think of a bunch of ways to simply some code I have using
this.

[Aahz]

Re: Function params with **? what do these mean?

In article <r56u12pb4ajvhs hvh40slb80di1bf eiagr@4ax.com>,
Dave Hansen <iddw@hotmail.c om> wrote:[color=blue]
>On 20 Mar 2006 12:46:43 -0800 in comp.lang.pytho n, "J Rice"
><rice.jeffrey@ gmail.com> wrote:[color=green]
>>
>>I'm sorry for such a basic question, but I haven't been able to phrase
>>a search that gets me an answer and my books are totally silent on
>>this. I have seen a number of python function defs that take
>>parameters of the form (**param1). Looks like a pointer... but my
>>books on python (basic as they are) don't make a mention. What is
>>this?[/color]
>
>It's a way of accepting a varying number of named arguments. In the
>function, the parameter becomes a dictionary with parameter names as
>the keys corresponding to the passed parameter values.
>
>It's harder to explain than understand. Try playing with the
>following function in the python interpreter:
>
> def test(a,b='b', *c, **d):
> print a,b,c,d[/color]

Personally, I think it's a Good Idea to stick with the semi-standard
names of *args and **kwargs to make searching easier...
--
Aahz (aahz@pythoncra ft.com) <*> http://www.pythoncraft.com/

"19. A language that doesn't affect the way you think about programming,
is not worth knowing." --Alan Perlis

[Dave Hansen]

Re: Function params with **? what do these mean?

On 20 Mar 2006 15:45:36 -0800 in comp.lang.pytho n,
aahz@pythoncraf t.com (Aahz) wrote:
[color=blue]
>In article <r56u12pb4ajvhs hvh40slb80di1bf eiagr@4ax.com>,
>Dave Hansen <iddw@hotmail.c om> wrote:[/color]
[...][color=blue][color=green]
>>It's harder to explain than understand. Try playing with the
>>following function in the python interpreter:
>>
>> def test(a,b='b', *c, **d):
>> print a,b,c,d[/color]
>
>Personally, I think it's a Good Idea to stick with the semi-standard
>names of *args and **kwargs to make searching easier...[/color]

Agreed (though "kwargs" kinda makes my skin crawl). I don't use these
features often in my code, but when I do, I follow the convention. The
example was just for illustrative purposes, and the names chosen for
easy typing.

It is important to note that using "args" and "kwargs" is a convention
rather than a requirement, analogous to "self". You can use different
identifiers, but future maintainers of your code will be annoyed.

But it won't affect the operation of the code. I found the test case
"test(a=1,b=2,c =3,d=4)" to be most edifying.

Regards,
-=Dave

--
Change is inevitable, progress is not.

[Ben Cartwright]

Re: Function params with **? what do these mean?

Dave Hansen wrote:[color=blue]
> On 20 Mar 2006 15:45:36 -0800 in comp.lang.pytho n,
> aahz@pythoncraf t.com (Aahz) wrote:[color=green]
> >Personally, I think it's a Good Idea to stick with the semi-standard
> >names of *args and **kwargs to make searching easier...[/color]
>
> Agreed (though "kwargs" kinda makes my skin crawl).[/color]

Coincidentally, "kwargs" is the sound my cat makes when coughing up a
hairball.

Fortunately, **kw is also semi-standard.

--Ben

[Dave Benjamin]

Re: Function params with **? what do these mean?

On Mon, 20 Mar 2006, Ben Cartwright wrote:
[color=blue]
> Dave Hansen wrote:[color=green]
>> On 20 Mar 2006 15:45:36 -0800 in comp.lang.pytho n,
>> aahz@pythoncraf t.com (Aahz) wrote:[color=darkred]
>>> Personally, I think it's a Good Idea to stick with the semi-standard
>>> names of *args and **kwargs to make searching easier...[/color]
>>
>> Agreed (though "kwargs" kinda makes my skin crawl).[/color]
>
> Coincidentally, "kwargs" is the sound my cat makes when coughing up a
> hairball.
>
> Fortunately, **kw is also semi-standard.[/color]

I prefer the semi-standard **kwds, myself. ;)

[Carl Banks]

Re: Function params with **? what do these mean?

Aahz wrote:[color=blue]
> Personally, I think it's a Good Idea to stick with the semi-standard
> names of *args and **kwargs to make searching easier...[/color]

I usually do stick to these names (since the I usually only use them
when forwarding arguments to another function, where such names are a
pretty good description), but I can't think of any particular reason to
search for all occurrences of them.

Carl Banks

[Scott David Daniels]

Re: Function params with **? what do these mean?

J Rice wrote:[color=blue]
> I'm sorry for such a basic question, but I haven't been able to phrase
> a search that gets me an answer and my books are totally silent on
> this. I have seen a number of python function defs that take
> parameters of the form (**param1). Looks like a pointer... but my
> books on python (basic as they are) don't make a mention. What is
> this?[/color]

At the risk of being thought of as beating a dead horse, this was a
_great_ way to ask this question. Others may note that nobody told
you you were abusing the newsgroup to do your work for you; you told
us how you tried to answer the question for yourself.

So, anyhow, thanks for taking the time to write your question properly.

--
-Scott David Daniels
scott.daniels@a cm.org

[Ben Finney]

Re: Function params with **? what do these mean?

Scott David Daniels <scott.daniels@ acm.org> writes:
[color=blue]
> At the risk of being thought of as beating a dead horse, this was a
> _great_ way to ask this question. [...]
> So, anyhow, thanks for taking the time to write your question properly.[/color]

Take that risk, please. There's enough lambasting of (and probably
much more private grumbling over) poor delivery of questions, but not
enough praise when *good* form is followed.

The latter does much more to show what we consider appropriate.

--
\ "Ice Water? Get some onions - that'll make your eyes water!" |
`\ -- Groucho Marx |
_o__) |
Ben Finney