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Re: Function __defaults__

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On 25-Apr-11 08:30 AM, Ken Seehart wrote:
> On 4/25/2011 4:59 AM, Colin J. Williams wrote:
>> On 24-Apr-11 13:07 PM, Ken Seehart wrote:
>>> On 4/24/2011 2:58 AM, Steven D'Aprano wrote:
>>>> Consider this in Python 3.1:
>>>>
>>>>
>>>>>>> def f(a=42):
>>>> ... return a
>>>> ...
>>>>>>> f()
>>>> 42
>>>>>>> f.__defaults__ = (23,)
>>>>>>> f()
>>>> 23
>>>>
>>>>
>>>> Is this an accident of implementation, or can I trust that changing
>>>> function defaults in this fashion is guaranteed to work?
>>>
>>> This is documented in python 3, so I would expect it to be stable (until
>>> python 4, that is)
>>> http://docs.python.org/py3k/whatsnew/3.0.html#operators-and-special-methods
>>>
>>> http://docs.python.org/py3k/library/inspect.html#types-and-members
>>>
>>> The f.__defaults__ attribute was previously known as f.func_defaults (in
>>> python 2.x), which has been around, documented and stable for quite a
>>> while.
>>>
>>> So it's probably just as safe as any other monkey patching technique. :)
>>>
>>> Best of luck,
>>> Ken
>>>
>>
>> Wouldn't it make more sense to return a dictionary instead of a tuple?
>>
>> Colin W.
>>
>
> I assume you mean making the value of f.__defaults__ a dictionary
> instead of a tuple.
>
> A dictionary would be slower to process since it would have to iterate
> the dictionary keys and assign arguments by name.
> Since argument defaults can only be applied to the rightmost contiguous
> sequence of zero or more parameters (excluding *args,**kwargs), a tuple
> is sufficient to cover all cases, so a dictionary would provide no
> advantage.
> Also, a dictionary would produce an unnecessary error case (if a key in
> the dictionary is not the name of an argument).
>
> Good question though.
>
> Cheers,
> Ken
>
I doubt that this functionality would be used in time critical work and 
so I suggest that efficiency is not a key consideration.

Loss of information is perhaps more important.  With the tuple being 
returned, the user is not informed that the value is associated with the 
name "a"

Colin W.

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Function __defaults__ Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-04-24 09:58 +0000
  Re: Function __defaults__ Terry Reedy <tjreedy@udel.edu> - 2011-04-24 12:48 -0400
  Re: Function __defaults__ Benjamin Kaplan <benjamin.kaplan@case.edu> - 2011-04-24 13:02 -0400
  Re: Function __defaults__ Daniel Kluev <dan.kluev@gmail.com> - 2011-04-25 04:18 +1100
  Re: Function __defaults__ Ken Seehart <ken@seehart.com> - 2011-04-24 10:07 -0700
    Re: Function __defaults__ Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-04-25 02:00 +0000
  Re: Function __defaults__ Ken Seehart <ken@seehart.com> - 2011-04-24 14:44 -0700
  Re: Function __defaults__ Daniel Kluev <dan.kluev@gmail.com> - 2011-04-25 08:47 +1100
  Re: Function __defaults__ Terry Reedy <tjreedy@udel.edu> - 2011-04-24 17:53 -0400
  Re: Function __defaults__ Ken Seehart <ken@seehart.com> - 2011-04-24 14:54 -0700
  Re: Function __defaults__ "Colin J. Williams" <cjw@ncf.ca> - 2011-04-25 07:59 -0400
  Re: Function __defaults__ Ken Seehart <ken@seehart.com> - 2011-04-25 05:30 -0700
  Re: Function __defaults__ "Colin J. Williams" <cjw@ncf.ca> - 2011-04-25 10:24 -0400

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