Function __defaults__

Colin J. Williams cjw at ncf.ca
Mon Apr 25 16:24:33 CEST 2011


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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