[Python-ideas] values in vs. values out

[Luc Goossens]

Hi Eric (and Rob, and Ben, ...),

Sorry maybe this was not clear from my mail but I am not so much  
interested in possible work-arounds but in why this asymmetry exists  
in the first place.
I mean is there a reason as to why it is the way it is, or is it just  
that nobody ever asked for anything else.

cheers,
Luc

On Jan 13, 2011, at 4:00 PM, Eric Smith wrote:

> On 01/13/2011 09:30 AM, Luc Goossens wrote:
>> Hi all,
>>
>> There's a striking asymmetry between the wonderful flexibility in
>> passing values into functions (positional args, keyword args, default
>> values, *args, **kwargs, ...) and the limited options for  
>> processing the
>> return values (assignment).
>> Hence, whenever I upgrade a function with a new keyword arg and a
>> default value, I do not have to change any of the existing calls,
>> whereas whenever I add a new element to its output tuple, I find  
>> myself
>> chasing all existing code to upgrade the corresponding assignments  
>> with
>> an additional (unused) variable.
>> So I was wondering whether this was ever discussed before (and  
>> recorded)
>> inside the Python community.
>> (naively what seems to be missing is the ability to use the  
>> assignment
>> machinery that binds functions' formal params to the given actual  
>> param
>> list also in the context of a return value assignment)
>
> You can achieve something similar with PEP 3132's Extended Iterable  
> Unpacking:
>
> >>> def f(): return 0, 1, 2, 3
> ...
> >>> a, b, c, d, *unused = f()
> >>> a, b, c, d, unused
> (0, 1, 2, 3, [])
>
> If you add more return values, they show up in unused.
>
> >>> def f(): return 0, 1, 2, 3, 4
> ...
> >>> a, b, c, d, *unused = f()   # note caller is unchanged
> >>> a, b, c, d, unused
> (0, 1, 2, 3, [4])
>
> Or you could return dicts.
>
> Eric.
>
>
>
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