[Python-ideas] Default arguments in Python - the return - running out of ideas but...

[Pascal Chambon]

Oleg Broytmann a écrit :
> On Tue, May 19, 2009 at 10:31:00PM +0200, Pascal Chambon wrote:
>   
>> I could content myself of the proposed solution :
>> @dynamic
>> def func (a, b = lambda : []):
>>    pass
>> But I just dislike the fact that the "dynamic" applies to all the  
>> defaults, even those which weren't supposed to be dynamic (and writing  
>> "lambda : lambda : []") doesn't look good).
>> Would there be any way of separating "to-be-called" lambdas from normal  
>> ones ? Except with a syntax like "b = dyn(lambda: [])" ?
>>     
>
> @dynamic('b')
> def func (a, b = lambda : []):
>     pass
>
> Oleg.
>   
*shame on me - /me must be tired this evening*

Well, with that solution we don't avoid some amount of boiler plate 
code, but imo the pros are
- we don't have to care about copy/deepcopy constraints, or giving the 
default argument expression as a string (with all the problems eval() 
might raise)
- most important for me : we have a pattern that could be broadcasted as 
a "standard practice", and thus warn newbies about the normal behaviour 
of default arguments.
If the "dynamic" (or any other proper name) decorator became part of the 
builtin ones, like staticmethod or classmethod, in my opinion newbies 
would quickly run into it, get used to it, and we'd have a common 
practice instead of the numerous ones currently possible to handle 
dynamic defaults (sentinels, other decorators...).

What do you think ? Is that worth the change compared to the sentinel 
stuff ?
In my opinion, it's clearly more explicit, and the newbie will never 
think "that programmer is dumb, he has put None as a default whereas we 
can directly put lists or any other expressions in the signature, we're 
not restricted to constants like in other languages" as it might 
currently be the case.

I'm eventually +1 on such a decorator/lambda mix (in the absence of more 
straightforward but language-changing syntax)

++
Pascal
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