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

[MRAB]

Steven D'Aprano wrote:
> On Thu, 14 May 2009 05:18:37 am CTO wrote:
> 
>> If you thought not reevaluating function expressions
>> was confusing for newbies, wait until you see what making up a new
>> kind of yield will do for them.
>>
>> Why not just push for some decorators that do this to be included in
>> stdlib? I see the utility, but not the point of adding extra syntax.
> 
> Even if a decorator solution can be made to work, it seems to me that 
> the difficulty with a decorator solution is that it is 
> all-or-nothing -- you can decorate the entire parameter list, or none 
> of the parameters, but not some of the parameters. You can bet that 
> people will say they want delayed evaluation of some default arguments 
> and compile-time evaluation of others, in the same function definition.
> 
> There are work-arounds, of course, but there are perfectly adequate 
> work-arounds for the lack of delayed evaluation defaults now, and it 
> hasn't stopped the complaints.
> 
> I'm going to suggest that any syntax should be applied to the formal 
> parameter name, not the default value. This feels right to me -- we're 
> saying that it's the formal parameter that is "special" for using 
> delayed semantics, not that the default object assigned to it is 
> special. Hence it should be the formal parameter that is tagged, not 
> the default value.
> 
> By analogy with the use of the unary-* operator, I suggest we use a new 
> unary-operator to indicate the new semantics. Inside the parameter 
> list, &x means to delay evaluation of the default argument to x to 
> runtime:
> 
> def parrot(a, b, x=[], &y=[], *args, **kwargs):
> 
> As a bonus, this will allow for a whole new series of bike-shedding 
> arguments about which specific operator should be used. *grin*
> 
> Tagging a parameter with unary-& but failing to specify a default value 
> should be a syntax error:
> 
> def parrot(&x, &y=[]):
> 
> Likewise for unary-& outside of a parameter list.
> 
> Bike-shedding away... *wink*
> 
Well, going back to 'def', it could mean 'deferred until call-time':

     def parrot(a, b, x=[], y=def [], *args, **kwargs):