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

[Terry Reedy]

Oleg Broytmann wrote:
> 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  

To repeat: I think one-usage default function objects defined by lambda 
are rather rare.  The most obvious is
def ident(ob): return ob
which, if used once, would likely be used more than once and defined as 
above, and which always could be so defined.  I suspect even more rare 
is such defaults used in the same function as a mutable default such as 
[] or {} that needs protecting.  And in such rare cases, one could 
either pull the function definition into a def statement or prefix it by 
a second lambda.

>> "lambda : lambda : []") doesn't look good).

and I would not suggest it unless what one wanted was for the default 
arg for each call, after the lambda call, to be 'lambda: []' and not '[]'.

>> 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

Or dynamic could have an optional explicit list.  But that option would 
be rarely needed, I think.

tjr