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):