On Mon, 26 Dec 2022 at 04:53, Steven D'Aprano <[email protected]> wrote:
>
> On Sat, Dec 24, 2022 at 11:34:19AM -0500, Shironeko wrote:
> >
> > Is the => syntax needed? as far as I can think of, the only time where
> > late evaluation is needed is when the expression references the other
> > arguments.
>
> You are missing the most common case, the motivating case, for
> late-bound defaults: mutable defaults.
>
> def spam(x, y=>[]):
> pass
Exactly - this is the most important reason. So the true reason for
the => syntax is: in order to gain late-bound defaults, we have to
distinguish them from early-bound defaults (because changing ALL
function default arguments to late-bound would be a massive breaking
change). There's no particular reason for it to be "=>" specifically,
and other syntax options have been considered, but it does need to be
something other than "=".
> The ability for default values to refer to other parameters is a Nice To
> Have, not a Must Have. It has been a very long time since I have read
> the PEP, and I don't remember whether it reviews other languages to see
> what functionality they provide for defaults, but I don't think many
> other languages allow you to set the default of one parameter to be
> another parameter.
JavaScript does, by nature of its extremely simplistic definition of
argument defaults.
function f(x=1, y=x) {console.log("--> x = ", x, ", y = ", y);}
f(5)
--> x = 5 , y = 5
f()
--> x = 1 , y = 1
f(42, undefined)
--> x = 42 , y = 42
f(undefined, 123)
--> x = 1 , y = 123
Ruby does:
$ irb
irb(main):001:1* def f(x=1, y=x)
irb(main):002:1* puts "x = #{x}, y = #{y}"
irb(main):003:0> end
=> :f
irb(main):004:0> f()
x = 1, y = 1
=> nil
irb(main):005:0> f(123)
x = 123, y = 123
=> nil
I suspect that, in each case, the rule is quite simple: the argument
default is evaluated in the context of the function's body. Exactly
the same as PEP 671 proposes.
Any other languages to test?
ChrisA
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