I think the example that Steven gave, and Stephen approximately repeats is
good.
def func(items=[], n=later len(items)):
items.append("Hello")
print(n)
func()
> I guess Chris will say 0 and David will say 1, but I might be wrong about
either of them.
This is correct. And even though using a (soft) keyword like this gets me
to -0, the semantics I want indeed are different. I only want the binding
to be evaluated when it is referenced. If it never gets referenced, the
compassion time asked side effects are skipped.
Of course, the first line of the function body could be `n = n`. But adding
that line isn't so much different from starting with `if n is None: n = ...
`
As to Stephen's comments on not having used the "evaluate on reference"
pattern, that's pretty much not having used dask.deferred.
The difference is that with Dask (or Haskell) everything stays lazy until
you explicitly call `.compute()` on something in the DAG of operations. I'd
prefer not to need that.
But then my not-a-proposal would need a way to have "a reference that isn't
a reference". I think the same keywords works.
def func(items=[], n=later len(items)):
items.append("Hello")
n = later n**3 # n remains lazy
# ... more stuff
print(n) # actually evaluate the cube of length
On Sat, Jun 18, 2022, 10:52 AM Stephen J. Turnbull <
stephenjturnb...@gmail.com> wrote:
> Chris wants syntax for the common pattern
>
> def foo(arg_with_new_empty_list_default=None):
> if arg_with_new_empty_list_default is None:
> arg_with_new_empty_list_default = []
> # do stuff
>
> I can't really guess how useful the "use point" version would be. It's not
> a pattern I've used, I use a zero-argument function very occasionally but I
> can't recall a case where I used a lambda
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