On Sun, Oct 24, 2021 at 9:52 PM Serhiy Storchaka <[email protected]> wrote:
>
> 23.10.21 19:07, Chris Angelico пише:
> > _missing = object()
> > def spaminate(thing, count=_missing):
> > if count is _missing: count = thing.getdefault()
> >
> > Proposal: Proper syntax and support for late-bound argument defaults.
> >
> > def spaminate(thing, count=:thing.getdefault()):
> > ...
>
> Few years ago I proposed a syntax for optional arguments without default
> value:
>
> def spaminate(thing, count=?):
> try:
> count
> except UnboundLocalError:
> count = thing.getdefault()
> ...
Ah yes, I'd forgotten about this proposal.
> It would help in cases in which we now use None or special singleton
> value. It is more general than late-bound arguments, because it can be
> used in cases in which the default argument cannot be expressed, like in
> getattr() and dict.pop().
True, but in the example you give here, I would definitely prefer
"count=>thing.getdefault()".
> The code for initialization of the default value is something
> complicated, but we can introduce special syntax for it:
>
> if unset count:
> count = thing.getdefault()
>
> or even
>
> count ?= thing.getdefault()
This is the perfect way to stir the pot on the None-coalescing
discussion, because now you're looking at a very real concept of null
value :)
(Although there's no way to return this null value from a function or
pass it to another, so the state of being unbound should be local to a
single code unit.)
> > 1) Inspecting the function would reveal the source code for the late-bound
> > value
> > 2) There is no value which can be passed as an argument to achieve the
> > same effect
>
> This is the largest problem of both ideas. The inspect module has no way
> to represent optional arguments without default value, and any solution
> will break user code which is not ready for this feature.
There kinda sorta is a way to do it, at least for keyword arguments:
def func_left(x, y=>len(x)): ...
def func_right(x, y=?):
if unset y: y = {}
else y = {"y": y}
return func_left(-x, **y)
But if there were a value that could be passed as an argument to
indicate "no value", then we'd come across the same problem of using
None as a default: sometimes, literally any value could be valid, and
we still need to signal the absence of a value.
ChrisA
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