On Sat, Sep 26, 2020 at 10:30 PM Ben Rudiak-Gould <[email protected]>
wrote:
> I don't understand the problem here.
>
> d[p=q] --> d.__{get,set,del}item__((), ..., p=q)
> d[1, p=q] --> d.__{get,set,del}item__((1), ..., p=q)
> d[1, 2, p=q] --> d.__{get,set,del}item__((1, 2), ..., p=q)
> d[1, 2, 3, p=q] --> d.__{get,set,del}item__((1, 2, 3), ..., p=q)
> d[1, 2, ..., n, p=q] --> d.__{get,set,del}item__((1, 2, ..., n),
> ..., p=q)
>
> Now obviously the n=1 case is a wart. But the n=0 case isn't a wart. It's
> just like n=2, n=3, etc.
>
> You can't tell the difference between a single tuple argument and n
> arguments for any n≠1. As far as I can tell the problem when n=0 is no more
> or less serious than the problem when n≥2. I don't see the point of adding
> another special case to the spec when it doesn't even solve the general
> problem.
>
The problem is that there is lots of existing code like this:
def __setitem__(self, index, value): ...
But the new features will almost certainly lead people to write new code
like this:
d={}
obj[**d] = "foo" # no kwd arguments provided here
...which, if someone does this arbitrarily against classes that use the
existing kind of code I gave at the first, will call (if the sentinel is ()
):
obj.__setitem__(())
...which could have all kinds of weird effects rather than giving an error,
as I think would be better.
Steven's response to that was essentially "well then, don't unpack
dictionaries against arbitrary subscriptable types" which I fully agree is
a perfectly legitimate response. But I think having no sentinel at all and
instead telling people "either provide a default argument for both index
AND value in your __setitem__ method, or neither" is also a perfectly
legitimate way forward.
Not sure which is better.
---
Ricky.
"I've never met a Kentucky man who wasn't either thinking about going home
or actually going home." - Happy Chandler
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