On Sun, 12 Jul 2020 at 10:47, Larry Hastings <la...@hastings.org> wrote:
> In that case, I'd like to make a specific pitch for "don't make '_' special".
> (I'm going to spell it '_' as it seems to be easier to read this way; ignore
> the quotes.)
Overall, this sounds mostly reasonable. I'm cutting nearly everything
here, because I don't have anything to add.
> One consideration: if you do use '_' multiple times in a single pattern, and
> you do refer to its value afterwards, what value should it get? Consider
> that Python already permits multiple assignments in a single expression:
>
> (x:="first", x:="middle", x:="last")
>
> After this expression is evaluated, x has been bound to the value "last". I
> could live with "it keeps the rightmost". I could also live with "the result
> is implementation-defined". I suspect it doesn't matter much, because the
> point of the idiom is that people don't care about the value.
The problem for me is specifically with variables *other* than `_` -
precisely because `_` has connotations of "don't care".
If I see
match expr:
case Point(x, x):
# what is x here?
I would very strongly expect that to mean that the two components of
Point were equal, and x was set to the common value. That's not what
Python does, so this would be a fairly easy mistake to make.
For what it's worth, it looks like Rust uses the same rule as the PEP
- multiple occurrences of the same variable are not allowed, but _ is
a wildcard that *can* be used multiple times, but isn't bound.
Paul
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