[Rhodri James <rho...@kynesim.co.uk>]
> I'm seriously going to maintain that I will forget the meaning of "case
> _:" quickly and regularly,
Actually, you won't - trust me ;-)
> just as I quickly and regularly forget to use
> "|" instead of "+" for set union. More accurately, I will quickly and
> regularly forget that in this one place, "_" is special.
Because that's the opposite of "accurate". There's nothing special
about "_" "in this one place". It's but a single application of that
"_" is used as a wildcard in _all_ matching contexts throughout the
PEP.
And it's not even new with this PEP. "_" is routinely used already in
lots of code to mean "the syntax requires a binding target here, but I
don't care about the binding", from
lists = [[] for _ in range(100)]
to
first, _, third = triple
The last is especially relevant, because that's already a form of destructuring.
The only thing new about this use of "_" in the PEP is that it
specifies no binding will occur. Binding does occur in the examples
above (because there's nothing AT ALL special about "_" now - it's
just a one-character identifier, and all the rest is convention,
including that the REPL uses it to store the value of the
last-displayed object).
>> See reply to Glenn. Can you give an example of a dotted name that is
>> not a constant value pattern? An example of a non-dotted name that is?
>> If you can't do either (and I cannot)), then that's simply what "if
> case long.chain.of.attributes:
That's a dotted name and so is a constant value pattern - read the PEP.
Every dotted name in a pattern is looked up using normal Python
name resolution rules, and the value is used for comparison by
equality with the matching expression (same as for literals).
> or more likely
>
> case (foo.x, foo.y)
Ditto.
> for the first. For the second, it's a no-brainer that you can't have a
> non-dotted name as a constant value pattern, since the current constant
> value pattern mandates a leading dot.
Not so. _Solme_ dot is necessary and sufficient to identify a
constant value pattern now. A leading dot is only _required_ in case
an intended constant value pattern would have no dots otherwise.
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