On Fri, 13 Nov 2020 at 09:39, David Mertz <[email protected]> wrote:
>
> I have read a great deal of discussion on the pattern matching PEPs and less
> formal discussions. It is possible I have overlooked some post in all of
> that, of course.
>
> ... OK, just saw Guido's "wait for new SC" comment, which I suppose applies
> to this too :-).
>
> One idea that I cannot recall seeing, but that seems to make sense to me and
> fit with Python's feel is using a WORD to distinguish between a variable
> value and a binding target. That is, instead of a special symbol prefixing
> or suffixing a name, either to indicate it is or is not a binding target. Of
> course, whether the extra word would be used for binding or for NOT binding
> is a question still.
If someone was prepared to pursue this to the level of writing a 3rd
competing PEP, the variant I would personally like to see written up
is the one where capture patterns are all prefixed with the keyword
`as`.
PEP 634 already uses the `PATTERN as NAME` syntax to combine other
match patterns with a capture pattern, and I'm going to be amending
PEP 642 to propose using `as` when embedding capture patterns inside
class patterns (`ATTR as NAME`) and mapping patterns (`KEY as NAME`).
>From there, it's an entirely plausible step to also require the `as`
prefix on capture patterns in sequence patterns and as top level
standalone patterns.
I personally don't think that extra step would be a good idea due to
the inconsistency with name binding and iterable unpacking in regular
assignment statements (if I liked the idea, I'd have already included
it in PEP 642), but I think "anchor match patterns in normal
expressions rather than assignment target syntax" is a credible enough
idea that the overall design process would benefit from having a
champion write it up.
> To me these read better than the punctuation characters. But I guess some
> folks have suggested enlisting 'as', which is a word, of course.
Indeed, and one that doesn't look too bad for top level patterns:
NOT_FOUND = 404
match http_code:
case 200:
print("OK document")
case NOT_FOUND: # use the variable value
print("Document not found")
case as other_code: # bind this name
print("Other HTTP code")
It starts to look a bit more strange when matching sequences, though:
match seq:
case as first, *middle, as last:
... # Or should that be "*as middle"?
Cheers,
Nick.
--
Nick Coghlan | [email protected] | Brisbane, Australia
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