If you look in PEP 622 you'll see that there was a rejected idea `if match
...` that's pretty similar. We nixed it because it just made the PEP
larger. For 3.11 we can consider something like this.
On Fri, Oct 23, 2020 at 4:12 PM Valentin Berlier <berlie...@gmail.com>
wrote:
> Pattern-matching is great. I think PEP 634 is on the right track, but it
> would be a waste to only use pattern-matching for choosing a branch in a
> match statement.
>
> Let’s look at Rust:
>
> if let [x, y] = my_array {
> ...
> }
>
> Rust "if let" constructs are an alternative to full-blown match
> statements that make it less verbose to match a single pattern.
>
> We have a similar problem. The syntax proposed by PEP 634 is pretty
> verbose for matching a single pattern:
>
> match my_list:
> case [x, y]:
> ...
>
> Two keywords and two indentation levels. We can do better:
>
> if [x, y] := my_list:
> ...
>
> Yes, your first intuition is right. This looks similar to the Rust
> version but would work completely differently. But hear me out. Let's
> look past my terrible example and focus on the idea.
>
> 1. The walrus operator was purposefully designed to create bindings and
> not to perform assignments to arbitrary lvalues.
> 2. Matching a pattern only introduces bindings as well, no assignments.
> 3. The current behavior of the walrus operator is equivalent to matching
> the right-side operand to an "irrefutable" capture pattern.
>
> Allowing the walrus operator to do pattern-matching would simply make
> the returned value conditional. If the pattern doesn't match, the walrus
> operator returns None.
>
> print(x := 42) # 42
> print(1 := 42) # None
>
> The current PEG parser would backtrack when encountering the walrus
> operator to interpret the left-side as a pattern.
>
> Finally, more examples:
>
> # Buy every pizza on the menu
> sum(
> price for food in menu
> if ({"type": "pizza", "price": price} := food)
> )
>
> # Download all images from document
> {url: download(url) for tag in html if (Img(src=url) := tag)}
>
> # Monitor service health
> while Response(status=200, json={"stats": stats}) := health_check():
> print(stats)
> time.sleep(5)
>
> I'm convinced that making the walrus operator a pattern-matching
> operator would turn it into the perfect companion for PEP 634. What do
> you think?
>
> References:
>
> - PEP 634: https://www.python.org/dev/peps/pep-0634/
> - Rust "if let": https://doc.rust-lang.org/book/ch06-03-if-let.html
>
> --
> Valentin
> _______________________________________________
> Python-ideas mailing list -- python-ideas@python.org
> To unsubscribe send an email to python-ideas-le...@python.org
> https://mail.python.org/mailman3/lists/python-ideas.python.org/
> Message archived at
> https://mail.python.org/archives/list/python-ideas@python.org/message/MJ7JHYKHKB2T4SCFV4TX4IMKUANUAF5B/
> Code of Conduct: http://python.org/psf/codeofconduct/
>
--
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
_______________________________________________
Python-ideas mailing list -- python-ideas@python.org
To unsubscribe send an email to python-ideas-le...@python.org
https://mail.python.org/mailman3/lists/python-ideas.python.org/
Message archived at
https://mail.python.org/archives/list/python-ideas@python.org/message/PVUQCU5722JVQC773OOEJ7I7FK3SFSS2/
Code of Conduct: http://python.org/psf/codeofconduct/
