Mailing List Archive: PEP 622: Structural Pattern Matching -- followup

> On 25 Jul 2020, at 03:44, Rob Cliffe <rob.cliffe@btinternet.com> wrote:
>
> Without arguing for or against allowing a capture variable, IMO rather
> than syntax like
> match <expr> into <var>:
> it would be far better (and not require a new keyword) to write this as
> with <var> as match <expr>:
>
> On 24/06/2020 20:38, Guido van Rossum wrote:
>> Everyone,
>>
>> If you've commented and you're worried you haven't been heard, please=20
>> add your issue *concisely* to this new thread. Note that the following=20
>> issues are already open and will be responded to separately; please=20
>> don't bother commenting on these until we've done so:
>>
>> - Alternative spellings for '|'
>> - Whether to add an 'else' clause (and how to indent it)
>> - A different token for wildcards instead of '_'
>> - What to do about the footgun of 'case foo' vs. 'case .foo'
>>
>> (Note that the last two could be combined, e.g. '?foo' or 'foo?' to=20
>> mark a variable binding and '?' for a wildcard.)
>>

We already have 'with' for contexts.

We already have patterns with 'as':
- except expression "as" identifier ":"
- "import" module "as" identifier
- "with" expression "as" target

So I think this would be quite confusing.
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On 24/06/2020 20:38, Guido van Rossum wrote:
> Everyone,
>
> If you've commented and you're worried you haven't been heard, please=20
> add your issue *concisely* to this new thread. Note that the following=20
> issues are already open and will be responded to separately; please=20
> don't bother commenting on these until we've done so:
>
> - Alternative spellings for '|'
> - Whether to add an 'else' clause (and how to indent it)
> - A different token for wildcards instead of '_'
> - What to do about the footgun of 'case foo' vs. 'case .foo'
>
> (Note that the last two could be combined, e.g. '?foo' or 'foo?' to=20
> mark a variable binding and '?' for a wildcard.)

Can we extend the syntax for the match statement from
match_stmt: "match" expression ':' NEWLINE INDENT case_block+ DEDENT
to
match_stmt: "match" expression ['as' NAME] ':' NEWLINE INDENT case_block+ DEDENT
with the same meaning as in "with"?
So the variable with the NAME assumes the value of the expression.

That way I won’t have to use the walrus at every case where I need the full result of the expression.
It could even be used to get rid of the walrus_pattern entirely.

Example:

match nested_data[index].attribute as foo:
...
case SomeClass(…):
bar(foo)
…
case AnotherClass(…):
baz(foo)
…
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