Description

sobolevn

opened on Sep 1, 2024

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Feature or enhancement

There's already a similar rule:

cpython/Grammar/python.gram

Line 1378 in 084e0f3

| or_pattern 'as' !NAME a=expression { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "invalid pattern target") }

But, it has two problems:

1. It does not work for cases like as a.b, because a matches NAME
2. It does not show rich error message, only a static one: invalid pattern target

My proposed change:

    | or_pattern 'as' a=expression {
        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(
            a, "cannot use pattern target as %s", _PyPegen_get_expr_name(a)) }

Why is it safe?

Here's how the parent rule is defined:

as_pattern[pattern_ty]:
    | pattern=or_pattern 'as' target=pattern_capture_target {
        _PyAST_MatchAs(pattern, target->v.Name.id, EXTRA) }
    | invalid_as_pattern

So, if pattern=or_pattern 'as' target=pattern_capture_target with a valid 'as' NAME is matched, we won't fall to the next invalid_ rule.

Proposed result:

>>> match 1:
...     case x as a.b: ...
...     
  File "<python-input-0>", line 2
    case x as a.b: ...
              ^^^
SyntaxError: cannot use pattern target as attribute

Refs #123440

Linked PRs

• gh-123562: Improve SyntaxError message for case ... as a.b #123563

Activity

1. sobolevn

   added

   type-featureA feature request or enhancementA feature request or enhancement

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   topic-parser

   on Sep 1, 2024
2. sobolevn

   self-assigned this

   on Sep 1, 2024
3. sobolevn

   added a commit that references this issue on Sep 1, 2024

   pythongh-123562: Improve `SyntaxError` message for `case ... as a.b`

   c0cf895
4. cfbolz commented on Sep 2, 2024

   

   cfbolz

   on Sep 2, 2024

   Contributor

   More actions

   Right, sorry, should have commented here too and not just on the PR. Imo it should be SyntaxError: cannot use attribute as pattern target

   Reacted by sobolevn
5. lysnikolaou

   added a commit that references this issue on Sep 2, 2024

   gh-123562: Improve `SyntaxError` message for `case ... as a.b` (#123563)

   23f159a
6. lysnikolaou

   closed this as completedon Sep 2, 2024
7. lysnikolaou commented on Sep 2, 2024

   

   lysnikolaou

   on Sep 2, 2024

   Member

   More actions

   Thanks @sobolevn!

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