Fix false positive used-before-assignment in pattern matching with a guard

[i-likecheese]

Type of Changes

	Type
✓	🐛 Bug fix

Description

Fix false positive used-before-assignment in pattern matching with a guard #5327

Closes #5327

[i-likecheese]

Any guidance on what would be a proper way of testing this?

[coveralls]

Pull Request Test Coverage Report for Build 3521229696

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.0002%) to 95.428%

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Totals
Change from base Build 3521160535:	0.0002%
Covered Lines:	17472
Relevant Lines:	18309

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💛 - Coveralls

[Pierre-Sassoulas]

Hey @i-likecheese , thank you for contributing. You can create a functional test using a configuration file like this so it's executed only for python 3.10. See the functional tests documentation

[jacobtylerwalls]

Thanks for tackling this false positive!

I'd prefer to solve it closer to the source instead of at the very last minute, so that the variables checker, which is already pretty hard to reason about, stays somewhat workable.

I suggest handling this case in _maybe_used_and_assigned_at_once, perhaps like:

diff --git a/pylint/checkers/variables.py b/pylint/checkers/variables.py
index 627e82ef0..7ad0c0913 100644
--- a/pylint/checkers/variables.py
+++ b/pylint/checkers/variables.py
@@ -1963,6 +1963,7 @@ class VariablesChecker(BaseChecker):
                             nodes.AugAssign,
                             nodes.Expr,
                             nodes.Return,
+                            nodes.Match,
                         ),
                     )
                     and VariablesChecker._maybe_used_and_assigned_at_once(defstmt)
@@ -2053,6 +2054,8 @@ class VariablesChecker(BaseChecker):
         """Check if `defstmt` has the potential to use and assign a name in the
         same statement.
         """
+        if isinstance(defstmt, nodes.Match):
+            return any(case.guard for case in defstmt.cases)
         if isinstance(defstmt.value, nodes.BaseContainer) and defstmt.value.elts:
             # The assignment must happen as part of the first element
             # e.g. "assert (x:= True), x"

[jacobtylerwalls]

This doesn't solve this false positive, because the .parent is nodes.Name:

match ("example", "one"):
    case (x, y) if x:  # <-- pylint complains about 'x' here
        print("woo!")
    case _:
        print("boo...")

[github-actions]

🤖 According to the primer, this change has no effect on the checked open source code. 🤖🎉

This comment was generated for commit be2bffe

[jacobtylerwalls]

Superseded by #7922.