Fix a false positive for undefined-loop-variable when a loop else raises or returns (#6480)

Co-authored-by: Pierre Sassoulas <[email protected]>

Author: jacobtylerwalls

ChangeLog

@@ -34,6 +34,11 @@ Release date: TBA
 
   Closes #5930
 
+* Fix a false positive for ``undefined-loop-variable`` when the ``else`` of a ``for``
+  loop raises or returns.
+
+  Closes #5971
+
 * Fix false positive for ``unused-variable`` for classes inside functions
   and where a metaclass is provided via a call.
 

doc/whatsnew/2.13.rst

@@ -607,6 +607,11 @@ Other Changes
 
   Closes #5769
 
+* Fix a false positive for ``undefined-loop-variable`` when the ``else`` of a ``for``
+  loop raises or returns.
+
+  Closes #5971
+
 * Fix false positive for ``unused-variable`` for classes inside functions
   and where a metaclass is provided via a call.
 

pylint/checkers/variables.py

@@ -2207,6 +2207,7 @@ def _loopvar_name(self, node: astroid.Name) -> None:
         scope = node.scope()
         # FunctionDef subclasses Lambda due to a curious ontology. Check both.
         # See https://github.com/PyCQA/astroid/issues/291
+        # pylint: disable-next=fixme
         # TODO: Revisit when astroid 3.0 includes the change
         if isinstance(scope, nodes.Lambda) and any(
             asmt.scope().parent_of(scope) for asmt in astmts
@@ -2252,11 +2253,16 @@ def _loopvar_name(self, node: astroid.Name) -> None:
         ):
             return
 
-        # For functions we can do more by inferring the length of the itered object
         if not isinstance(assign, nodes.For):
             self.add_message("undefined-loop-variable", args=node.name, node=node)
             return
+        if any(
+            isinstance(else_stmt, (nodes.Return, nodes.Raise))
+            for else_stmt in assign.orelse
+        ):
+            return
 
+        # For functions we can do more by inferring the length of the itered object
         try:
             inferred = next(assign.iter.infer())
         except astroid.InferenceError:

tests/functional/u/undefined/undefined_loop_variable.py

@@ -91,6 +91,22 @@ def handle_line(layne):
         handle_line(layne)
 
 
+def for_else_returns(iterable):
+    for thing in iterable:
+        break
+    else:
+        return
+    print(thing)
+
+
+def for_else_raises(iterable):
+    for thing in iterable:
+        break
+    else:
+        raise Exception
+    print(thing)
+
+
 lst = []
 lst2 = [1, 2, 3]