Fix false positive for `undefined-loop-variable` for lambda in first of two loops (#6479)

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>

4 files changed, 34 insertions, 1 deletions

diff --git a/ChangeLog b/ChangeLog
index 2775fb3ad..c52bf9033 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -20,6 +20,11 @@ What's New in Pylint 2.13.8?
 ============================
 Release date: TBA
 
+* Fix a false positive for ``undefined-loop-variable`` for a variable used in a lambda
+  inside the first of multiple loops.
+
+  Closes #6419
+
 * Fix a crash when linting a file that passes an integer ``mode=`` to
   ``open``
 
diff --git a/doc/whatsnew/2.13.rst b/doc/whatsnew/2.13.rst
index ffac2c217..1d86a6fd9 100644
--- a/doc/whatsnew/2.13.rst
+++ b/doc/whatsnew/2.13.rst
@@ -620,6 +620,11 @@ Other Changes
 
   Closes #5930
 
+* Fix a false positive for ``undefined-loop-variable`` for a variable used in a lambda
+  inside the first of multiple loops.
+
+  Closes #6419
+
 * Fix a crash when linting a file that passes an integer ``mode=`` to
   ``open``
 
diff --git a/pylint/checkers/variables.py b/pylint/checkers/variables.py
index 9b2b2bd7a..2e4f4d70f 100644
--- a/pylint/checkers/variables.py
+++ b/pylint/checkers/variables.py
@@ -2205,7 +2205,10 @@ class VariablesChecker(BaseChecker):
         # the usage is safe because the function will not be defined either if
         # the variable is not defined.
         scope = node.scope()
-        if isinstance(scope, nodes.FunctionDef) and any(
+        # FunctionDef subclasses Lambda due to a curious ontology. Check both.
+        # See https://github.com/PyCQA/astroid/issues/291
+        # 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
         ):
             return
diff --git a/tests/functional/u/undefined/undefined_loop_variable.py b/tests/functional/u/undefined/undefined_loop_variable.py
index 956773e31..0270c6b53 100644
--- a/tests/functional/u/undefined/undefined_loop_variable.py
+++ b/tests/functional/u/undefined/undefined_loop_variable.py
@@ -103,3 +103,23 @@ bigger = [
     ]
     for item in lst
 ]
+
+
+def lambda_in_first_of_two_loops():
+    """https://github.com/PyCQA/pylint/issues/6419"""
+    my_list = []
+    for thing in my_list:
+        print_it = lambda: print(thing)  # pylint: disable=cell-var-from-loop
+        print_it()
+
+    for thing in my_list:
+        print(thing)
+
+
+def variable_name_assigned_in_body_of_second_loop():
+    for alias in tuple(bigger):
+        continue
+    for _ in range(3):
+        alias = True
+        if alias:
+            print(alias)