Fix used-before-assignment for assignment expressions in lambda (#5530)

Author: DanielNoord

ChangeLog

@@ -50,6 +50,11 @@ Release date: TBA
 
 * ``used-before-assignment`` now checks names in try blocks.
 
+* Fixed false positive with ``used-before-assignment`` for assignment expressions
+  in lambda statements.
+
+  Closes #5360, #3877
+
 * Some files in ``pylint.testutils`` were deprecated. In the future imports should be done from the
   ``pylint.testutils.functional`` namespace directly.
 

doc/whatsnew/2.13.rst

@@ -86,6 +86,11 @@ Other Changes
 
 * ``used-before-assignment`` now checks names in try blocks.
 
+* Fixed false positive with ``used-before-assignment`` for assignment expressions
+  in lambda statements.
+
+  Closes #5360, #3877
+
 * Require Python ``3.6.2`` to run pylint.
 
   Closes #5065

pylint/checkers/variables.py

@@ -1402,10 +1402,6 @@ def _check_consumer(
                         "used-before-assignment", args=node.name, node=node
                     )
 
-                elif current_consumer.scope_type == "lambda":
-                    self.add_message("undefined-variable", args=node.name, node=node)
-                    return (VariableVisitConsumerAction.CONSUME, found_nodes)
-
         elif self._is_only_type_assignment(node, defstmt):
             self.add_message("undefined-variable", args=node.name, node=node)
             return (VariableVisitConsumerAction.CONSUME, found_nodes)

tests/functional/u/undefined/undefined_variable_py38.py

@@ -97,3 +97,14 @@ def no_parameters_in_function_default() -> None:
 COMPREHENSION_FIVE = {i: (else_assign_2 := i) if False else 0 for i in range(10)}
 
 print(else_assign_2)  # [undefined-variable]
+
+
+# Tests for assignment expressions in lambda statements
+
+things = []
+sorted_things = sorted(
+    things,
+    key=lambda thing: x_0
+    if (x_0 := thing.this_value) < (x_1 := thing.that_value)
+    else x_1,
+)