Raise a warning on Final instance attributes outside __init__ (PEP 591) (PY-34945)

GitOrigin-RevId: 74f4aab3a879d74815945a6f33d5a66afabd25c7

python/src/com/jetbrains/python/inspections/PyFinalInspection.kt

@@ -71,8 +71,12 @@ class PyFinalInspection : PyInspection() {
             registerProblem(node.nameIdentifier, "'${(it as PyFunction).qualifiedName}' is marked as '@final' and should not be overridden")
           }
 
-        if (!PyiUtil.isInsideStub(node) && isFinal(node) && PyiUtil.isOverload(node, myTypeEvalContext)) {
-          registerProblem(node.nameIdentifier, "'@final' should be placed on the implementation")
+        if (!PyiUtil.isInsideStub(node)) {
+          if (isFinal(node) && PyiUtil.isOverload(node, myTypeEvalContext)) {
+            registerProblem(node.nameIdentifier, "'@final' should be placed on the implementation")
+          }
+
+          checkInstanceFinalsOutsideInit(node)
         }
       }
       else if (isFinal(node)) {
@@ -158,6 +162,16 @@ class PyFinalInspection : PyInspection() {
       }
     }
 
+    private fun checkInstanceFinalsOutsideInit(method: PyFunction) {
+      if (PyUtil.isInit(method)) return
+
+      val instanceAttributes = mutableMapOf<String, PyTargetExpression>()
+      PyClassImpl.collectInstanceAttributes(method, instanceAttributes)
+      instanceAttributes.values.forEach {
+        if (isFinal(it)) registerProblem(it, "'Final' attribute should be declared in class body or '__init__'")
+      }
+    }
+
     private fun checkFinalIsOuterMost(node: PyReferenceExpression) {
       if (isTopLevelInAnnotationOrTypeComment(node)) return
       (node.parent as? PySubscriptionExpression)?.let {

python/testSrc/com/jetbrains/python/inspections/PyFinalInspectionTest.java

@@ -272,6 +272,22 @@ public class PyFinalInspectionTest extends PyInspectionTestCase {
     );
   }
 
+  // PY-34945
+  public void testFinalInstanceAttributes() {
+    runWithLanguageLevel(
+      LanguageLevel.PYTHON36,
+      () -> doTestByText("from typing_extensions import Final\n" +
+                         "\n" +
+                         "class A:\n" +
+                         "    def __init__(self):\n" +
+                         "        self.a: Final[str] = \"str\"\n" +
+                         "\n" +
+                         "    def method(self):\n" +
+                         "        <warning descr=\"'Final' attribute should be declared in class body or '__init__'\">self.a</warning>: Final[int] = 10\n" +
+                         "        <warning descr=\"'Final' attribute should be declared in class body or '__init__'\">self.b</warning>: Final[int] = 10")
+    );
+  }
+
   @NotNull
   @Override
   protected Class<? extends PyInspection> getInspectionClass() {