From f7790f9d89b01a57407bc645b50bf57b9075f02d Mon Sep 17 00:00:00 2001
From: sobolevn <mail@sobolevn.me>
Date: Sat, 18 Mar 2023 18:59:06 +0300
Subject: [PATCH 1/2] [stubtest] Respect `@final` runtime decorator, enforce it
 in stubs

---
 mypy/stubtest.py          | 10 ++++++++++
 mypy/test/teststubtest.py | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/mypy/stubtest.py b/mypy/stubtest.py
index df1c246a82d6f..b74005ccb6193 100644
--- a/mypy/stubtest.py
+++ b/mypy/stubtest.py
@@ -419,6 +419,16 @@ class SubClass(runtime):  # type: ignore[misc]
         # Examples: ctypes.Array, ctypes._SimpleCData
         pass
 
+    # Runtime class might be annotated with `@final`:
+    if getattr(runtime, "__final__", False) and not stub.is_final:
+        yield Error(
+            object_path,
+            "has `__final__` attribute, but isn't marked with @final in the stub",
+            stub,
+            runtime,
+            stub_desc=repr(stub),
+        )
+
 
 def _verify_metaclass(
     stub: nodes.TypeInfo, runtime: type[Any], object_path: list[str]
diff --git a/mypy/test/teststubtest.py b/mypy/test/teststubtest.py
index 6bb4dfb2c937d..d39812b5f9b6b 100644
--- a/mypy/test/teststubtest.py
+++ b/mypy/test/teststubtest.py
@@ -1139,6 +1139,45 @@ def test_not_subclassable(self) -> Iterator[Case]:
             error="CannotBeSubclassed",
         )
 
+    @collect_cases
+    def test_has_runtime_final_decorator(self) -> Iterator[Case]:
+        yield Case(
+            stub="from typing_extensions import final",
+            runtime="from typing_extensions import final",
+            error=None,
+        )
+        yield Case(
+            stub="""
+            @final
+            class A: ...
+            """,
+            runtime="""
+            @final
+            class A: ...
+            """,
+            error=None,
+        )
+        yield Case(  # Runtime can miss `@final` decorator
+            stub="""
+            @final
+            class B: ...
+            """,
+            runtime="""
+            class B: ...
+            """,
+            error=None,
+        )
+        yield Case(  # Stub cannot miss `@final` decorator
+            stub="""
+            class C: ...
+            """,
+            runtime="""
+            @final
+            class C: ...
+            """,
+            error="C",
+        )
+
     @collect_cases
     def test_name_mangling(self) -> Iterator[Case]:
         yield Case(

From f118b722f7de8e71a78195614b3e5641d492f688 Mon Sep 17 00:00:00 2001
From: sobolevn <mail@sobolevn.me>
Date: Sun, 19 Mar 2023 15:56:51 +0300
Subject: [PATCH 2/2] Address review

---
 mypy/stubtest.py | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mypy/stubtest.py b/mypy/stubtest.py
index b74005ccb6193..b0ef94e62480d 100644
--- a/mypy/stubtest.py
+++ b/mypy/stubtest.py
@@ -420,7 +420,12 @@ class SubClass(runtime):  # type: ignore[misc]
         pass
 
     # Runtime class might be annotated with `@final`:
-    if getattr(runtime, "__final__", False) and not stub.is_final:
+    try:
+        runtime_final = getattr(runtime, "__final__", False)
+    except Exception:
+        runtime_final = False
+
+    if runtime_final and not stub.is_final:
         yield Error(
             object_path,
             "has `__final__` attribute, but isn't marked with @final in the stub",
@@ -1349,7 +1354,7 @@ def verify_typealias(
         "__origin__",
         "__args__",
         "__orig_bases__",
-        "__final__",
+        "__final__",  # Has a specialized check
         # Consider removing __slots__?
         "__slots__",
     }