stubtest: error if a class should be decorated with @Final (#12091)

mypy/stubtest.py

@@ -254,6 +254,24 @@ def verify_typeinfo(
         yield Error(object_path, "is not a type", stub, runtime, stub_desc=repr(stub))
         return
 
+    try:
+        class SubClass(runtime):  # type: ignore
+            pass
+    except TypeError:
+        # Enum classes are implicitly @final
+        if not stub.is_final and not issubclass(runtime, enum.Enum):
+            yield Error(
+                object_path,
+                "cannot be subclassed at runtime, but isn't marked with @final in the stub",
+                stub,
+                runtime,
+                stub_desc=repr(stub),
+            )
+    except Exception:
+        # The class probably wants its subclasses to do something special.
+        # Examples: ctypes.Array, ctypes._SimpleCData
+        pass
+
     # Check everything already defined in the stub
     to_check = set(stub.names)
     # There's a reasonable case to be made that we should always check all dunders, but it's

mypy/test/teststubtest.py

@@ -691,8 +691,13 @@ def test_special_dunders(self) -> Iterator[Case]:
         )
         if sys.version_info >= (3, 6):
             yield Case(
-                stub="class C:\n  def __init_subclass__(cls, e: int, **kwargs: int) -> None: ...",
-                runtime="class C:\n  def __init_subclass__(cls, e, **kwargs): pass",
+                stub=(
+                    "class C:\n"
+                    "  def __init_subclass__(\n"
+                    "    cls, e: int = ..., **kwargs: int\n"
+                    "  ) -> None: ...\n"
+                ),
+                runtime="class C:\n  def __init_subclass__(cls, e=1, **kwargs): pass",
                 error=None,
             )
         if sys.version_info >= (3, 9):
@@ -702,6 +707,28 @@ def test_special_dunders(self) -> Iterator[Case]:
                 error=None,
             )
 
+    def test_not_subclassable(self) -> None:
+        output = run_stubtest(
+            stub=(
+                "class CanBeSubclassed: ...\n"
+                "class CanNotBeSubclassed:\n"
+                "  def __init_subclass__(cls) -> None: ...\n"
+            ),
+            runtime=(
+                "class CanNotBeSubclassed:\n"
+                "  def __init_subclass__(cls): raise TypeError('nope')\n"
+                # ctypes.Array can be subclassed, but subclasses must define a few
+                # special attributes, e.g. _length_
+                "from ctypes import Array as CanBeSubclassed\n"
+            ),
+            options=[],
+        )
+        assert (
+            "CanNotBeSubclassed cannot be subclassed at runtime,"
+            " but isn't marked with @final in the stub"
+        ) in output
+        assert "CanBeSubclassed cannot be subclassed" not in output
+
     @collect_cases
     def test_name_mangling(self) -> Iterator[Case]:
         yield Case(