stubtest: analyze metaclass of types, refs #13327

Author: sobolevn

mypy/stubtest.py

@@ -375,6 +375,40 @@ class SubClass(runtime):  # type: ignore
         # Examples: ctypes.Array, ctypes._SimpleCData
         pass
 
+    # Check metaclass. We exclude protocols, because of how complex
+    # their implementation is in different versions of python.
+    # Enums are also hard, ignoring.
+    if not stub.is_protocol and not stub.is_enum:
+        runtime_metaclass = type(runtime)
+        if runtime_metaclass is not type and stub.metaclass_type is None:
+            # This means that runtime has a custom metaclass, but a stub does not.
+            yield Error(
+                object_path,
+                "metaclass missmatch",
+                stub,
+                runtime,
+                stub_desc="Missing metaclass",
+                runtime_desc=f"Exiting metaclass: {runtime_metaclass}",
+            )
+        elif (
+            runtime_metaclass is type
+            and stub.metaclass_type is not None
+            # We ignore extra `ABCMeta` metaclass on stubs, this might be typing hack.
+            # We also ignore `builtins.type` metaclass as an implementation detail in mypy.
+            and not mypy.types.is_named_instance(
+                stub.metaclass_type, ("abc.ABCMeta", "builtins.type")
+            )
+        ):
+            # This means that our stub has a metaclass that is not present at runtime.
+            yield Error(
+                object_path,
+                "metaclass missmatch",
+                stub,
+                runtime,
+                stub_desc=f"Existing metaclass: {stub.metaclass_type.type.fullname}",
+                runtime_desc="Missing metaclass",
+            )
+
     # Check everything already defined on the stub class itself (i.e. not inherited)
     to_check = set(stub.names)
     # Check all public things on the runtime class

mypy/test/teststubtest.py

@@ -1264,6 +1264,56 @@ def test_type_var(self) -> Iterator[Case]:
             )
             yield Case(stub="C = ParamSpec('C')", runtime="C = ParamSpec('C')", error=None)
 
+    @collect_cases
+    def test_metaclass_match(self) -> Iterator[Case]:
+        yield Case(stub="class Meta(type): ...", runtime="class Meta(type): ...", error=None)
+        yield Case(stub="class A0: ...", runtime="class A0: ...", error=None)
+        yield Case(
+            stub="class A1(metaclass=Meta): ...",
+            runtime="class A1(metaclass=Meta): ...",
+            error=None,
+        )
+        yield Case(stub="class A2: ...", runtime="class A2(metaclass=Meta): ...", error="A2")
+        yield Case(stub="class A3(metaclass=Meta): ...", runtime="class A3: ...", error="A3")
+
+        # With inheritance:
+        yield Case(
+            stub="""
+            class I1(metaclass=Meta): ...
+            class S1(I1): ...
+            """,
+            runtime="""
+            class I1(metaclass=Meta): ...
+            class S1(I1): ...
+            """,
+            error=None,
+        )
+        yield Case(
+            stub="""
+            class I2(metaclass=Meta): ...
+            class S2: ...  # missing inheritance
+            """,
+            runtime="""
+            class I2(metaclass=Meta): ...
+            class S2(I2): ...
+            """,
+            error="S2",
+        )
+
+    @collect_cases
+    def test_metaclass_abcmeta(self) -> Iterator[Case]:
+        # Handling abstract metaclasses is special:
+        yield Case(stub="from abc import ABCMeta", runtime="from abc import ABCMeta", error=None)
+        yield Case(
+            stub="class A1(metaclass=ABCMeta): ...",
+            runtime="class A1(metaclass=ABCMeta): ...",
+            error=None,
+        )
+        # Stubs cannot miss abstract metaclass:
+        yield Case(stub="class A2: ...", runtime="class A2(metaclass=ABCMeta): ...", error="A2")
+        # But, stubs can add extra abstract metaclass, this might be a typing hack:
+        yield Case(stub="class A3(metaclass=ABCMeta): ...", runtime="class A3: ...", error=None)
+
 
 def remove_color_code(s: str) -> str:
     return re.sub("\\x1b.*?m", "", s)  # this works!