Allow stubtest to raise errors on abstract state mismatch (#13323)

Co-authored-by: Alex Waygood <[email protected]>
Co-authored-by: Shantanu <[email protected]>

mypy/stubtest.py

@@ -840,6 +840,18 @@ def verify_funcitem(
         if not callable(runtime):
             return
 
+    if isinstance(stub, nodes.FuncDef):
+        stub_abstract = stub.abstract_status == nodes.IS_ABSTRACT
+        runtime_abstract = getattr(runtime, "__isabstractmethod__", False)
+        # The opposite can exist: some implementations omit `@abstractmethod` decorators
+        if runtime_abstract and not stub_abstract:
+            yield Error(
+                object_path,
+                "is inconsistent, runtime method is abstract but stub is not",
+                stub,
+                runtime,
+            )
+
     for message in _verify_static_class_methods(stub, runtime, object_path):
         yield Error(object_path, "is inconsistent, " + message, stub, runtime)
 

mypy/test/teststubtest.py

@@ -1268,6 +1268,104 @@ def test_type_var(self) -> Iterator[Case]:
             )
             yield Case(stub="C = ParamSpec('C')", runtime="C = ParamSpec('C')", error=None)
 
+    @collect_cases
+    def test_abstract_methods(self) -> Iterator[Case]:
+        yield Case(
+            stub="from abc import abstractmethod",
+            runtime="from abc import abstractmethod",
+            error=None,
+        )
+        yield Case(
+            stub="""
+            class A1:
+                def some(self) -> None: ...
+            """,
+            runtime="""
+            class A1:
+                @abstractmethod
+                def some(self) -> None: ...
+            """,
+            error="A1.some",
+        )
+        yield Case(
+            stub="""
+            class A2:
+                @abstractmethod
+                def some(self) -> None: ...
+            """,
+            runtime="""
+            class A2:
+                @abstractmethod
+                def some(self) -> None: ...
+            """,
+            error=None,
+        )
+        # Runtime can miss `@abstractmethod`:
+        yield Case(
+            stub="""
+            class A3:
+                @abstractmethod
+                def some(self) -> None: ...
+            """,
+            runtime="""
+            class A3:
+                def some(self) -> None: ...
+            """,
+            error=None,
+        )
+
+    @collect_cases
+    def test_abstract_properties(self) -> Iterator[Case]:
+        yield Case(
+            stub="from abc import abstractmethod",
+            runtime="from abc import abstractmethod",
+            error=None,
+        )
+        # Ensure that `@property` also can be abstract:
+        yield Case(
+            stub="""
+            class AP1:
+                def some(self) -> int: ...
+            """,
+            runtime="""
+            class AP1:
+                @property
+                @abstractmethod
+                def some(self) -> int: ...
+            """,
+            error="AP1.some",
+        )
+        yield Case(
+            stub="""
+            class AP2:
+                @property
+                @abstractmethod
+                def some(self) -> int: ...
+            """,
+            runtime="""
+            class AP2:
+                @property
+                @abstractmethod
+                def some(self) -> int: ...
+            """,
+            error=None,
+        )
+        # Runtime can miss `@abstractmethod`:
+        yield Case(
+            stub="""
+            class AP3:
+                @property
+                @abstractmethod
+                def some(self) -> int: ...
+            """,
+            runtime="""
+            class AP3:
+                @property
+                def some(self) -> int: ...
+            """,
+            error=None,
+        )
+
 
 def remove_color_code(s: str) -> str:
     return re.sub("\\x1b.*?m", "", s)  # this works!