stubtest: error for read-only property at runtime, but not in stub (#12291)

Author: AlexWaygood

mypy/stubtest.py

@@ -773,6 +773,18 @@ def verify_var(
             yield Error(object_path, "is not present at runtime", stub, runtime)
         return
 
+    if (
+        stub.is_initialized_in_class
+        and is_read_only_property(runtime)
+        and (stub.is_settable_property or not stub.is_property)
+    ):
+        yield Error(
+            object_path,
+            "is read-only at runtime but not in the stub",
+            stub,
+            runtime
+        )
+
     runtime_type = get_mypy_type_of_runtime_value(runtime)
     if (
         runtime_type is not None
@@ -805,7 +817,14 @@ def verify_overloadedfuncdef(
         return
 
     if stub.is_property:
-        # We get here in cases of overloads from property.setter
+        # Any property with a setter is represented as an OverloadedFuncDef
+        if is_read_only_property(runtime):
+            yield Error(
+                object_path,
+                "is read-only at runtime but not in the stub",
+                stub,
+                runtime
+            )
         return
 
     if not is_probably_a_function(runtime):
@@ -848,7 +867,7 @@ def verify_typevarexpr(
         yield None
 
 
-def _verify_property(stub: nodes.Decorator, runtime: Any) -> Iterator[str]:
+def _verify_readonly_property(stub: nodes.Decorator, runtime: Any) -> Iterator[str]:
     assert stub.func.is_property
     if isinstance(runtime, property):
         return
@@ -923,7 +942,7 @@ def verify_decorator(
         yield Error(object_path, "is not present at runtime", stub, runtime)
         return
     if stub.func.is_property:
-        for message in _verify_property(stub, runtime):
+        for message in _verify_readonly_property(stub, runtime):
             yield Error(object_path, message, stub, runtime)
         return
 
@@ -1044,6 +1063,10 @@ def is_probably_a_function(runtime: Any) -> bool:
     )
 
 
+def is_read_only_property(runtime: object) -> bool:
+    return isinstance(runtime, property) and runtime.fset is None
+
+
 def safe_inspect_signature(runtime: Any) -> Optional[inspect.Signature]:
     try:
         return inspect.signature(runtime)

mypy/test/teststubtest.py

@@ -547,12 +547,12 @@ def test_property(self) -> Iterator[Case]:
             stub="""
             class Good:
                 @property
-                def f(self) -> int: ...
+                def read_only_attr(self) -> int: ...
             """,
             runtime="""
             class Good:
                 @property
-                def f(self) -> int: return 1
+                def read_only_attr(self): return 1
             """,
             error=None,
         )
@@ -592,6 +592,38 @@ class BadReadOnly:
             """,
             error="BadReadOnly.f",
         )
+        yield Case(
+            stub="""
+            class Y:
+                @property
+                def read_only_attr(self) -> int: ...
+                @read_only_attr.setter
+                def read_only_attr(self, val: int) -> None: ...
+            """,
+            runtime="""
+            class Y:
+                @property
+                def read_only_attr(self): return 5
+            """,
+            error="Y.read_only_attr",
+        )
+        yield Case(
+            stub="""
+            class Z:
+                @property
+                def read_write_attr(self) -> int: ...
+                @read_write_attr.setter
+                def read_write_attr(self, val: int) -> None: ...
+            """,
+            runtime="""
+            class Z:
+                @property
+                def read_write_attr(self): return self._val
+                @read_write_attr.setter
+                def read_write_attr(self, val): self._val = val
+            """,
+            error=None,
+        )
 
     @collect_cases
     def test_var(self) -> Iterator[Case]:
@@ -630,6 +662,32 @@ def __init__(self):
             """,
             error=None,
         )
+        yield Case(
+            stub="""
+            class Y:
+                read_only_attr: int
+            """,
+            runtime="""
+            class Y:
+                @property
+                def read_only_attr(self): return 5
+            """,
+            error="Y.read_only_attr",
+        )
+        yield Case(
+            stub="""
+            class Z:
+                read_write_attr: int
+            """,
+            runtime="""
+            class Z:
+                @property
+                def read_write_attr(self): return self._val
+                @read_write_attr.setter
+                def read_write_attr(self, val): self._val = val
+            """,
+            error=None,
+        )
 
     @collect_cases
     def test_type_alias(self) -> Iterator[Case]: