Check __setattr__ when property is not settable (#9196)

* Create test for __setattr__ usage with property

* Check __setattr__ before calling read_only_property

* Look through MRO for setattr definition

* Create test case with setattr and incompatible type assignment

Author: Kamil Turek

mypy/checkmember.py

@@ -553,7 +553,8 @@ def analyze_var(name: str,
             return mx.chk.handle_partial_var_type(typ, mx.is_lvalue, var, mx.context)
         if mx.is_lvalue and var.is_property and not var.is_settable_property:
             # TODO allow setting attributes in subclass (although it is probably an error)
-            mx.msg.read_only_property(name, itype.type, mx.context)
+            if info.get('__setattr__') is None:
+                mx.msg.read_only_property(name, itype.type, mx.context)
         if mx.is_lvalue and var.is_classvar:
             mx.msg.cant_assign_to_classvar(name, mx.context)
         t = get_proper_type(expand_type_by_instance(typ, itype))
@@ -563,7 +564,8 @@ def analyze_var(name: str,
             if mx.is_lvalue:
                 if var.is_property:
                     if not var.is_settable_property:
-                        mx.msg.read_only_property(name, itype.type, mx.context)
+                        if info.get('__setattr__') is None:
+                            mx.msg.read_only_property(name, itype.type, mx.context)
                 else:
                     mx.msg.cant_assign_to_method(mx.context)
 

mypy/test/testcheck.py

@@ -90,6 +90,7 @@
     'check-reports.test',
     'check-errorcodes.test',
     'check-annotated.test',
+    'check-setattr.test',
     'check-parameter-specification.test',
 ]
 

test-data/unit/check-setattr.test

@@ -0,0 +1,39 @@
+[case testSetAttrWithProperty]
+from typing import Any
+class Foo:
+    _a = 1
+    def __setattr__(self, name: str, value: Any) -> None:
+        self._a = value
+    @property
+    def a(self) -> int:
+        return self._a
+f = Foo()
+f.a = 2
+[builtins fixtures/property.pyi]
+
+[case testInheritedSetAttrWithProperty]
+from typing import Any
+class Foo:
+    _a = 1
+    def __setattr__(self, name: str, value: Any) -> None:
+        self._a = value
+class Bar(Foo):
+    @property
+    def a(self) -> int:
+        return self._a
+f = Bar()
+f.a = 2
+[builtins fixtures/property.pyi]
+
+[case testSetAttrWithIncompatibleType]
+from typing import Any
+class Foo:
+    _a = 1
+    def __setattr__(self, name: str, value: Any) -> None:
+        self._a = value
+    @property
+    def a(self) -> int:
+        return self._a
+f = Foo()
+f.a = 'hello'  # E: Incompatible types in assignment (expression has type "str", variable has type "int")
+[builtins fixtures/property.pyi]