Fix bug with read-only implementations of attributes on dataclasses.

pure_interface/interface.py

@@ -402,11 +402,20 @@ def _ensure_everything_is_abstract(attributes):
     return namespace, functions, interface_method_signatures, interface_attribute_names
 
 
+def _readonly_attribute(name, namespace):
+    # if the attribute is a property with a getter but no setter then it is read-only
+    prop = namespace.get(name)
+    if isinstance(prop, property):
+        if prop.fset is None:
+            return True
+    return False
+
+
 def _ensure_annotations(names, namespace):
     # annotations need to be kept in order, add base-class names first
     annotations = {}
     for name in names:
-        if name not in annotations:
+        if name not in annotations and not _readonly_attribute(name, namespace):
             annotations[name] = Any
     annotations.update(namespace.get('__annotations__', {}))
     namespace['__annotations__'] = annotations

tests/test_dataclass_support.py

@@ -53,3 +53,19 @@ def foo(self):
         f = Foo(a=1, b='two')
         self.assertEqual(1, f.a)
         self.assertEqual('two', f.b)
+
+    def test_read_only_attr(self):
+        @dataclass
+        class RoFoo(IFoo):
+            c: int
+
+            @property
+            def b(self):
+                return 'str'
+
+            def foo(self):
+                return 'a={}, b={}, c={}'.format(self.a, self.b, self.c)
+
+        f = RoFoo(a=1, c=3)
+        self.assertEqual(1, f.a)
+        self.assertEqual('str', f.b)