fix(python): user defined __jsii_proxy_class attributes are not preserved

packages/@jsii/python-runtime/src/jsii/_runtime.py

@@ -168,7 +168,7 @@ def implements(*interfaces: Type[Any]) -> Callable[[T], T]:
     def deco(cls):
         cls.__jsii_type__ = getattr(cls, "__jsii_type__", None)
         cls.__jsii_ifaces__ = getattr(cls, "__jsii_ifaces__", []) + list(interfaces)
-        cls.__jsii_proxy_class__ = lambda: getattr(cls, "__jsii_proxy_class__", None)
+        cls.__jsii_proxy_class__ = getattr(cls, "__jsii_proxy_class__", lambda: None)
 
         # https://github.com/agronholm/typeguard/issues/479
         cls.__protocol_attrs__ = getattr(cls, "__protocol_attrs__", [])

packages/@jsii/python-runtime/tests/test_python.py

@@ -27,6 +27,37 @@ def test_inheritance_maintained(self):
 
         assert base_names == ["DerivedStruct", "MyFirstStruct"]
 
+
+class TestImplementsInterface:
+
+    def test_jsii_proxy_class_defaults_to_none(self) -> None:
+        @jsii.implements(IBaz)
+        class MyBaz:
+            pass
+
+        klass = getattr(MyBaz, "__jsii_proxy_class__")()
+        assert klass == None
+
+    def test_jsii_proxy_class_preserves_user_defined_attribute(self) -> None:
+
+        class _MyBazProxy:
+            def baz_method(self) -> str:
+                return "_MyBazProxy"
+
+        @jsii.implements(IBaz)
+        class MyBaz:
+
+            @staticmethod
+            def __jsii_proxy_class__():
+                return _MyBazProxy
+
+            def baz_method(self) -> str:
+                return "MyBaz"
+
+        klass = getattr(MyBaz, "__jsii_proxy_class__")()
+        instance = klass()
+        assert instance.baz_method() == "_MyBazProxy"
+
     def test_implements_interface(self) -> None:
         """Checks that jsii-generated classes correctly implement the relevant jsii-generated interfaces."""