fix(python): correctly handle interface declarations on returned objects

packages/jsii-calc/lib/compliance.ts

@@ -2208,3 +2208,31 @@ export class RootStructValidator {
 
     private constructor() { }
 }
+
+/**
+ * Returns a subclass of a known class which implements an interface.
+ */
+export interface IReturnJsii976 {
+    readonly foo: number;
+}
+
+export class BaseJsii976 { }
+
+export class SomeTypeJsii976 {
+
+    static returnReturn(): IReturnJsii976 {
+        class Derived extends BaseJsii976 implements IReturnJsii976 {
+            public readonly foo = 333
+        }
+
+        return new Derived();
+    }
+
+    static returnAnonymous(): any {
+        class Derived implements IReturnJsii976 {
+            public readonly foo = 1337;
+        }
+
+        return new Derived();
+    }
+}

packages/jsii-calc/test/assembly.jsii

@@ -1236,6 +1236,20 @@
       ],
       "name": "AugmentableClass"
     },
+    "jsii-calc.BaseJsii976": {
+      "assembly": "jsii-calc",
+      "docs": {
+        "stability": "experimental"
+      },
+      "fqn": "jsii-calc.BaseJsii976",
+      "initializer": {},
+      "kind": "class",
+      "locationInModule": {
+        "filename": "lib/compliance.ts",
+        "line": 2219
+      },
+      "name": "BaseJsii976"
+    },
     "jsii-calc.Bell": {
       "assembly": "jsii-calc",
       "docs": {
@@ -5140,6 +5154,37 @@
       ],
       "name": "IRandomNumberGenerator"
     },
+    "jsii-calc.IReturnJsii976": {
+      "assembly": "jsii-calc",
+      "docs": {
+        "stability": "experimental",
+        "summary": "Returns a subclass of a known class which implements an interface."
+      },
+      "fqn": "jsii-calc.IReturnJsii976",
+      "kind": "interface",
+      "locationInModule": {
+        "filename": "lib/compliance.ts",
+        "line": 2215
+      },
+      "name": "IReturnJsii976",
+      "properties": [
+        {
+          "abstract": true,
+          "docs": {
+            "stability": "experimental"
+          },
+          "immutable": true,
+          "locationInModule": {
+            "filename": "lib/compliance.ts",
+            "line": 2216
+          },
+          "name": "foo",
+          "type": {
+            "primitive": "number"
+          }
+        }
+      ]
+    },
     "jsii-calc.IReturnsNumber": {
       "assembly": "jsii-calc",
       "docs": {
@@ -8654,6 +8699,54 @@
       ],
       "name": "SingletonStringEnum"
     },
+    "jsii-calc.SomeTypeJsii976": {
+      "assembly": "jsii-calc",
+      "docs": {
+        "stability": "experimental"
+      },
+      "fqn": "jsii-calc.SomeTypeJsii976",
+      "initializer": {},
+      "kind": "class",
+      "locationInModule": {
+        "filename": "lib/compliance.ts",
+        "line": 2221
+      },
+      "methods": [
+        {
+          "docs": {
+            "stability": "experimental"
+          },
+          "locationInModule": {
+            "filename": "lib/compliance.ts",
+            "line": 2231
+          },
+          "name": "returnAnonymous",
+          "returns": {
+            "type": {
+              "primitive": "any"
+            }
+          },
+          "static": true
+        },
+        {
+          "docs": {
+            "stability": "experimental"
+          },
+          "locationInModule": {
+            "filename": "lib/compliance.ts",
+            "line": 2223
+          },
+          "name": "returnReturn",
+          "returns": {
+            "type": {
+              "fqn": "jsii-calc.IReturnJsii976"
+            }
+          },
+          "static": true
+        }
+      ],
+      "name": "SomeTypeJsii976"
+    },
     "jsii-calc.StableClass": {
       "assembly": "jsii-calc",
       "docs": {
@@ -10913,5 +11006,5 @@
     }
   },
   "version": "0.20.3",
-  "fingerprint": "1F+uskR3++T5mjRcWge9oG3H/jJvXm1C3IhR1AwsBTE="
+  "fingerprint": "umMeNAH41pX11GUAjHkw6RTyAwGCeTqRNJ2vPv5aeM0="
 }

packages/jsii-python-runtime/src/jsii/_reference_map.py

@@ -49,6 +49,7 @@ def resolve(self, kernel, ref):
         # First we need to check our reference map to see if we have any instance that
         # already matches this reference.
         try:
+            # TODO: Handle discovery of possible new interfaces on the ObjRef
             return self._refs[ref.ref]
         except KeyError:
             pass
@@ -69,6 +70,13 @@ def resolve(self, kernel, ref):
             # then assign our reference to __jsii_ref__
             inst = klass.__new__(klass)
             inst.__jsii_ref__ = ref
+
+            if ref.interfaces is not None:
+                return InterfaceDynamicProxy([inst] + self.build_interface_proxies_for_ref(ref))
+            else:
+                return inst
+
+        # Legacy code path - Kernel invariant ought to guarantee that class_fqn can't be Struct (they're interfaces)
         elif class_fqn in _data_types:
             # Data types have been serialized by-reference (see aws/jsii#400).
             # We retrieve all of its properties right now and then construct a value
@@ -86,8 +94,9 @@ def resolve(self, kernel, ref):
 
             return data_type(**python_props)
         elif class_fqn in _enums:
-            inst = _enums[class_fqn]
+            return _enums[class_fqn]
         elif class_fqn == "Object" and ref.interfaces is not None:
+            # If any one interface is a struct, all of them are guaranteed to be (Kernel invariant)
             if any(fqn in _data_types for fqn in ref.interfaces):
                 # Ugly delayed import here because I can't solve the cyclic
                 # package dependency right now :(.
@@ -100,32 +109,34 @@ def resolve(self, kernel, ref):
                     }) for struct in structs]
                 return StructDynamicProxy(insts)
             else:
-                ifaces = [_interfaces[fqn] for fqn in ref.interfaces]
-                classes = [iface.__jsii_proxy_class__() for iface in ifaces]
-                insts = [klass.__new__(klass) for klass in classes]
-                for inst in insts:
-                    inst.__jsii_ref__ = ref
-                return InterfaceDynamicProxy(insts)
+                return InterfaceDynamicProxy(self.build_interface_proxies_for_ref(ref))
         else:
             raise ValueError(f"Unknown type: {class_fqn}")
 
-        return inst
-
     def resolve_id(self, id):
         return self._refs[id]
 
+    def build_interface_proxies_for_ref(self, ref):
+        if ref.interfaces is None:
+            raise AssertionError("Attempted to create interface proxies for ObjectRef without interfaces!")
+        ifaces = [_interfaces[fqn] for fqn in ref.interfaces]
+        classes = [iface.__jsii_proxy_class__() for iface in ifaces]
+        insts = [klass.__new__(klass) for klass in classes]
+        for inst in insts:
+            inst.__jsii_ref__ = ref
+        return insts
+
 
 class InterfaceDynamicProxy(object):
     def __init__(self, delegates):
         self._delegates = delegates
 
     def __getattr__(self, name):
         for delegate in self._delegates:
-            try:
+            if hasattr(delegate, name):
                 return getattr(delegate, name)
-            except NameError:
-                pass
-        return None
+        type_info = "+".join([str(delegate.__class__) for delegate in self._delegates])
+        raise AttributeError(f"'%s' object has no attribute '%s'" % (type_info, name))
 
 
 class StructDynamicProxy(object):
@@ -134,11 +145,10 @@ def __init__(self, delegates):
 
     def __getattr__(self, name):
         for delegate in self._delegates:
-            try:
+            if hasattr(delegate, name):
                 return getattr(delegate, name)
-            except NameError:
-                pass
-        return None
+        type_info = "+".join([str(delegate.__class__) for delegate in self._delegates])
+        raise AttributeError("'%s' object has no attribute '%s'" % (type_info, name))
 
     def __eq__(self, rhs) -> bool:
         if len(self._delegates) == 1:

packages/jsii-python-runtime/tests/test_compliance.py

@@ -48,14 +48,14 @@
     EraseUndefinedHashValues,
     EraseUndefinedHashValuesOptions,
     VariadicMethod,
-    RootStruct,
     RootStructValidator,
     StructPassing,
     TopLevelStruct,
     SecondLevelStruct,
     StructA,
     StructB,
-    StructUnionConsumer
+    StructUnionConsumer,
+    SomeTypeJsii976
 )
 from scope.jsii_calc_lib import IFriendly, EnumFromScopedModule, Number
 
@@ -1013,6 +1013,24 @@ def test_can_pass_nested_struct_as_dict():
         }
     )
 
+# https://github.com/aws/jsii/issues/976
+def test_return_subclass_that_implements_interface_976():
+    obj = SomeTypeJsii976.return_return()
+    assert obj.foo == 333
+
+def test_return_subclass_that_implements_interface_976_raises_attributeerror_when_using_non_existent_method():
+    obj = SomeTypeJsii976.return_return()
+    try:
+        print(obj.not_a_real_method_I_swear)
+        failed = False
+    except AttributeError as err:
+        failed = True
+        assert err.args[0] == "'<class 'jsii_calc.BaseJsii976'>+<class 'jsii_calc._IReturnJsii976Proxy'>' object has no attribute 'not_a_real_method_I_swear'"
+    assert failed
+
+def test_return_anonymous_implementation_of_interface():
+    assert SomeTypeJsii976.return_anonymous() is not None
+
 @jsii.implements(IBellRinger)
 class PythonBellRinger:
     def your_turn(self, bell):