Python: Ignore internal properties when unmarshaling (#5251)

CHANGELOG.md

@@ -2,7 +2,10 @@ CHANGELOG
 =========
 
 ## HEAD (Unreleased)
-_(none)_
+
+- Python SDK: Avoid raising an error when internal properties don't match the
+  expected type.
+  [#5251](https://github.com/pulumi/pulumi/pull/5251)
 
 ## 2.9.1 (2020-08-127
 

sdk/python/lib/pulumi/runtime/rpc.py

@@ -255,6 +255,14 @@ def deserialize_properties(props_struct: struct_pb2.Struct, keep_unknowns: Optio
     # since we can only set secret outputs on top level properties.
     output = {}
     for k, v in list(props_struct.items()):
+        # Unilaterally skip properties considered internal by the Pulumi engine.
+        # These don't actually contribute to the exposed shape of the object, do
+        # not need to be passed back to the engine, and often will not match the
+        # expected type we are deserializing into.
+        # Keep "__provider" as it's the property name used by Python dynamic providers.
+        if k.startswith("__") and k != "__provider":
+            continue
+
         value = deserialize_property(v, keep_unknowns)
         # We treat values that deserialize to "None" as if they don't exist.
         if value is not None:

sdk/python/lib/test/test_next_serialize.py

@@ -913,6 +913,20 @@ class DeserializationTests(unittest.TestCase):
         self.assertEqual(val["listWithMap"]["value"][0]["regular"], "a normal value")
         self.assertEqual(val["listWithMap"]["value"][0]["secret"], "a secret value")
 
+    def test_internal_property(self):
+        all_props = struct_pb2.Struct()
+        all_props["a"] = "b"
+        all_props["__defaults"] = []
+        all_props["c"] = {"foo": "bar", "__defaults": []}
+        all_props["__provider"] = "serialized_dynamic_provider"
+        all_props["__other"] = "baz"
+
+        val = rpc.deserialize_properties(all_props)
+        self.assertEqual({
+            "a": "b",
+            "c": {"foo": "bar"},
+            "__provider": "serialized_dynamic_provider",
+        }, val)
 
 @input_type
 class FooArgs: