move stored error into own variable

google/api_core/grpc_helpers.py

@@ -99,7 +99,7 @@ def __init__(self, wrapped, prefetch_first_result=True):
             pass
         except grpc.RpcError as exc:
             # If the pre-fetch fails, store exception to be raised on next() call.
-            self._stored_first_result = exc
+            self._stored_first_error = exc
 
     def __iter__(self) -> Iterator[P]:
         """This iterator is also an iterable that returns itself."""
@@ -115,9 +115,11 @@ def __next__(self) -> P:
             if hasattr(self, "_stored_first_result"):
                 result = self._stored_first_result
                 del self._stored_first_result
-                if isinstance(result, Exception):
-                    raise result
                 return result
+            elif hasattr(self, "_stored_first_error"):
+                exc = self._stored_first_error
+                del self._stored_first_error
+                raise exc
             return next(self._wrapped)
         except grpc.RpcError as exc:
             # If the stream has already returned data, we cannot recover here.

tests/unit/test_grpc_helpers.py

@@ -104,7 +104,7 @@ def test_ctor_w_rpc_error_on_prefetch(self):
         wrapped.__next__.side_effect = exc
 
         obj = self._make_one(wrapped)
-        assert obj._stored_first_result is exc
+        assert obj._stored_first_error is exc
         with pytest.raises(exceptions.GoogleAPICallError):
             next(obj)
 
@@ -330,7 +330,7 @@ def test_wrap_stream_errors_iterator_initialization():
     wrapped_callable = grpc_helpers._wrap_stream_errors(callable_)
 
     it = wrapped_callable(1, 2, three="four")
-    assert it._stored_first_result is grpc_error
+    assert it._stored_first_error is grpc_error
 
     callable_.assert_called_once_with(1, 2, three="four")
     # should raise on first iteration