Store the callback in the interceptor input

Non-ideal

Author: dandavison

temporalio/client.py

@@ -990,11 +990,6 @@ async def _start_update_with_start(
             wait_for_stage=wait_for_stage,
         )
 
-        input = StartWorkflowUpdateWithStartInput(
-            start_workflow_input=start_workflow_operation._start_workflow_input,
-            update_workflow_input=update_input,
-        )
-
         def on_start(
             start_response: temporalio.api.workflowservice.v1.StartWorkflowExecutionResponse,
         ):
@@ -1006,7 +1001,11 @@ def on_start(
                 )
             )
 
-        setattr(input, "_on_start_callback", on_start)
+        input = StartWorkflowUpdateWithStartInput(
+            start_workflow_input=start_workflow_operation._start_workflow_input,
+            update_workflow_input=update_input,
+            _on_start_callback=on_start,
+        )
 
         update_handle = await self._impl.start_workflow_update_with_start(input)
         # TODO https://github.com/temporalio/sdk-python/issues/682
@@ -5134,6 +5133,9 @@ class StartWorkflowUpdateWithStartInput:
 
     start_workflow_input: UpdateWithStartStartWorkflowInput
     update_workflow_input: UpdateWithStartUpdateWorkflowInput
+    _on_start_callback: Callable[
+        [temporalio.api.workflowservice.v1.StartWorkflowExecutionResponse], None
+    ]
 
 
 @dataclass
@@ -5891,16 +5893,10 @@ async def start_workflow_update_with_start(
             ],
         )
 
-        on_start = getattr(input, "_on_start_callback", None)
-        if not on_start:
-            raise RuntimeError(
-                "Missing on_start callback. Please report this as a bug at https://github.com/temporalio/sdk-python/issues."
-            )
-        seen_start = False
-
         # Repeatedly try to invoke ExecuteMultiOperation until the update
         # reaches user-provided wait stage or is at least ACCEPTED (as of the
         # time of this writing, the user cannot specify sooner than ACCEPTED)
+        seen_start = False
         while True:
             try:
                 multiop_response = (
@@ -5964,7 +5960,7 @@ async def start_workflow_update_with_start(
             [start_response] = start_responses
             [update_response] = update_responses
             if not seen_start:
-                on_start(start_response)
+                input._on_start_callback(start_response)
                 seen_start = True
             known_outcome = (
                 update_response.outcome if update_response.HasField("outcome") else None