From 1d7c2fa60f717dea7239970d96f7d4bf96842039 Mon Sep 17 00:00:00 2001
From: ImSheridan <xiaoyudong0512@gmail.com>
Date: Fri, 30 Mar 2018 02:11:55 +0800
Subject: [PATCH] Raise a nicer error message when trying to call gradients
 with while loop (#18052)

* Produce a nicer error message when trying to call gradients on a while loop without properly serializing graph via MetaGraphDef

* Fix syntax and lint error

* Fix minor intent: Wrong continued indentation (add 2 spaces)
---
 tensorflow/python/ops/control_flow_ops.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tensorflow/python/ops/control_flow_ops.py b/tensorflow/python/ops/control_flow_ops.py
index 1278768d8bdc9f..710287012eadea 100644
--- a/tensorflow/python/ops/control_flow_ops.py
+++ b/tensorflow/python/ops/control_flow_ops.py
@@ -833,6 +833,9 @@ def __init__(self, forward_ctxt, outer_grad_state):
     if outer_grad_state:
       outer_forward_ctxt = outer_grad_state.forward_context
     else:
+      if not hasattr(forward_ctxt, 'outer_context'):
+        raise ValueError("Failed to call gradients on a while loop without"
+                         "properly serializing graph via MetaGraphDef")
       outer_forward_ctxt = forward_ctxt.outer_context
 
     # Add the forward loop counter.