From c13f77173fe1d83069f585e046a2b358e65b4c76 Mon Sep 17 00:00:00 2001
From: Ben Jones <RealSalmon@users.noreply.github.com>
Date: Wed, 21 Mar 2018 11:13:05 -0500
Subject: [PATCH] add UsePreviousValue support for parameters when updating a
 CloudFormation stack (#1504)

---
 moto/cloudformation/responses.py              | 13 ++++++---
 .../test_cloudformation_stack_crud_boto3.py   | 27 +++++++++++++++++++
 2 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/moto/cloudformation/responses.py b/moto/cloudformation/responses.py
index 73d1d2c2..a1295a20 100644
--- a/moto/cloudformation/responses.py
+++ b/moto/cloudformation/responses.py
@@ -222,17 +222,24 @@ class CloudFormationResponse(BaseResponse):
         role_arn = self._get_param('RoleARN')
         template_url = self._get_param('TemplateURL')
         stack_body = self._get_param('TemplateBody')
+        stack = self.cloudformation_backend.get_stack(stack_name)
         if self._get_param('UsePreviousTemplate') == "true":
-            stack_body = self.cloudformation_backend.get_stack(
-                stack_name).template
+            stack_body = stack.template
         elif not stack_body and template_url:
             stack_body = self._get_stack_from_s3_url(template_url)
 
+        incoming_params = self._get_list_prefix("Parameters.member")
         parameters = dict([
             (parameter['parameter_key'], parameter['parameter_value'])
             for parameter
-            in self._get_list_prefix("Parameters.member")
+            in incoming_params if 'parameter_value' in parameter
         ])
+        previous = dict([
+            (parameter['parameter_key'], stack.parameters[parameter['parameter_key']])
+            for parameter
+            in incoming_params if 'use_previous_value' in parameter
+        ])
+        parameters.update(previous)
         # boto3 is supposed to let you clear the tags by passing an empty value, but the request body doesn't
         # end up containing anything we can use to differentiate between passing an empty value versus not
         # passing anything. so until that changes, moto won't be able to clear tags, only update them.
diff --git a/tests/test_cloudformation/test_cloudformation_stack_crud_boto3.py b/tests/test_cloudformation/test_cloudformation_stack_crud_boto3.py
index fb9b549c..1dbf80fb 100644
--- a/tests/test_cloudformation/test_cloudformation_stack_crud_boto3.py
+++ b/tests/test_cloudformation/test_cloudformation_stack_crud_boto3.py
@@ -276,6 +276,33 @@ def test_create_stack_from_s3_url():
         json.loads(dummy_template_json, object_pairs_hook=OrderedDict))
 
 
+@mock_cloudformation
+def test_update_stack_with_previous_value():
+    name = 'update_stack_with_previous_value'
+    cf_conn = boto3.client('cloudformation', region_name='us-east-1')
+    cf_conn.create_stack(
+        StackName=name, TemplateBody=dummy_template_yaml_with_ref,
+        Parameters=[
+            {'ParameterKey': 'TagName', 'ParameterValue': 'foo'},
+            {'ParameterKey': 'TagDescription', 'ParameterValue': 'bar'},
+        ]
+    )
+    cf_conn.update_stack(
+        StackName=name, UsePreviousTemplate=True,
+        Parameters=[
+            {'ParameterKey': 'TagName', 'UsePreviousValue': True},
+            {'ParameterKey': 'TagDescription', 'ParameterValue': 'not bar'},
+        ]
+    )
+    stack = cf_conn.describe_stacks(StackName=name)['Stacks'][0]
+    tag_name = [x['ParameterValue'] for x in stack['Parameters']
+                if x['ParameterKey'] == 'TagName'][0]
+    tag_desc = [x['ParameterValue'] for x in stack['Parameters']
+                if x['ParameterKey'] == 'TagDescription'][0]
+    assert tag_name == 'foo'
+    assert tag_desc == 'not bar'
+
+
 @mock_cloudformation
 @mock_s3
 @mock_ec2