Deleted Stack Fix and ValidationError for DescribeStacks.

* stop backed from trying to iterate over empty list of deleted stacks.
  * Update DescribeStacks to throw ValidationError if stack name or id doesn't exist.

moto/cloudformation/exceptions.py

@@ -1,6 +1,26 @@
 from __future__ import unicode_literals
+from boto.exception import BotoServerError
+from jinja2 import Template
+
 
 
 class UnformattedGetAttTemplateException(Exception):
     description = 'Template error: resource {0} does not support attribute type {1} in Fn::GetAtt'
     status_code = 400
+
+
+class ValidationError(BotoServerError):
+    def __init__(self, name_or_id):
+        template = Template(STACK_DOES_NOT_EXIST_RESPONSE)
+        super(ValidationError, self).__init__(status=400, reason='Bad Request',
+                                              body=template.render(name_or_id=name_or_id))
+
+STACK_DOES_NOT_EXIST_RESPONSE = """<ErrorResponse xmlns="http://cloudformation.amazonaws.com/doc/2010-05-15/">
+  <Error>
+    <Type>Sender</Type>
+    <Code>ValidationError</Code>
+    <Message>Stack:{{ name_or_id }} does not exist</Message>
+  </Error>
+  <RequestId>cf4c737e-5ae2-11e4-a7c9-ad44eEXAMPLE</RequestId>
+</ErrorResponse>
+"""

moto/cloudformation/models.py

@@ -5,6 +5,7 @@ from moto.core import BaseBackend
 
 from .parsing import ResourceMap, OutputMap
 from .utils import generate_stack_id
+from .exceptions import ValidationError
 
 
 class FakeStack(object):
@@ -50,10 +51,12 @@ class CloudFormationBackend(BaseBackend):
             for stack in stacks:
                 if stack.name == name_or_stack_id or stack.stack_id == name_or_stack_id:
                     return [stack]
-            deleted_stacks = self.deleted_stacks.values()
+            if self.deleted_stacks:
-            for stack in deleted_stacks:
+                deleted_stacks = self.deleted_stacks.values()
-                if stack.stack_id == name_or_stack_id:
+                for stack in deleted_stacks:
-                    return [stack]
+                    if stack.stack_id == name_or_stack_id:
+                        return [stack]
+            raise ValidationError(name_or_stack_id)
         else:
             return stacks