Fix update command with change sets for multiple stacks

Previously the update command would exit if any change sets status
was not equal to READY. However, when a stack does not contain
any updates, it will not be READY since there is nothing to execute.
This should not prevent other change sets to be executed.

[Resolves Sceptre#723]

sceptre/cli/update.py

@@ -52,31 +52,46 @@ def update_command(ctx, path, change_set, verbose, yes):
     plan = SceptrePlan(context)
 
     if change_set:
+        delete_change_set = True
+
         change_set_name = "-".join(["change-set", uuid1().hex])
         plan.create_change_set(change_set_name)
         try:
             # Wait for change set to be created
             statuses = plan.wait_for_cs_completion(change_set_name)
             # Exit if change set fails to create
+            atleast_one_ready = False
             for status in list(statuses.values()):
-                if status != StackChangeSetStatus.READY:
+                if status != StackChangeSetStatus.READY \
+                        and status != StackChangeSetStatus.NO_CHANGES:
+                    # TODO: Print an informative message to the user.
                     exit(1)
+                if status == StackChangeSetStatus.READY:
+                    atleast_one_ready = True
+
+            if not atleast_one_ready:
+                write("No changes detected", context.output_format)
+                exit(0)
 
             # Describe changes
             descriptions = plan.describe_change_set(change_set_name)
             for description in list(descriptions.values()):
+                status = description.get("Status")
+                reason = description.get("StatusReason")
+                if status == "FAILED" and "submitted information didn't contain changes" in reason:
+                    continue
                 if not verbose:
                     description = simplify_change_set_description(description)
                 write(description, context.output_format)
 
             # Execute change set if happy with changes
             if yes or click.confirm("Proceed with stack update?"):
                 plan.execute_change_set(change_set_name)
-        except Exception as e:
-            raise e
+                delete_change_set = False
         finally:
             # Clean up by deleting change set
-            plan.delete_change_set(change_set_name)
+            if delete_change_set:
+                plan.delete_change_set(change_set_name)
     else:
         confirmation("update", yes, command_path=path)
         responses = plan.update()

sceptre/plan/actions.py

@@ -884,6 +884,7 @@ def _get_cs_status(self, change_set_name):
         cs_description = self.describe_change_set(change_set_name)
 
         cs_status = cs_description["Status"]
+        cs_reason = cs_description.get("StatusReason")
         cs_exec_status = cs_description["ExecutionStatus"]
         possible_statuses = [
             "CREATE_PENDING", "CREATE_IN_PROGRESS",
@@ -915,6 +916,12 @@ def _get_cs_status(self, change_set_name):
                 cs_exec_status in ["UNAVAILABLE", "AVAILABLE"]
         ):
             return StackChangeSetStatus.PENDING
+        elif (
+                cs_status == "FAILED" and
+                cs_reason is not None and
+                "submitted information didn't contain changes" in cs_reason
+        ):
+            return StackChangeSetStatus.NO_CHANGES
         elif (
                 cs_status in ["DELETE_COMPLETE", "FAILED"] or
                 cs_exec_status in [

sceptre/stack_status.py

@@ -25,3 +25,4 @@ class StackChangeSetStatus(object):
     PENDING = "pending"
     READY = "ready"
     DEFUNCT = "defunct"
+    NO_CHANGES = "no changes"

tests/test_actions.py

@@ -1088,6 +1088,17 @@ def test_get_cs_status_handles_all_statuses(
                 )
                 assert response == returns[i]
 
+        mock_describe_change_set.return_value = {
+            "Status": "FAILED",
+            "StatusReason": "The submitted information didn't contain changes. "
+                            "Submit different information to create a change set.",
+            "ExecutionStatus": "UNAVAILABLE"
+        }
+        response = self.actions._get_cs_status(
+            sentinel.change_set_name
+        )
+        assert response == scss.NO_CHANGES
+
         for status in return_values['Status']:
             mock_describe_change_set.return_value = {
                 "Status": status,