[Resolves #1388] Handle failures gracefully in stack outputs

sceptre/resolvers/stack_output.py

@@ -6,7 +6,12 @@
 
 from botocore.exceptions import ClientError
 
-from sceptre.exceptions import DependencyStackMissingOutputError, StackDoesNotExistError
+from sceptre.exceptions import (
+    DependencyStackMissingOutputError,
+    StackDoesNotExistError,
+    SceptreException,
+)
+
 from sceptre.helpers import normalise_path, sceptreise_path
 from sceptre.resolvers import Resolver
 
@@ -108,7 +113,13 @@ def setup(self):
         """
         Adds dependency to a Stack.
         """
-        dep_stack_name, self.output_key = self.argument.split("::")
+        try:
+            dep_stack_name, self.output_key = self.argument.split("::")
+        except ValueError as err:
+            raise SceptreException(
+                "!stack_output arg should match STACK_NAME::OUTPUT_KEY"
+            ) from err
+
         self.dependency_stack_name = sceptreise_path(normalise_path(dep_stack_name))
         self.stack.dependencies.append(self.dependency_stack_name)
 
@@ -120,7 +131,6 @@ def resolve(self):
         :rtype: str
         """
         self.logger.debug("Resolving Stack output: {0}".format(self.argument))
-
         friendly_stack_name = self.dependency_stack_name.replace(TEMPLATE_EXTENSION, "")
 
         stack = next(
@@ -170,10 +180,24 @@ def resolve(self):
 
         stack_argument = arguments[0]
         if len(arguments) > 1:
-            extra_args = arguments[1].split("::", 2)
-            profile, region, sceptre_role = extra_args + (3 - len(extra_args)) * [None]
+            try:
+                extra_args = arguments[1].split("::", 2)
+                profile, region, sceptre_role = extra_args + (3 - len(extra_args)) * [
+                    None
+                ]
+            except ValueError as err:
+                message = (
+                    "!stack_output_external second arg should be "
+                    "in the format 'profile::region::sceptre_role'"
+                )
+                raise SceptreException(message) from err
+
+        try:
+            dependency_stack_name, output_key = stack_argument.split("::")
+        except ValueError as err:
+            message = "!stack_output_external arg should match STACK_NAME::OUTPUT_KEY"
+            raise SceptreException(message) from err
 
-        dependency_stack_name, output_key = stack_argument.split("::")
         return self._get_output_value(
             dependency_stack_name,
             output_key,

tests/test_resolvers/test_stack_output.py

@@ -5,6 +5,8 @@
 
 from sceptre.exceptions import DependencyStackMissingOutputError
 from sceptre.exceptions import StackDoesNotExistError
+from sceptre.exceptions import SceptreException
+
 from botocore.exceptions import ClientError
 
 from sceptre.connection_manager import ConnectionManager
@@ -50,6 +52,16 @@ def test_resolver(self, mock_get_output_value):
             sceptre_role="dependency_sceptre_role",
         )
 
+    @patch("sceptre.resolvers.stack_output.StackOutput._get_output_value")
+    def test_resolver__badly_formatted(self, mock_get_output_value):
+        stack = MagicMock(spec=Stack)
+        stack.name = "my/stack"
+
+        stack_output_resolver = StackOutput("not_a_valid_stack_output", stack)
+
+        with pytest.raises(SceptreException, match="STACK_NAME::OUTPUT_KEY"):
+            stack_output_resolver.setup()
+
     @patch("sceptre.resolvers.stack_output.StackOutput._get_output_value")
     def test_resolver_with_existing_dependencies(self, mock_get_output_value):
         stack = MagicMock(spec=Stack)
@@ -171,6 +183,18 @@ def test_resolve(self, mock_get_output_value):
         )
         assert stack.dependencies == []
 
+    @patch("sceptre.resolvers.stack_output.StackOutput._get_output_value")
+    def test_resolve__badly_formatted(self, mock_get_output_value):
+        stack = MagicMock(spec=Stack)
+        stack.name = "my/stack"
+
+        stack_output_external_resolver = StackOutputExternal(
+            "not_a_valid_stack_output", stack
+        )
+
+        with pytest.raises(SceptreException, match="STACK_NAME::OUTPUT_KEY"):
+            stack_output_external_resolver.resolve()
+
     @patch("sceptre.resolvers.stack_output.StackOutputExternal._get_output_value")
     def test_resolve_with_args(self, mock_get_output_value):
         stack = MagicMock(spec=Stack)