[Resolves #359] Implements use previous paramater feature

docs/_source/docs/stack_config.rst

@@ -86,6 +86,13 @@ values/resolvers. Lists of values/resolvers will be formatted into an AWS
 compatible comma separated string e.g. \ ``value1,value2,value3``. Lists can
 contain a mixture of values and resolvers.
 
+A parameter can also be configured to use the previous value. You can do so by
+making the value a dictionary. The values supported in the dictionary are
+``initial_value`` and ``use_previous_value``. When creating stacks, setting
+``initial_value`` is required, but can be left out for stack updates. The value
+set at ``initial_value`` value will only be used during creation, or when
+setting ``use_previous_value`` to false.
+
 Syntax:
 
 .. code-block:: yaml
@@ -102,6 +109,14 @@ Syntax:
      <parameter5_name>:
        - !<resolver_name> <resolver_value>
        - "value1"
+     <parameter6_name>:
+       initial_value: "value"
+       use_previous_value: <boolean>
+     <parameter7_name>:
+       initial_value:
+         - "value1"
+         - !<resolver_name> <resolver_value>
+       use_previous_value: <boolean>
 
 Example:
 
@@ -117,6 +132,11 @@ Example:
        - "sg-12345678"
        - !stack_output security-groups::BaseSecurityGroupId
        - !file_contents /file/with/security_group_id.txt
+     security_group_whitelist:
+       initial_value:
+         - "127.0.0.0/24"
+         - "127.0.1.0/24"
+       use_previous_value: true
 
 protected
 ~~~~~~~~~

docs/_source/docs/templates.rst

@@ -72,7 +72,7 @@ Stack:
 
 Template `dns-extras.j2`:
 
-.. code-block:: yaml
+.. code-block:: jinja
 
     AWSTemplateFormatVersion: '2010-09-09'
     Description: 'Add Route53 - CNAME and ALIAS records'

sceptre/exceptions.py

@@ -166,3 +166,10 @@ class InvalidAWSCredentialsError(SceptreException):
     Error raised when AWS credentials are invalid.
     """
     pass
+
+
+class InvalidParameterError(SceptreException):
+    """
+    Error raised when parameters are invalid.
+    """
+    pass

sceptre/plan/actions.py

@@ -27,6 +27,7 @@
 from sceptre.exceptions import UnknownStackChangeSetStatusError
 from sceptre.exceptions import StackDoesNotExistError
 from sceptre.exceptions import ProtectedStackError
+from sceptre.exceptions import InvalidParameterError
 
 
 class StackActions(object):
@@ -59,7 +60,7 @@ def create(self):
         self.logger.info("%s - Creating Stack", self.stack.name)
         create_stack_kwargs = {
             "StackName": self.stack.external_name,
-            "Parameters": self._format_parameters(self.stack.parameters),
+            "Parameters": self._format_parameters(self.stack.parameters, create=True),
             "Capabilities": ['CAPABILITY_IAM', 'CAPABILITY_NAMED_IAM', 'CAPABILITY_AUTO_EXPAND'],
             "NotificationARNs": self.stack.notifications,
             "Tags": [
@@ -640,7 +641,7 @@ def get_status(self):
         except StackDoesNotExistError:
             return "PENDING"
 
-    def _format_parameters(self, parameters):
+    def _format_parameters(self, parameters, create=False):
         """
         Converts CloudFormation parameters to the format used by Boto3.
 
@@ -653,12 +654,27 @@ def _format_parameters(self, parameters):
         for name, value in parameters.items():
             if value is None:
                 continue
+            formatted_parameter = dict(ParameterKey=name)
             if isinstance(value, list):
-                value = ",".join(value)
-            formatted_parameters.append({
-                "ParameterKey": name,
-                "ParameterValue": value
-            })
+                formatted_parameter['ParameterValue'] = ",".join(value)
+            elif isinstance(value, dict):
+                initial_value = value.get('initial_value')
+                use_previous_value = value.get('use_previous_value', False)
+                if not isinstance(use_previous_value, bool):
+                    raise InvalidParameterError("'use_previous_value' must be a boolean")
+                if (create is True or use_previous_value is False) and initial_value is None:
+                    raise InvalidParameterError("'initial_value' is required when creating a new "
+                                                "stack or when 'use_previous_value' is false")
+                if create is True or use_previous_value is False:
+                    if isinstance(initial_value, list):
+                        formatted_parameter['ParameterValue'] = ",".join(initial_value)
+                    else:
+                        formatted_parameter['ParameterValue'] = value.get('initial_value')
+                else:
+                    formatted_parameter['UsePreviousValue'] = use_previous_value
+            else:
+                formatted_parameter['ParameterValue'] = value
+            formatted_parameters.append(formatted_parameter)
 
         return formatted_parameters
 

tests/test_actions.py

@@ -18,6 +18,7 @@
 from sceptre.exceptions import UnknownStackChangeSetStatusError
 from sceptre.exceptions import StackDoesNotExistError
 from sceptre.exceptions import ProtectedStackError
+from sceptre.exceptions import InvalidParameterError
 
 
 class TestStackActions(object):
@@ -703,7 +704,7 @@ def test_unlock_calls_set_stack_policy_with_policy(
             "tests/fixtures/stack_policies/unlock.json"
         )
 
-    def test_format_parameters_with_sting_values(self):
+    def test_format_parameters_with_string_values(self):
         parameters = {
             "key1": "value1",
             "key2": "value2",
@@ -733,6 +734,20 @@ def test_format_parameters_with_none_values(self):
         )
         assert sorted_formatted_parameters == []
 
+    def test_format_parameters_with_empty_dict_value(self):
+        parameter = {
+            "key": dict()
+        }
+        with pytest.raises(InvalidParameterError):
+            self.actions._format_parameters(parameter)
+
+    def test_format_parameters_with_non_bool_previous_value(self):
+        parameter = {
+            "key": dict(use_previous_value='fosho')
+        }
+        with pytest.raises(InvalidParameterError):
+            self.actions._format_parameters(parameter)
+
     def test_format_parameters_with_none_and_string_values(self):
         parameters = {
             "key1": "value1",
@@ -815,6 +830,77 @@ def test_format_parameters_with_none_list_and_string_values(self):
             {"ParameterKey": "key2", "ParameterValue": "value4"},
         ]
 
+    def test_format_parameters_with_string_and_dict_values(self):
+        parameters = {
+            "key1": "value1",
+            "key2": {"initial_value": "value2"}
+        }
+        formatted_parameters = self.actions._format_parameters(parameters)
+        sorted_formatted_parameters = sorted(
+            formatted_parameters,
+            key=lambda x: x["ParameterKey"]
+        )
+        assert sorted_formatted_parameters == [
+            {"ParameterKey": "key1", "ParameterValue": "value1"},
+            {"ParameterKey": "key2", "ParameterValue": "value2"}
+        ]
+
+    def test_format_parameters_with_dict_string_and_list_values(self):
+        parameters = {
+            "key1": {"initial_value": ["value1", "value2"]},
+            "key2": {"initial_value": "value3"}
+        }
+        formatted_parameters = self.actions._format_parameters(parameters)
+        sorted_formatted_parameters = sorted(
+            formatted_parameters,
+            key=lambda x: x["ParameterKey"]
+        )
+        assert sorted_formatted_parameters == [
+            {"ParameterKey": "key1", "ParameterValue": "value1,value2"},
+            {"ParameterKey": "key2", "ParameterValue": "value3"}
+        ]
+
+    def test_format_parameters_with_string_and_previous_values(self):
+        parameters = {
+            "key1": "value1",
+            "key2": {"use_previous_value": True},
+            "key3": {"initial_value": "value3", "use_previous_value": True}
+        }
+        formatted_parameters = self.actions._format_parameters(parameters)
+        sorted_formatted_parameters = sorted(
+            formatted_parameters,
+            key=lambda x: x["ParameterKey"]
+        )
+        assert sorted_formatted_parameters == [
+            {"ParameterKey": "key1", "ParameterValue": "value1"},
+            {"ParameterKey": "key2", "UsePreviousValue": True},
+            {"ParameterKey": "key3", "UsePreviousValue": True}
+        ]
+
+    def test_format_parameters_with_create_and_previous_without_initial_values(self):
+        parameters = {
+            "key1": "value1",
+            "key2": {"use_previous_value": True},
+            "key3": {"initial_value": "value3", "use_previous_value": True}
+        }
+        with pytest.raises(InvalidParameterError):
+            self.actions._format_parameters(parameters, create=True)
+
+    def test_format_parameters_with_create_and_previous_values(self):
+        parameters = {
+            "key1": "value1",
+            "key2": {"initial_value": "value2", "use_previous_value": True}
+        }
+        formatted_parameters = self.actions._format_parameters(parameters, create=True)
+        sorted_formatted_parameters = sorted(
+            formatted_parameters,
+            key=lambda x: x["ParameterKey"]
+        )
+        assert sorted_formatted_parameters == [
+            {"ParameterKey": "key1", "ParameterValue": "value1"},
+            {"ParameterKey": "key2", "ParameterValue": "value2"}
+        ]
+
     @patch("sceptre.plan.actions.StackActions._describe")
     def test_get_status_with_created_stack(self, mock_describe):
         mock_describe.return_value = {