[Resolve #1436] Handle exception in differ (#1437)

See also Issue #1336. Before this change, in the event of Parameters
being entered of types other than str (e.g. bool, presumably float,
int etc), the differ would bomb out with a confusing AttributeError
error when trying to rstrip newline characters.

This changes to instead raise a SceptreException at that point, with an
informative error message to help the caller understand what they did
wrong.

CONTRIBUTING.md

@@ -85,7 +85,7 @@ $ git clone git@github.org:<github_username>/sceptre.git
    [virtual environment](http://docs.python-guide.org/en/latest/dev/virtualenvs/))
 
 ```bash
-   $ poetry install --all-extras -v
+$ poetry install --all-extras -v
 ```
 
 4. Create a branch for local development:

sceptre/diffing/stack_differ.py

@@ -19,6 +19,7 @@
 from cfn_tools import ODict
 from yaml import Dumper
 
+from sceptre.exceptions import SceptreException
 from sceptre.plan.actions import StackActions
 from sceptre.stack import Stack
 
@@ -173,9 +174,18 @@ def _extract_parameters_from_generated_stack(self, stack: Stack) -> dict:
             if value is None:
                 continue
 
-            if isinstance(value, list):
-                value = ",".join(item.rstrip("\n") for item in value)
-            formatted_parameters[key] = value.rstrip("\n")
+            try:
+                if isinstance(value, list):
+                    value = ",".join(item.rstrip("\n") for item in value)
+                formatted_parameters[key] = value.rstrip("\n")
+            # Other unexpected data can get through and this would blow up the differ
+            # and lead to quite confusing exceptions being raised. This check here could
+            # be removed in a future version of Sceptre if the reader class did sanity checking.
+            except AttributeError:
+                raise SceptreException(
+                    f"Parameter '{key}' whose value is {value} "
+                    f"is of type {type(value)} and not expected here"
+                )
 
         return formatted_parameters
 
@@ -361,7 +371,7 @@ def __init__(
         self,
         show_no_echo=False,
         *,
-        universal_template_loader: Callable[[str], Tuple[dict, str]] = cfn_flip.load
+        universal_template_loader: Callable[[str], Tuple[dict, str]] = cfn_flip.load,
     ):
         """Initializes a DeepDiffStackDiffer.
 
@@ -409,7 +419,7 @@ def __init__(
         self,
         show_no_echo=False,
         *,
-        universal_template_loader: Callable[[str], Tuple[dict, str]] = cfn_flip.load
+        universal_template_loader: Callable[[str], Tuple[dict, str]] = cfn_flip.load,
     ):
         """Initializes a DifflibStackDiffer.
 

tests/test_diffing/test_stack_differ.py

@@ -16,6 +16,7 @@
     DeepDiffStackDiffer,
     DifflibStackDiffer,
 )
+from sceptre.exceptions import SceptreException
 from sceptre.plan.actions import StackActions
 from sceptre.stack import Stack
 
@@ -343,6 +344,17 @@ def test_diff__generated_stack_has_none_for_parameter_value__its_treated_like_it
             self.expected_deployed_config, expected_generated
         )
 
+    def test_diff__generated_stack_has_a_bool(
+        self,
+    ):
+        self.parameters_on_stack_config["new"] = True
+        message = (
+            "Parameter 'new' whose value is True is of type "
+            "<class 'bool'> and not expected here"
+        )
+        with pytest.raises(SceptreException, match=message):
+            self.differ.diff(self.actions)
+
     def test_diff__stack_exists_with_same_config_but_template_does_not__compares_identical_configs(
         self,
     ):