Merge pull request #456 from douglasjacobsen/update_is_node_error

Update `is` node error

lib/ramble/ramble/expander.py

@@ -597,6 +597,8 @@ def perform_math_eval(self, in_str):
         except MathEvaluationError as e:
             logger.debug(f'   Math input is: "{in_str}"')
             logger.debug(e)
+        except RambleSyntaxError as e:
+            raise RambleSyntaxError(f'{str(e)} in "{in_str}"')
 
         return in_str
 
@@ -711,6 +713,9 @@ def _eval_comparisons(self, node):
         if len(node.ops) == 1 and isinstance(node.ops[0], ast.In):
             return self._eval_comp_in(node)
 
+        if len(node.ops) == 1 and isinstance(node.ops[0], ast.Is):
+            raise RambleSyntaxError('Encountered unsupported operator `is`')
+
         # Try to evaluate the comparison logic, if not return the node as is.
         try:
             cur_left = self.eval_math(node.left)