feat: Error on undefined template variables

Previously, if a Jinja2 template contained a variable that was not
defined, the template would fail to render silently, and the raw,
un-rendered string would be returned as the value. This could lead to
unexpected behavior and make debugging difficult.

This change modifies the Jinja2 environment to use ,
which raises an  when a template variable is not defined.
This error is now caught and re-raised as a  with a more
informative message, making it clear which variable is causing the issue.

A new test has been added to verify that an error is raised when a
template contains an undefined variable.

Author: kthhrv

src/envars/main.py

@@ -3,7 +3,8 @@
 from collections import deque
 
 import yaml
-from jinja2 import Environment, meta
+from jinja2 import Environment, StrictUndefined, meta
+from jinja2.exceptions import UndefinedError
 
 from .aws_cloudformation import CloudFormationExports
 from .aws_kms import AWSKMSAgent
@@ -377,7 +378,7 @@ def _get_resolved_variables(
 
     # Template substitution with Jinja2
     sorted_order = _check_for_circular_dependencies(resolved_vars)
-    jinja_env = Environment(autoescape=True)
+    jinja_env = Environment(autoescape=True, undefined=StrictUndefined)
     rendered = {}
     for var_name in sorted_order:
         value = resolved_vars[var_name]
@@ -387,8 +388,8 @@ def _get_resolved_variables(
                 context = {"env": os.environ}
                 context.update(rendered)
                 rendered[var_name] = template.render(context)
-            except Exception:
-                rendered[var_name] = value
+            except UndefinedError as e:
+                raise ValueError(f"Error rendering template for variable '{var_name}': {e}") from e
         else:
             rendered[var_name] = value
     resolved_vars = rendered

tests/test_main.py

@@ -333,3 +333,59 @@ def test_resolve_cloudformation_export(tmp_path, monkeypatch):
     assert "MY_EXPORT" in resolved_vars
     assert resolved_vars["MY_EXPORT"] == "my-cf-export-value"
     mock_cf_exports.get_export_value.assert_called_once_with("my-cf-export")
+
+
+def test_resolve_jinja2_template(tmp_path, monkeypatch):
+    """Test that Jinja2 templates are resolved correctly."""
+    yaml_content = """
+configuration:
+  environments:
+    - dev
+  locations:
+    - local: "local"
+
+environment_variables:
+  GREETING:
+    default: "Hello"
+  NAME:
+    default: "World"
+  MESSAGE:
+    default: "{{ GREETING }}, {{ NAME }}!"
+  SHELL_VAR:
+    default: "Value is: {{ env.get('MY_SHELL_VAR', 'default') }}"
+"""
+    file_path = create_yaml_file(tmp_path, yaml_content)
+    manager = load_from_yaml(file_path)
+
+    # Test with shell variable set
+    monkeypatch.setenv("MY_SHELL_VAR", "from_shell")
+    resolved_vars = _get_resolved_variables(manager, loc="local", env="dev", decrypt=True)
+    assert resolved_vars["MESSAGE"] == "Hello, World!"
+    assert resolved_vars["SHELL_VAR"] == "Value is: from_shell"
+
+    # Test with shell variable not set (should use default)
+    monkeypatch.delenv("MY_SHELL_VAR", raising=False)
+    resolved_vars = _get_resolved_variables(manager, loc="local", env="dev", decrypt=True)
+    assert resolved_vars["SHELL_VAR"] == "Value is: default"
+
+
+def test_resolve_template_with_undefined_variable_raises_error(tmp_path):
+    """Test that a template with an undefined variable raises a ValueError."""
+    yaml_content = """
+configuration:
+  environments:
+    - dev
+  locations:
+    - local: "local"
+
+environment_variables:
+  MESSAGE:
+    default: "Hello, {{ UNDEFINED_VAR }}"
+"""
+    file_path = create_yaml_file(tmp_path, yaml_content)
+    manager = load_from_yaml(file_path)
+
+    with pytest.raises(
+        ValueError, match="Error rendering template for variable 'MESSAGE': 'UNDEFINED_VAR' is undefined"
+    ):
+        _get_resolved_variables(manager, loc="local", env="dev", decrypt=True)