Updated based on PR comments

tests/app/test_utils.py

@@ -3,12 +3,14 @@ from datetime import date, datetime
 import pytest
 from freezegun import freeze_time
 
-from app.enums import ServicePermissionType
+from app.enums import ServicePermissionType, TemplateType
 from app.utils import (
     get_midnight_in_utc,
     get_public_notify_type_text,
+    get_template_instance,
     midnight_n_days_ago,
 )
+from notifications_utils.template import HTMLEmailTemplate, SMSMessageTemplate
 
 
 @pytest.mark.parametrize(
@@ -52,3 +54,87 @@ def test_get_public_notify_type_text():
     assert (
         get_public_notify_type_text(ServicePermissionType.UPLOAD_DOCUMENT) == "document"
     )
+
+@pytest.mark.parametrize(
+    "template_type, expected_class",
+    [
+        (TemplateType.SMS, SMSMessageTemplate),
+        (TemplateType.EMAIL, HTMLEmailTemplate),
+    ],
+)
+def test_get_template_instance_with_none_values(template_type, expected_class):
+    """Test that get_template_instance handles None values safely for both template types"""
+    template = {
+        "template_type": template_type,
+        "id": "test-id",
+        "content": "Test content",
+        "subject": "Test subject" if template_type == TemplateType.EMAIL else None,
+    }
+
+    result = get_template_instance(template, values=None)
+    assert isinstance(result, expected_class)
+
+    result_default = get_template_instance(template)
+    assert isinstance(result_default, expected_class)
+
+
+@pytest.mark.parametrize(
+    "template_type, expected_class",
+    [
+        (TemplateType.SMS, SMSMessageTemplate),
+        (TemplateType.EMAIL, HTMLEmailTemplate),
+    ],
+)
+def test_get_template_instance_with_actual_values(template_type, expected_class):
+    """Test that get_template_instance works normally with actual values"""
+    template = {
+        "template_type": template_type,
+        "id": "test-id",
+        "content": "Hello ((name))",
+        "subject": "Test subject" if template_type == TemplateType.EMAIL else None,
+    }
+
+    values = {"name": "World"}
+
+    result = get_template_instance(template, values=values)
+    assert isinstance(result, expected_class)
+
+
+def test_get_template_instance_invalid_template_type():
+    """Test that get_template_instance raises KeyError for invalid template types"""
+    template = {
+        "template_type": "INVALID_TYPE",
+        "id": "test-id",
+        "content": "Test content",
+    }
+
+    with pytest.raises(KeyError):
+        get_template_instance(template, values=None)
+
+
+@pytest.mark.parametrize(
+    "template_type, values",
+    [
+        (TemplateType.SMS, None),
+        (TemplateType.SMS, {}),
+        (TemplateType.SMS, {"name": "test"}),
+        (TemplateType.EMAIL, None),
+        (TemplateType.EMAIL, {}),
+        (TemplateType.EMAIL, {"name": "test"}),
+    ],
+)
+def test_get_template_instance_comprehensive(template_type, values):
+    """Comprehensive test covering all combinations of template types and value scenarios"""
+    template = {
+        "template_type": template_type,
+        "id": "test-id",
+        "content": "Test content ((name))" if values and "name" in values else "Test content",
+        "subject": "Test subject" if template_type == TemplateType.EMAIL else None,
+    }
+
+    result = get_template_instance(template, values=values)
+
+    if template_type == TemplateType.SMS:
+        assert isinstance(result, SMSMessageTemplate)
+    else:
+        assert isinstance(result, HTMLEmailTemplate)