fix(variable): treat unexported struct fields as nil instead of panicking

pongo2_issues_test.go

@@ -2471,3 +2471,26 @@ func TestBugAndOrPrecedence(t *testing.T) {
 		})
 	}
 }
+
+type issue204Item struct{ index string }
+
+func (i issue204Item) String() string { return "Item-" + i.index }
+
+// Issue #204: accessing an unexported struct field from a template must not panic.
+func TestIssue204UnexportedStructField(t *testing.T) {
+	items := []interface{}{issue204Item{"0"}, issue204Item{"1"}, issue204Item{"2"}}
+
+	for _, tplStr := range []string{
+		`{% for item in items %}{% if item.index %}x{% endif %}{% endfor %}`,
+		`{% for item in items %}{{ item["index"] }}{% endfor %}`,
+		`{% for item in items %}{{ item.index }}{% endfor %}`,
+	} {
+		tpl, err := pongo2.FromString(tplStr)
+		if err != nil {
+			t.Fatalf("parse error for %q: %v", tplStr, err)
+		}
+		if _, err := tpl.Execute(pongo2.Context{"items": items}); err != nil {
+			t.Fatalf("execute error for %q: %v", tplStr, err)
+		}
+	}
+}

variable.go

@@ -381,7 +381,12 @@
 func (vr *variableResolver) resolveIdentifier(current reflect.Value, part *variablePart) (reflect.Value, bool, error) {
 	switch current.Kind() {
 	case reflect.Struct:
-		return current.FieldByName(part.s), false, nil
+		field := current.FieldByName(part.s)
+		if field.IsValid() && !field.CanInterface() {
+			// Unexported fields are not accessible from templates (Django renders them empty).
+			return reflect.Value{}, true, nil
+		}
+		return field, false, nil
 	case reflect.Map:
 		return current.MapIndex(reflect.ValueOf(part.s)), false, nil
 	default:
@@ -391,7 +396,7 @@
 }
 
 // resolveSubscript resolves a subscript access (e.g., foo[bar]).
 func (vr *variableResolver) resolveSubscript(
 	ctx *ExecutionContext,
 	current reflect.Value,
 	part *variablePart,
@@ -417,7 +422,12 @@
 		}
 		return reflect.Value{}, true, nil
 	case reflect.Struct:
-		return current.FieldByName(sv.String()), false, nil
+		field := current.FieldByName(sv.String())
+		if field.IsValid() && !field.CanInterface() {
+			// Unexported fields are not accessible from templates (Django renders them empty).
+			return reflect.Value{}, true, nil
+		}
+		return field, false, nil
 	case reflect.Map:
 		if sv.IsNil() {
 			return reflect.Value{}, true, nil