diff --git a/bin/tools/yaml_tag.py b/bin/tools/yaml_tag.py
index 648fd56206e..e12aecdcd96 100644
--- a/bin/tools/yaml_tag.py
+++ b/bin/tools/yaml_tag.py
@@ -4,7 +4,7 @@ import logging
 class YamlTag(object):
     """
     Superclass for constructors of custom tags defined in yaml file.
-    __str__ are overriden in subclass and used for serialization in module recorder.
+    __str__ is overriden in subclass and used for serialization in module recorder.
     """
     def __init__(self, **kwargs):
         self.__dict__.update(kwargs)
@@ -89,14 +89,14 @@ class Eval(YamlTag):
         self.expression = expression
         super(Eval, self).__init__()
     def __str__(self):
-        return 'eval(%s)' % (str(self.expr,))
+        return '!eval %s' % str(self.expression)
     
 class Ref(YamlTag):
     def __init__(self, expr="False", *args, **kwargs):
         self.expr = expr
         super(Ref, self).__init__(*args, **kwargs)
     def __str__(self):
-        return 'ref(%s)' % (str(self.expr,))
+        return 'ref(%s)' % repr(self.expr)
     
 class IrSet(YamlTag):
     def __init__(self):