Add a 'string literal' serialization method and use it in the lambda function…

Add a 'string literal' serialization method and use it in the lambda function mapping table when the value to xml serialize is of basestring type. Also a couple of drive-by pep8 fixes.

common/lib/xmodule/xmodule/xml_module.py

@@ -88,6 +88,16 @@ def serialize_field(value):
     return json.dumps(value, cls=EdxJSONEncoder)
 
 
+def serialize_string_literal(value):
+    """
+    Assert that the value is a base string and - if it is - simply return it
+    """
+    if not isinstance(value, basestring):
+        raise Exception('Value {0} is not of type basestring!'.format(value))
+
+    return value
+
+
 def deserialize_field(field, value):
     """
     Deserialize the string version to the value stored internally.
@@ -166,7 +176,7 @@ class XmlDescriptor(XModuleDescriptor):
         for field in set(cls.fields + cls.lms.fields):
             if field.name == attr:
                 from_xml = lambda val: deserialize_field(field, val)
-                to_xml = lambda val : serialize_field(val)
+                to_xml = lambda val: serialize_string_literal(val) if isinstance(val, basestring) else serialize_field(val)
                 return AttrMap(from_xml, to_xml)
 
         return AttrMap()
@@ -254,7 +264,7 @@ class XmlDescriptor(XModuleDescriptor):
         definition, children = cls.definition_from_xml(definition_xml, system)
         if definition_metadata:
             definition['definition_metadata'] = definition_metadata
-        definition['filename'] = [ filepath, filename ]
+        definition['filename'] = [filepath, filename]
 
         return definition, children
 
@@ -280,7 +290,6 @@ class XmlDescriptor(XModuleDescriptor):
                 metadata[attr] = attr_map.from_xml(val)
         return metadata
 
-
     @classmethod
     def apply_policy(cls, metadata, policy):
         """
@@ -374,7 +383,6 @@ class XmlDescriptor(XModuleDescriptor):
         """
         return True
 
-
     def export_to_xml(self, resource_fs):
         """
         Returns an xml string representing this module, and all modules