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

@@ -81,13 +81,23 @@ class AttrMap(_AttrMapBase):
 
 def serialize_field(value):
     """
-        Return a string version of the value (where value is the JSON-formatted, internally stored value).
+    Return a string version of the value (where value is the JSON-formatted, internally stored value).
 
-        By default, this is the result of calling json.dumps on the input value.
-        """
+    By default, this is the result of calling json.dumps on the input 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.
@@ -126,7 +136,7 @@ class XmlDescriptor(XModuleDescriptor):
     """
 
     xml_attributes = Dict(help="Map of unhandled xml attributes, used only for storage between import and export",
-        default={}, scope=Scope.settings)
+                          default={}, scope=Scope.settings)
 
     # Extension to append to filename paths
     filename_extension = 'xml'
@@ -141,23 +151,23 @@ class XmlDescriptor(XModuleDescriptor):
     # understand?  And if we do, is this the place?
     # Related: What's the right behavior for clean_metadata?
     metadata_attributes = ('format', 'graceperiod', 'showanswer', 'rerandomize',
-        'start', 'due', 'graded', 'display_name', 'url_name', 'hide_from_toc',
-        'ispublic',  # if True, then course is listed for all users; see
-        'xqa_key',  # for xqaa server access
-        'giturl',  # url of git server for origin of file
-        # information about testcenter exams is a dict (of dicts), not a string,
-        # so it cannot be easily exportable as a course element's attribute.
-        'testcenter_info',
-        # VS[compat] Remove once unused.
-        'name', 'slug')
+                           'start', 'due', 'graded', 'display_name', 'url_name', 'hide_from_toc',
+                           'ispublic',  # if True, then course is listed for all users; see
+                           'xqa_key',  # for xqaa server access
+                           'giturl',  # url of git server for origin of file
+                           # information about testcenter exams is a dict (of dicts), not a string,
+                           # so it cannot be easily exportable as a course element's attribute.
+                           'testcenter_info',
+                           # VS[compat] Remove once unused.
+                           'name', 'slug')
 
     metadata_to_strip = ('data_dir',
-            'tabs', 'grading_policy', 'published_by', 'published_date',
-            'discussion_blackouts', 'testcenter_info',
-           # VS[compat] -- remove the below attrs once everything is in the CMS
-           'course', 'org', 'url_name', 'filename',
-           # Used for storing xml attributes between import and export, for roundtrips
-           'xml_attributes')
+                         'tabs', 'grading_policy', 'published_by', 'published_date',
+                         'discussion_blackouts', 'testcenter_info',
+                         # VS[compat] -- remove the below attrs once everything is in the CMS
+                         'course', 'org', 'url_name', 'filename',
+                         # Used for storing xml attributes between import and export, for roundtrips
+                         'xml_attributes')
 
     metadata_to_export_to_policy = ('discussion_topics')
 
@@ -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