Provide a path for default xml to load upon creation.

mentoring/light_children.py

@@ -55,6 +55,16 @@ from .utils import XBlockWithChildrenFragmentsMixin
 log = logging.getLogger(__name__)
 
 
+def get_xml_content(block):
+    """
+    Because we get the XML content dynamically, we can't use the String field's
+    default kwarg to fall back on. Therefore any time we need to access the
+    xml_content's value, we need to do it through this function.
+    """
+    default = getattr(block, 'default_xml_content', '')
+    value = getattr(block, 'xml_content', '')
+    return value or default
+
 # Classes ###########################################################
 
 class LightChildrenMixin(XBlockWithChildrenFragmentsMixin):
@@ -83,7 +93,7 @@ class LightChildrenMixin(XBlockWithChildrenFragmentsMixin):
         log.debug('parse_xml called')
         block = runtime.construct_xblock_from_class(cls, keys)
         cls.init_block_from_node(block, node, node.items())
-        block.xml_content = getattr(block, 'xml_content', '') or etree.tostring(node)
+        block.xml_content = get_xml_content(block) or etree.tostring(node)
         return block
 
     @classmethod
@@ -126,11 +136,12 @@ class LightChildrenMixin(XBlockWithChildrenFragmentsMixin):
         Load light children from the `xml_content` attribute
         """
         self.light_children = []
-        if not hasattr(self, 'xml_content') or not self.xml_content:
+        xml_content = get_xml_content(self)
+        if not xml_content:
             return
 
         parser = etree.XMLParser(remove_comments=True)
-        node = etree.parse(StringIO(self.xml_content), parser=parser).getroot()
+        node = etree.parse(StringIO(xml_content), parser=parser).getroot()
         LightChildrenMixin.init_block_from_node(self, node, node.items())
 
     def get_children_objects(self):

mentoring/mentoring.py

@@ -33,7 +33,7 @@ from xblock.core import XBlock
 from xblock.fields import Boolean, Scope, String, Integer, Float, List
 from xblock.fragment import Fragment
 
-from .light_children import XBlockWithLightChildren
+from .light_children import XBlockWithLightChildren, get_xml_content
 from .title import TitleBlock
 from .header import SharedHeaderBlock
 from .html import HTMLBlock
@@ -71,6 +71,7 @@ class MentoringBlock(XBlockWithLightChildren, StepParentMixin):
     enforce_dependency = Boolean(help="Should the next step be the current block to complete?",
                                  default=False, scope=Scope.content, enforce_type=True)
     display_submit = Boolean(help="Allow to submit current block?", default=True, scope=Scope.content)
+    # See get_xml_content from light_children.
     xml_content = String(help="XML content", default='', scope=Scope.content)
     weight = Float(help="Defines the maximum total grade of the block.",
                    default=1, scope=Scope.content, enforce_type=True)
@@ -373,7 +374,7 @@ class MentoringBlock(XBlockWithLightChildren, StepParentMixin):
         fragment = Fragment()
         fragment.add_content(render_template('templates/html/mentoring_edit.html', {
             'self': self,
-            'xml_content': self.xml_content or self.default_xml_content,
+            'xml_content': get_xml_content(self)
         }))
         fragment.add_javascript_url(
             self.runtime.local_resource_url(self, 'public/js/mentoring_edit.js'))