Make decision about export file splitting overrideable

* create split_to_file() function
* chapters, html, problems always split
* course never split
* others based on size of subtree for now

common/lib/xmodule/xmodule/capa_module.py

@@ -550,6 +550,7 @@ class CapaDescriptor(RawDescriptor):
 
     module_class = CapaModule
 
+    # VS[compat]
     # TODO (cpennington): Delete this method once all fall 2012 course are being
     # edited in the cms
     @classmethod
@@ -558,3 +559,7 @@ class CapaDescriptor(RawDescriptor):
             'problems/' + path[8:],
             path[8:],
         ]
+    @classmethod
+    def split_to_file(cls, xml_object):
+        '''Problems always written in their own files'''
+        return True

common/lib/xmodule/xmodule/html_module.py

@@ -42,3 +42,8 @@ class HtmlDescriptor(RawDescriptor):
     def file_to_xml(cls, file_object):
         parser = etree.HTMLParser()
         return etree.parse(file_object, parser).getroot()
+
+    @classmethod
+    def split_to_file(cls, xml_object):
+        # never include inline html
+        return True

common/lib/xmodule/xmodule/seq_module.py

@@ -121,3 +121,17 @@ class SequenceDescriptor(MakoModuleDescriptor, XmlDescriptor):
             xml_object.append(
                 etree.fromstring(child.export_to_xml(resource_fs)))
         return xml_object
+
+    @classmethod
+    def split_to_file(cls, xml_object):
+        # Note: if we end up needing subclasses, can port this logic there.
+        yes = ('chapter',)
+        no = ('course',)
+
+        if xml_object.tag in yes:
+            return True
+        elif xml_object.tag in no:
+            return False
+
+        # otherwise maybe--delegate to superclass.
+        return XmlDescriptor.split_to_file(xml_object)

common/lib/xmodule/xmodule/xml_module.py

@@ -213,6 +213,20 @@ class XmlDescriptor(XModuleDescriptor):
                                                  name=name,
                                                  ext=cls.filename_extension)
 
+    @classmethod
+    def split_to_file(cls, xml_object):
+        '''
+        Decide whether to write this object to a separate file or not.
+
+        xml_object: an xml definition of an instance of cls.
+
+        This default implementation will split if this has more than 7
+        descendant tags.
+
+        Can be overridden by subclasses.
+        '''
+        return len(list(xml_object.iter())) > 7
+
     def export_to_xml(self, resource_fs):
         """
         Returns an xml string representing this module, and all modules
@@ -233,14 +247,14 @@ class XmlDescriptor(XModuleDescriptor):
 
         xml_object.tag = self.category
         xml_object.set('slug', self.name)
-        # Put content in a separate file if it's large (has more than 5 descendent tags)
-        if len(list(xml_object.iter())) > 5:
 
+        if self.split_to_file(xml_object):
+            # Put this object in it's own file
             filepath = self.__class__._format_filepath(self.category, self.name)
             resource_fs.makedir(os.path.dirname(filepath), allow_recreate=True)
             with resource_fs.open(filepath, 'w') as file:
                 file.write(etree.tostring(xml_object, pretty_print=True))
-
+            # ...and remove all of its children here
             for child in xml_object:
                 xml_object.remove(child)