Push dependency on mitxmako up out of mako_module

common/lib/xmodule/xmodule/mako_module.py

-from x_module import XModuleDescriptor
-from mitxmako.shortcuts import render_to_string
+from x_module import XModuleDescriptor, DescriptorSystem
+
+
+class MakoDescriptorSystem(DescriptorSystem):
+    def __init__(self, render_template, *args, **kwargs):
+        self.render_template = render_template
+        super(MakoDescriptorSystem, self).__init__(*args, **kwargs)
 
 
 class MakoModuleDescriptor(XModuleDescriptor):
@@ -12,6 +17,11 @@ class MakoModuleDescriptor(XModuleDescriptor):
     the descriptor as the `module` parameter to that template
     """
 
+    def __init__(self, system, definition=None, **kwargs):
+        if getattr(system, 'render_template', None) is None:
+            raise TypeError('{system} must have a render_template function in order to use a MakoDescriptor'.format(system=system))
+        super(MakoModuleDescriptor, self).__init__(system, definition, **kwargs)
+
     def get_context(self):
         """
         Return the context to render the mako template with
@@ -19,4 +29,4 @@ class MakoModuleDescriptor(XModuleDescriptor):
         return {'module': self}
 
     def get_html(self):
-        return render_to_string(self.mako_template, self.get_context())
+        return self.system.render_template(self.mako_template, self.get_context())

common/lib/xmodule/xmodule/modulestore/mongo.py

 import pymongo
 from importlib import import_module
-from xmodule.x_module import XModuleDescriptor, DescriptorSystem
+from xmodule.x_module import XModuleDescriptor
+from xmodule.mako_module import MakoDescriptorSystem
+from mitxmako.shortcuts import render_to_string
 
 from . import ModuleStore, Location
 from .exceptions import ItemNotFoundError, InsufficientSpecificationError
@@ -54,7 +56,7 @@ class MongoModuleStore(ModuleStore):
 
         # TODO (cpennington): Pass a proper resources_fs to the system
         return XModuleDescriptor.load_from_json(
-            item, DescriptorSystem(self.get_item, None), self.default_class)
+            item, MakoDescriptorSystem(load_item=self.get_item, resources_fs=None, render_template=render_to_string), self.default_class)
 
     def create_item(self, location):
         """