Requesting an xmodule field in wrong app context raises exception rather than assertion

common/lib/xmodule/xmodule/exceptions.py

@@ -31,3 +31,10 @@ class SerializationError(Exception):
     def __init__(self, location, msg):
         super(SerializationError, self).__init__(msg)
         self.location = location
+
+
+class UndefinedContext(Exception):
+    """
+    Tried to access an xmodule field which needs a different context (runtime) to have a value.
+    """
+    pass

common/lib/xmodule/xmodule/modulestore/mixed.py

@@ -15,6 +15,7 @@ from xmodule.modulestore.exceptions import ItemNotFoundError, InvalidLocationErr
 from uuid import uuid4
 from xmodule.modulestore.mongo.base import MongoModuleStore
 from xmodule.modulestore.split_mongo.split import SplitMongoModuleStore
+from xmodule.exceptions import UndefinedContext
 
 log = logging.getLogger(__name__)
 
@@ -207,9 +208,10 @@ class MixedModuleStore(ModuleStoreWriteBase):
         """
         Get the course_id from the block or from asking its store. Expensive.
         """
-        # HACK: check xmodule_runtime to avoid an assertion error
-        if block.xmodule_runtime is not None and block.course_id is not None:
+        try:
             return block.course_id
+        except UndefinedContext:
+            pass
         try:
             course = store._get_course_for_item(block.scope_ids.usage_id)
             if course is not None:
@@ -262,9 +264,10 @@ class MixedModuleStore(ModuleStoreWriteBase):
                     blocks = store.get_items(location)
                     if len(blocks) == 1:
                         block = blocks[0]
-                        # HACK violate abstraction to avoid assertion error
-                        if block.xmodule_runtime is not None and block.course_id is not None:
+                        try:
                             return block.course_id
+                        except UndefinedContext:
+                            pass
                 except ItemNotFoundError:
                     pass
         # if we get here, it must be in a Locator based store, but we won't be able to find

common/lib/xmodule/xmodule/x_module.py

@@ -26,6 +26,7 @@ from xmodule.errortracker import exc_info_to_str
 from xmodule.modulestore import Location
 from xmodule.modulestore.exceptions import ItemNotFoundError, InsufficientSpecificationError, InvalidLocationError
 from xmodule.modulestore.locator import BlockUsageLocator
+from xmodule.exceptions import UndefinedContext
 
 
 log = logging.getLogger(__name__)
@@ -795,7 +796,8 @@ class XModuleDescriptor(XModuleMixin, HTMLSnippet, ResourceTemplates, XBlock):
         Returns the XModule corresponding to this descriptor. Expects that the system
         already supports all of the attributes needed by xmodules
         """
-        assert self.xmodule_runtime is not None
+        if self.xmodule_runtime is None:
+            raise UndefinedContext()
         assert self.xmodule_runtime.error_descriptor_class is not None
         if self.xmodule_runtime.xmodule_instance is None:
             try: