choose last item in history when choices are exhausted

Raising an exception in the case that all choices have been exhausted in
the randomize module breaks things so set it to the last seen problem
instead.

common/lib/xmodule/xmodule/randomize_module.py

@@ -83,14 +83,16 @@ class RandomizeModule(RandomizeFields, XModule):
                     log.debug('using randrange for %s' % str(self.location))
                 self.choice = choice
             else:
-                # we're in a bad state - possible solution below tries to
-                # restore last failed problem from history (raise exc for now)
-                # try:
-                #     last_failed = json.loads(self.history or '[]')[-1]
-                #     self.choice = last_failed
-                # except IndexError:
-                #     raise Exception('No choices left!!!')
-                raise Exception('No choices left!!!')
+                # Student has seen all problems. Ideally we autogenerate a fake
+                # CapaModule problem and leave it until history gets wiped by
+                # an instructor/TA but for now we use the last seen choice from
+                # history.
+                log.error("No choices left!!! Attempting to use last "
+                          "item in history")
+                try:
+                    self.choice = self.history[-1]
+                except IndexError:
+                    log.error("Failed to load last item in history")
 
         if self.choice is not None:
             self.child_descriptor = all_choices.get(self.choice)
@@ -126,14 +128,14 @@ class RandomizeModule(RandomizeFields, XModule):
         return [self.child_descriptor]
 
     def student_view(self, context):
-        if self.choice is not None:
+        if self.child is None:
+            # raise error instead?  In fact, could complain on descriptor load
+            return Fragment(content=u"<div>Nothing to randomize between</div>")
+        else:
             child_loc = self.child.location.url()
             if child_loc not in self.history:
                 self.history.append(child_loc)
             self.save()
-        if self.child is None:
-            # raise error instead?  In fact, could complain on descriptor load
-            return Fragment(content=u"<div>Nothing to randomize between</div>")
         child_html = self.child.render('student_view', context)
         if self.system.user_is_staff:
             choices = self.get_choices().keys()