Fix listen_* methods without valid last_object provided (#24)

* Fix listen_* methods without valid last_object provided

* Reorganized last_object method

src/core.py

@@ -168,42 +168,35 @@ def recent_blobs(self):
 
     def _listen(self, last_object, object_type, blocking=True, interval=15, query=None):
         if last_object is None:
-            last_object = next(self._recent(object_type, query=query))
+            last_object = next(self._recent(object_type, query=query), None)
             # If there are no elements (even first element): just get new samples from now on
-            if last_object is not None:
-                last_id = last_object.id
-                last_time = last_object.upload_time
         elif isinstance(last_object, MWDBObject):
             # If we are requesting for typed objects, we should additionally check the object type
             if object_type is not MWDBObject and not isinstance(last_object, object_type):
                 raise TypeError("latest_object type must be 'str' or '{}'".format(object_type.__name__))
             # If object instance provided: get ID from instance
-            last_id = last_object.id
-            last_time = last_object.upload_time
         else:
             # If not: first check whether object exists in repository
-            last_obj = self._query(object_type, last_object, raise_not_found=True)
-            last_id = last_obj.id
-            last_time = last_obj.upload_time
+            last_object = self._query(object_type, last_object, raise_not_found=True)
 
         while True:
             objects = []
             for obj in self._recent(object_type, query=query):
-                if obj.id == last_id:
-                    break
-
-                if obj.upload_time < last_time:
-                    raise RuntimeError(
-                        "Newly fetched object [{}] is older than the pivot [{}]".format(
-                            obj.id, last_id
+                if last_object:
+                    if obj.id == last_object.id:
+                        break
+
+                    if obj.upload_time < last_object.upload_time:
+                        raise RuntimeError(
+                            "Newly fetched object [{}] is older than the pivot [{}]".format(
+                                obj.id, last_object.id
+                            )
                         )
-                    )
                 objects.append(obj)
 
             # Return fetched objects in reversed order (from oldest to latest)
             for obj in objects[::-1]:
-                last_id = obj.id
-                last_time = obj.upload_time
+                last_object = obj
                 yield obj
             if blocking:
                 time.sleep(interval)