Delete lib log missing (pandas-dev#551)

* Added log for non existing libraries when trying to delete

* Added logging functionality when deleting a library

* Added logging functionality when deleting a library

arctic/arctic.py

@@ -1,10 +1,10 @@
 import logging
 import os
+import re
+import threading
 
 import pymongo
 from pymongo.errors import OperationFailure, AutoReconnect
-import threading
-
 from ._util import indent
 from .auth import authenticate, get_auth
 from .decorators import mongo_retry
@@ -233,6 +233,8 @@ def delete_library(self, library):
         """
         l = ArcticLibraryBinding(self, library)
         colname = l.get_top_level_collection().name
+        if not [c for c in l._db.collection_names(False) if re.match(r"^{}([\.].*)?$".format(colname), c)]:
+            logger.info('Nothing to delete. Arctic library %s does not exist.' % colname)
         logger.info('Dropping collection: %s' % colname)
         l._db.drop_collection(colname)
         for coll in l._db.collection_names():