introduice uuid type check

xtreemfs_client/osd.py

@@ -4,7 +4,9 @@ class OSD(object):
     it keeps track of the folders saved on the OSD as well as the size of the folders.
     """
 
-    def __init__(self, uuid):
+    def __init__(self, uuid: str):
+        if not isinstance(uuid, str):
+            raise ValueError("OSD uuid must be str!")
         self.uuid = uuid
         self.total_folder_size = 0
         self.folders = {}